LTE air interface

LTE Air Interface

Training Manual Contents

Issue 01 (2010-05-01) Huawei Proprietary and Confidential

Copyright © Huawei Technologies Co., Ltd

i

Contents

1 The Air Interface ............................................................................................................................. 1-1

1.1 Evolution of Cellular Networks .................................................................................................................... 1-2

1.1.1 First Generation Mobile Systems ......................................................................................................... 1-2

1.1.2 Second Generation Mobile Systems..................................................................................................... 1-2

1.1.3 Third Generation Mobile Systems ....................................................................................................... 1-4

1.1.4 Fourth Generation Mobile Systems...................................................................................................... 1-5

1.2 3GPP Releases............................................................................................................................................... 1-6

1.2.1 Pre-Release 99...................................................................................................................................... 1-6

1.2.2 Release 99 ............................................................................................................................................ 1-7

1.2.3 Release 4 .............................................................................................................................................. 1-7

1.2.4 Release 5 .............................................................................................................................................. 1-7

1.2.5 Release 6 .............................................................................................................................................. 1-7

1.2.6 Release 7 .............................................................................................................................................. 1-8

1.2.7 Release 8 .............................................................................................................................................. 1-9

1.2.8 Release 9 and Beyond ........................................................................................................................ 1-10

1.3 Radio Interface Techniques ......................................................................................................................... 1-10

1.3.1 Frequency Division Multiple Access ................................................................................................. 1-10

1.3.2 Time Division Multiple Access .......................................................................................................... 1-11

1.3.3 Code Division Multiple Access .......................................................................................................... 1-11

1.3.4 Orthogonal Frequency Division Multiple Access .............................................................................. 1-12

1.4 Transmission Modes .................................................................................................................................... 1-12

1.4.1 Frequency Division Duplex ............................................................................................................... 1-13

1.4.2 Time Division Duplex ........................................................................................................................ 1-13

1.5 Spectrum Usage........................................................................................................................................... 1-14

1.5.1 Frequency Bands ................................................................................................................................ 1-14

1.5.2 Existing Mobile Deployment ............................................................................................................. 1-16

1.5.3 LTE Release 8 Bands ......................................................................................................................... 1-17

1.6 Channel Coding in LTE ............................................................................................................................... 1-20

1.6.1 Transport Block CRC ......................................................................................................................... 1-20

1.6.2 Code Block Segmentation and CRC Attachment ............................................................................... 1-21

1.6.3 Channel Coding.................................................................................................................................. 1-23

1.6.4 Rate Matching .................................................................................................................................... 1-28

Contents

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Training Manual

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Issue 01 (2010-05-01)

1.6.5 Code Block Concatenation ................................................................................................................. 1-29

1.7 Principles of OFDM .................................................................................................................................... 1-30

1.7.2 Frequency Division Multiplexing ...................................................................................................... 1-30

1.7.1 OFDM Subcarriers ............................................................................................................................. 1-31

1.7.2 Fast Fourier Transforms ..................................................................................................................... 1-31

1.7.3 LTE FFT Sizes ................................................................................................................................... 1-32

1.7.4 OFDM Symbol Mapping ................................................................................................................... 1-32

1.7.5 Time Domain Interference ................................................................................................................. 1-33

1.7.6 OFDM Advantages and Disadvantages .............................................................................................. 1-35

2 LTE Physical Layer ......................................................................................................................... 2-1

2.1 The Uu Interface............................................................................................................................................ 2-3

2.2 LTE Radio Interface Protocols ...................................................................................................................... 2-3

2.2.1 Control and User Plane Protocols ........................................................................................................ 2-4

2.2.2 Non Access Stratum ............................................................................................................................. 2-4

2.2.3 RRC...................................................................................................................................................... 2-7

2.2.4 PDCP.................................................................................................................................................... 2-7

2.2.5 RLC ...................................................................................................................................................... 2-8

2.2.6 MAC .................................................................................................................................................... 2-8

2.2.7 Physical ................................................................................................................................................ 2-9

2.3 LTE Channel Structure .................................................................................................................................. 2-9

2.3.1 Logical Channels................................................................................................................................ 2-10

2.3.2 Transport Channels............................................................................................................................. 2-11

2.3.3 Physical Channels .............................................................................................................................. 2-12

2.3.4 Radio Channels .................................................................................................................................. 2-13

2.3.5 Channel Mapping ............................................................................................................................... 2-13

2.4 LTE Frame Structure ................................................................................................................................... 2-15

2.4.1 Type 1 Radio Frames, Slots and Subframes ....................................................................................... 2-15

2.4.2 Type 2 Radio Frames, Slots and Subframes ....................................................................................... 2-17

2.5 OFDM Signal Generation ........................................................................................................................... 2-18

2.5.1 Codewords, Layers and Antenna Ports............................................................................................... 2-19

2.5.2 Scrambling ......................................................................................................................................... 2-20

2.5.3 Modulation Mapper ............................................................................................................................ 2-21

2.5.4 Layer Mapper ..................................................................................................................................... 2-22

2.5.5 Precoding ........................................................................................................................................... 2-23

2.5.6 Resource Element Mapper ................................................................................................................. 2-26

2.5.7 OFDM Signal Generation .................................................................................................................. 2-26

2.6 Downlink OFDMA ..................................................................................................................................... 2-26

2.6.1 General OFDMA Structure ................................................................................................................ 2-26

2.6.2 Physical Resource Blocks and Resource Elements ............................................................................ 2-27

2.7 LTE Physical Signals................................................................................................................................... 2-28

2.8 Downlink Reference Signals ....................................................................................................................... 2-31

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2.8.1 Cell Specific Reference Signals ......................................................................................................... 2-31

2.8.2 MBSFN Reference Signals ................................................................................................................ 2-33

2.8.3 UE Specific Reference Signals .......................................................................................................... 2-34

2.9 Downlink LTE Physical Channels ............................................................................................................... 2-34

2.9.1 PBCH (Physical Broadcast Channel) ................................................................................................. 2-34

2.9.2 PCFICH (Physical Control Format Indicator Channel) ..................................................................... 2-35

2.9.3 PDCCH (Physical Downlink Control Channel) ................................................................................. 2-37

2.9.4 PHICH (Physical Hybrid ARQ Indicator Channel)............................................................................ 2-40

2.9.5 PDSCH (Physical Downlink Shared Channel)................................................................................... 2-41

2.10 Downlink Control Signaling ..................................................................................................................... 2-42

2.10.1 DCI Format 0 ................................................................................................................................... 2-42

2.10.2 DCI Format 1 ................................................................................................................................... 2-43

2.10.3 DCI Format 1A ................................................................................................................................ 2-43

2.10.4 DCI Format 1B................................................................................................................................. 2-44

2.10.5 DCI Format 1C................................................................................................................................. 2-44

2.10.6 DCI Format 1D ................................................................................................................................ 2-45

2.10.7 DCI Format 2 ................................................................................................................................... 2-45

2.10.8 DCI Format 2A ................................................................................................................................ 2-46

2.10.9 DCI Format 3 ................................................................................................................................... 2-46

2.10.10 DCI Format 3A .............................................................................................................................. 2-46

2.11 LTE Cell Search Procedure ....................................................................................................................... 2-47

2.11.1 Cell Search ....................................................................................................................................... 2-47

2.11.2 PSS Correlation ................................................................................................................................ 2-48

2.11.3 SSS Correlation ................................................................................................................................ 2-49

2.11.4 Master Information Block ................................................................................................................ 2-50

2.11.5 System Information Messages.......................................................................................................... 2-50

2.11.6 PLMN Selection ............................................................................................................................... 2-55

2.11.7 Cell Selection ................................................................................................................................... 2-57

2.12 Uplink Transmission Technique ................................................................................................................ 2-58

2.12.1 SC-FDMA Signal Generation .......................................................................................................... 2-58

2.13 OFDMA Verses SC-FDMA....................................................................................................................... 2-61

2.14 Uplink LTE Physical Channels.................................................................................................................. 2-61

2.14.1 PRACH (Physical Random Access Channel)................................................................................... 2-62

2.14.2 PUSCH (Physical Uplink Shared Channel) ..................................................................................... 2-66

2.14.3 PUCCH (Physical Uplink Control Channel) .................................................................................... 2-68

2.15 Timing Relationships................................................................................................................................. 2-69

2.16 Uplink Reference Signals .......................................................................................................................... 2-70

2.16.1 Demodulation Reference Signal....................................................................................................... 2-71

2.16.2 Sounding Reference Signal .............................................................................................................. 2-72

2.17 Uplink Control Signaling .......................................................................................................................... 2-75

2.17.1 PUCCH Format 1 ............................................................................................................................. 2-75

2.17.2 PUCCH Format 1a and 1b ............................................................................................................... 2-76

Contents

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Training Manual

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Issue 01 (2010-05-01)

2.18 LTE Random Access Procedure ................................................................................................................ 2-78

2.18.1 RRC Connection .............................................................................................................................. 2-78

2.18.2 PRACH Preambles ........................................................................................................................... 2-79

2.18.3 Random Access Procedure Initialization .......................................................................................... 2-80

2.18.4 Random Access Response Window ................................................................................................. 2-82

2.18.5 Random Access Response ................................................................................................................ 2-82

2.18.6 Uplink Transmission ........................................................................................................................ 2-83

2.19 Uplink Power Control ............................................................................................................................... 2-84

2.19.1 PUSCH Power Control .................................................................................................................... 2-84

2.19.2 PUCCH Power Control .................................................................................................................... 2-85

2.19.3 PRACH Power Control .................................................................................................................... 2-86

2.20 Paging Procedures ..................................................................................................................................... 2-86

2.20.1 Discontinuous Reception for Paging ................................................................................................ 2-86

2.20.2 Paging Frame ................................................................................................................................... 2-87

2.21 HARQ Operation....................................................................................................................................... 2-88

2.21.1 Retransmission Types ....................................................................................................................... 2-88

2.21.2 HARQ Methods ............................................................................................................................... 2-88

2.21.3 HARQ in LTE .................................................................................................................................. 2-90

2.21.4 HARQ In the Downlink ................................................................................................................... 2-91

2.21.5 HARQ In the Uplink ........................................................................................................................ 2-91

2.21.6 ACK NACK Timing ......................................................................................................................... 2-92

2.22 Diversity Options ...................................................................................................................................... 2-94

2.22.1 SU-MIMO and MU-MIMO ............................................................................................................. 2-94

2.22.2 MIMO and Transmission Options .................................................................................................... 2-94

2.22.3 MIMO Modes .................................................................................................................................. 2-95

2.22.4 Spatial Multiplexing in LTE ............................................................................................................. 2-96

2.22.5 Feedback Reporting ......................................................................................................................... 2-98

3 Dynamic Resource Allocation ..................................................................................................... 3-1

3.1 Scheduling Principles and Signaling ............................................................................................................. 3-2

3.1.1 QoS in Packet Switched Networks....................................................................................................... 3-3

3.1.2 Key Factors Influencing Scheduling .................................................................................................... 3-4

3.1.3 Scheduling Methods ............................................................................................................................. 3-4

3.1.4 Downlink Scheduling ........................................................................................................................... 3-5

3.1.5 PDSCH Resource Allocation ............................................................................................................... 3-6

3.1.6 Modulation and Coding Scheme .......................................................................................................... 3-7

3.1.7 Uplink Scheduling................................................................................................................................ 3-9

3.2 Scheduler Interaction..................................................................................................................................... 3-9

3.2.1 Radio Bearers ....................................................................................................................................... 3-9

3.2.2 Scheduler Interaction with Layer 2 and Layer 1 .................................................................................. 3-9

3.3 Dynamic and Semi-persistent Scheduling ................................................................................................... 3-10

3.3.1 Dynamic Scheduling .......................................................................................................................... 3-11

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3.3.2 Downlink Semi-persistent Scheduling ............................................................................................... 3-11

3.3.3 Uplink Semi-persistent Scheduling .................................................................................................... 3-12

4 Intra LTE Mobility .......................................................................................................................... 4-1

4.1 Intra-LTE Mobility ........................................................................................................................................ 4-2

4.1.1 Idle State - Cell Reselection ................................................................................................................. 4-2

4.1.2 Active State Mobility ........................................................................................................................... 4-4

4.1.3 Handover Procedure ............................................................................................................................. 4-5

4.2 Reporting Options ......................................................................................................................................... 4-6

4.2.1 Measurement Configuration Parameter................................................................................................ 4-6

4.2.2 Report Configuration Parameter .......................................................................................................... 4-7

4.3 Mobility Measurements................................................................................................................................. 4-8

4.3.1 Measurement Gaps ............................................................................................................................... 4-8

4.3.2 Gap Configuration................................................................................................................................ 4-9

4.3.3 UE Measurements ................................................................................................................................ 4-9

5 Glossary ............................................................................................................................................. 5-1

Contents

LTE Air Interface

Training Manual

vi Huawei Proprietary and Confidential


Issue 01 (2010-05-01)

LTE Air Interface

Training Manual Figures



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Figures

Figure 1-1 Evolution of Cellular Networks ........................................................................................................ 1-2

Figure 1-2 Second Generation Mobile Systems ................................................................................................. 1-3

Figure 1-3 Third Generation Mobile Systems .................................................................................................... 1-5

Figure 1-4 Forth Generation Mobile System ...................................................................................................... 1-6

Figure 1-5 3GPP Releases .................................................................................................................................. 1-6

Figure 1-6 HSDPA ............................................................................................................................................. 1-7

Figure 1-7 HSUPA ............................................................................................................................................. 1-8

Figure 1-8 HSPA+ (Release 7) ........................................................................................................................... 1-9

Figure 1-9 Release 8 HSPA+ and LTE ............................................................................................................... 1-9

Figure 1-10 Release 9 and Beyond ................................................................................................................... 1-10

Figure 1-11 Radio Interface Techniques ........................................................................................................... 1-10

Figure 1-12 Frequency Division Multiple Access ............................................................................................ 1-11

Figure 1-13 Time Division Multiple Access ..................................................................................................... 1-11

Figure 1-14 Code Division Multiple Access .................................................................................................... 1-12

Figure 1-15 Orthogonal Frequency Division Multiple Access ......................................................................... 1-12

Figure 1-16 Frequency Division Duplex .......................................................................................................... 1-13

Figure 1-17 Time Division Duplex ................................................................................................................... 1-13

Figure 1-18 GSM Deployments ....................................................................................................................... 1-16

Figure 1-19 Key UMTS Deployment Bands .................................................................................................... 1-17

Figure 1-20 EARFCN Calculation ................................................................................................................... 1-19

Figure 1-21 Example Downlink EARFCN Calculation ................................................................................... 1-19

Figure 1-22 Summary of LTE Transport Channel Processing .......................................................................... 1-20

Figure 1-23 Cyclic Redundancy Check Concept.............................................................................................. 1-21

Figure 1-24 CRC Parity Bits ............................................................................................................................ 1-21

Figure 1-25 Code Block Segmentation and CRC Attachment.......................................................................... 1-22

Figure 1-26 Example Calculation for Segmentation and Filler Bits. ................................................................ 1-22

Figures

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Figure 1-27 Repetition Coding of the HI.......................................................................................................... 1-24

Figure 1-28 Basic ½ Rate Convolutional Coder ............................................................................................... 1-25

Figure 1-29 Convolutional Coding Trellis........................................................................................................ 1-25

Figure 1-30 Example of Viterbi Decoding ....................................................................................................... 1-26

Figure 1-31 Initializing Tail Biting Convolutional Coding .............................................................................. 1-27

Figure 1-32 LTE 1/3 Rate Tail Biting Convolutional Coding........................................................................... 1-27

Figure 1-33 LTE Turbo Coding ........................................................................................................................ 1-28

Figure 1-34 LTE Rate Matching ....................................................................................................................... 1-28

Figure 1-35 Code Block Concatenation ........................................................................................................... 1-29

Figure 1-36 Use of OFDM in LTE ................................................................................................................... 1-30

Figure 1-37 FDM Carriers ................................................................................................................................ 1-30

Figure 1-38 OFDM Subcarriers........................................................................................................................ 1-31

Figure 1-39 Inverse Fast Fourier Transform ..................................................................................................... 1-31

Figure 1-40 Fast Fourier Transform ................................................................................................................. 1-32

Figure 1-41 OFDM Symbol Mapping .............................................................................................................. 1-33

Figure 1-42 OFDM PAPR (Peak to Average Power Ratio) .............................................................................. 1-33

Figure 1-43 Delay Spread ................................................................................................................................. 1-34

Figure 1-44 Inter Symbol Interference ............................................................................................................. 1-34

Figure 1-45 Cyclic Prefix ................................................................................................................................. 1-35

Figure 2-1 The LTE Air Interface ....................................................................................................................... 2-3

Figure 2-2 LTE Control Plane and User Plane ................................................................................................... 2-3

Figure 2-3 E-UTRA Protocols ............................................................................................................................ 2-4

Figure 2-4 NAS Signaling .................................................................................................................................. 2-4

Figure 2-5 Main RRC Functions ........................................................................................................................ 2-7

Figure 2-6 PDCP Functions ................................................................................................................................ 2-7

Figure 2-7 RLC Modes and Functions ............................................................................................................... 2-8

Figure 2-8 Medium Access Control Functions ................................................................................................... 2-9

Figure 2-9 Physical Layer Functions .................................................................................................................. 2-9

Figure 2-10 LTE Channels ............................................................................................................................... 2-10

Figure 2-11 Location of Channels .................................................................................................................... 2-10

Figure 2-12 BCCH and PCCH Logical Channels ............................................................................................ 2-10

Figure 2-13 CCCH and DCCH Signaling ........................................................................................................ 2-11

Figure 2-14 Dedicated Traffic Channel ............................................................................................................ 2-11

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Figure 2-15 LTE Release 8 Transport Channels ............................................................................................... 2-12

Figure 2-16 Radio Channel .............................................................................................................................. 2-13

Figure 2-17 Downlink Channel Mapping ......................................................................................................... 2-14

Figure 2-18 Uplink Channel Mapping.............................................................................................................. 2-15

Figure 2-19 LTE Frame Structure..................................................................................................................... 2-16

Figure 2-20 Normal and Extended Cyclic Prefix ............................................................................................. 2-16

Figure 2-21 Normal CP Configuration ............................................................................................................. 2-17

Figure 2-22 Type 2 TDD Radio Frame ............................................................................................................. 2-18

Figure 2-23 Downlink Physical Layer Processing ........................................................................................... 2-19

Figure 2-24 Codeword, Layer and Antenna Port Mapping............................................................................... 2-19

Figure 2-25 Scrambling in LTE ........................................................................................................................ 2-20

Figure 2-26 LTE Scrambling Code Generation ................................................................................................ 2-20

Figure 2-27 BPSK, QPSK and 16QAM Modulation Mapper .......................................................................... 2-21

Figure 2-28 64QAM Modulation Mapper ........................................................................................................ 2-21

Figure 2-29 LTE Precoding Options................................................................................................................. 2-24

Figure 2-30 Example of the Downlink Signal Generation Equation ................................................................ 2-26

Figure 2-31 OFDMA in LTE ............................................................................................................................ 2-27

Figure 2-32 Physical Resource Block and Resource Element .......................................................................... 2-28

Figure 2-33 Downlink Cell ID ......................................................................................................................... 2-29

Figure 2-34 PSS and SSS Location for FDD.................................................................................................... 2-29

Figure 2-35 PSS and SSS Location for TDD ................................................................................................... 2-30

Figure 2-36 SSS Scrambling ............................................................................................................................ 2-31

Figure 2-37 Reference Signals - One Antenna Port .......................................................................................... 2-32

Figure 2-38 Reference Signal Physical Cell ID Offset ..................................................................................... 2-32

Figure 2-39 Reference Signals - Two Antenna Ports (Normal CP) .................................................................. 2-32

Figure 2-40 Reference Signals - Four Antenna Ports (Normal CP).................................................................. 2-33

Figure 2-41 MBSFN Reference Signals ........................................................................................................... 2-34

Figure 2-42 UE Specific Reference Signals ..................................................................................................... 2-34

Figure 2-43 Broadcast Signaling ...................................................................................................................... 2-35

Figure 2-44 MIB to PBCH Mapping (FDD and Normal CP) ........................................................................... 2-35

Figure 2-45 CFI to PCFICH Mapping .............................................................................................................. 2-36

Figure 2-46 FDD Downlink Control Region .................................................................................................... 2-37

Figure 2-47 REG to CCE and PDCCH Mapping ............................................................................................. 2-38

Figures

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Figure 2-48 PDCCH to Control Region Mapping ............................................................................................ 2-38

Figure 2-49 CCE Allocation Levels ................................................................................................................. 2-39

Figure 2-50 Common and UE-Specific Search Spaces .................................................................................... 2-39

Figure 2-51 PHICH Mapping ........................................................................................................................... 2-40

Figure 2-52 Extended PHICH Example ........................................................................................................... 2-41

Figure 2-53 Generic PDSCH Mapping ............................................................................................................ 2-41

Figure 2-54 Initial Procedures .......................................................................................................................... 2-47

Figure 2-55 PSS and SSS for Cell Search (FDD Mode) .................................................................................. 2-47

Figure 2-56 Physical Cell Identities ................................................................................................................. 2-48

Figure 2-57 PSS Correlation ............................................................................................................................ 2-48

Figure 2-58 SSS Correlation Example ............................................................................................................. 2-49

Figure 2-59 PBCH and the Master Information Block ..................................................................................... 2-50

Figure 2-60 System Information Block Type 1 ................................................................................................ 2-51

Figure 2-61 Example of SI Mapping ................................................................................................................ 2-52









Figure 2-70 PLMN Selection ........................................................................................................................... 2-55

Figure 2-71 LTE Cell Selection ........................................................................................................................ 2-57

Figure 2-72 SC-FDMA Subcarrier Mapping Concept ...................................................................................... 2-59

Figure 2-73 SC-FDMA Signal Generation ....................................................................................................... 2-60

Figure 2-74 SC-FDMA and the eNB ................................................................................................................ 2-60

Figure 2-75 Example of the Uplink Signal Generation Equation ..................................................................... 2-61

Figure 2-76 Release 8 Uplink Physical Channels............................................................................................. 2-62

Figure 2-77 PRACH Preamble ......................................................................................................................... 2-62

Figure 2-78 PRACH Guard Period ................................................................................................................... 2-63

Figure 2-79 PRACH FDD Formats .................................................................................................................. 2-64

Figure 2-80 PRACH Configuration .................................................................................................................. 2-64

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Figure 2-81 PRACH Configuration and Preamble Sequences Per Cell ........................................................... 2-66

Figure 2-82 PUSCH Mapping .......................................................................................................................... 2-67

Figure 2-83 Multiplexing Control Signaling .................................................................................................... 2-67

Figure 2-84 Mapping to Physical Resource Blocks for PUCCH ...................................................................... 2-68

Figure 2-85 FDD Timing .................................................................................................................................. 2-69

Figure 2-86 Example of TDD Configuration 2 ................................................................................................ 2-70

Figure 2-87 Uplink Reference Signals ............................................................................................................. 2-70

Figure 2-88 DRS Sequence Group Selection ................................................................................................... 2-71

Figure 2-89 Uplink Demodulation Reference Signal (Normal CP).................................................................. 2-72

Figure 2-90 Uplink Demodulation Reference Signal (Extended CP) ............................................................... 2-72

Figure 2-91 Requirement for SRS .................................................................................................................... 2-73

Figure 2-92 Example of SRS Frequency Hopping ........................................................................................... 2-73

Figure 2-93 Example SRS Allocation .............................................................................................................. 2-74

Figure 2-94 PUCCH Format 1a and 1b (Normal CP) ...................................................................................... 2-76

Figure 2-95 PUCCH Format 2 (Normal CP) .................................................................................................... 2-77

Figure 2-96 PUCCH Format 2 (Extended CP) ................................................................................................. 2-77

Figure 2-97 PUCCH Format 2a and 2b ACK/NACK Coding .......................................................................... 2-78

Figure 2-98 Overall Random Access Procedure ............................................................................................... 2-78

Figure 2-99 Random Access RRC Signaling Procedure .................................................................................. 2-79

Figure 2-100 PRACH Probing ......................................................................................................................... 2-79

Figure 2-101 Allocating Preambles to Group A and Group B .......................................................................... 2-81

Figure 2-102 Random Access Response Window ............................................................................................ 2-82

Figure 2-103 MAC Random Access Response ................................................................................................ 2-82

Figure 2-104 Random Access - Assigned UL-SCH.......................................................................................... 2-83

Figure 2-105 MAC Contention Resolution ...................................................................................................... 2-84

Figure 2-106 Uplink Power Control ................................................................................................................. 2-84

Figure 2-107 Paging Issues .............................................................................................................................. 2-86

Figure 2-108 System with DRX Reception of Paging ...................................................................................... 2-87

Figure 2-109 ARQ Verses HARQ .................................................................................................................... 2-88

Figure 2-110 Basic Concept of SAW................................................................................................................ 2-89

Figure 2-111 HARQ Parallel Processes ............................................................................................................ 2-89

Figure 2-112 HARQ Methods .......................................................................................................................... 2-89

Figure 2-113 Example of Redundancy Versions and Soft Bits ......................................................................... 2-90

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Figure 2-114 FDD HARQ Processes ................................................................................................................ 2-91

Figure 2-115 Downlink FDD HARQ Timing ................................................................................................... 2-93

Figure 2-116 Uplink FDD HARQ Timing ........................................................................................................ 2-93

Figure 2-117 SU-MIMO and MU-MIMO ........................................................................................................ 2-94

Figure 2-118 Spatial Multiplexing MIMO ....................................................................................................... 2-95

Figure 2-119 Spatial Multiplexing Interference Issues ..................................................................................... 2-95

Figure 2-120 MIMO Space Time Coding......................................................................................................... 2-96

Figure 2-121 AMS Concept.............................................................................................................................. 2-96

Figure 2-122 PDSCH Processing ..................................................................................................................... 2-97

Figure 2-123 Feedback Reporting .................................................................................................................... 2-98

Figure 2-124 4-bit CQI Table ........................................................................................................................... 2-98

Figure 3-1 IP Scheduling .................................................................................................................................... 3-2

Figure 3-2 Basic Scheduling in a Cell ................................................................................................................ 3-2

Figure 3-3 Packet Classifier and Packet Scheduler ............................................................................................ 3-3

Figure 3-4 Key Factors Influencing Scheduling ................................................................................................. 3-4

Figure 3-5 Possible Scheduling Method ............................................................................................................. 3-4

Figure 3-6 Type 0 Resource Allocation .............................................................................................................. 3-6



Figure 3-9 Using the TBS Size ........................................................................................................................... 3-8

Figure 3-10 Scheduler Interaction .................................................................................................................... 3-10

Figure 3-11 Dynamic Scheduling ..................................................................................................................... 3-11

Figure 3-12 Semi Persistent Scheduling ........................................................................................................... 3-12

Figure 4-1 Intra-LTE Mobility............................................................................................................................ 4-2

Figure 4-2 Intra-Frequency and Inter-frequency ................................................................................................ 4-2

Figure 4-3 Sintrasearch Parameter............................................................................................................................ 4-3

Figure 4-4 Impact to Treselection ....................................................................................................................... 4-4

Figure 4-5 Ranking Equation ............................................................................................................................. 4-4

Figure 4-6 Intra-LTE Mobility............................................................................................................................ 4-5

Figure 4-7 LTE Handover Procedure.................................................................................................................. 4-5

Figure 4-8 Measurement Configuration Parameters ........................................................................................... 4-6

Figure 4-9 Report Configuration Parameters ..................................................................................................... 4-7

Figure 4-10 Periodic and Event Reporting ......................................................................................................... 4-8

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Figure 4-11 Non Gap Assisted ............................................................................................................................ 4-8

Figure 4-12 Gap Assisted ................................................................................................................................... 4-9

Figure 4-13 Gap Configuration .......................................................................................................................... 4-9

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Tables

Table 1-1 2.5G and 2.75G GSM/GPRS Systems ................................................................................................ 1-3

Table 1-2 IMT Advanced Features ..................................................................................................................... 1-5

Table 1-3 GSM Frequency Bands ..................................................................................................................... 1-14

Table 1-4 UMTS FDD Frequency Bands ......................................................................................................... 1-15

Table 1-5 UMTS TDD Frequency Bands ......................................................................................................... 1-15

Table 1-6 LTE Release 8 Frequency Bands ...................................................................................................... 1-18

Table 1-7 Transport Channel Coding Options .................................................................................................. 1-23

Table 1-8 Control Information Coding Options ................................................................................................ 1-23

Table 1-9 CFI Encoding.................................................................................................................................... 1-24

Table 1-10 Convolutional Coding Example...................................................................................................... 1-25

Table 1-11 Standard Convolutional Coding Verses Tail Biting Convolutional Coding .................................... 1-27

Table 1-12 LTE Sub-block Interleaver.............................................................................................................. 1-29

Table 1-13 LTE Channel and FFT Sizes ........................................................................................................... 1-32

Table 2-1 NAS EMM and ESM Procedures ....................................................................................................... 2-5

Table 2-2 Downlink CP Parameters .................................................................................................................. 2-17

Table 2-3 Type 2 Radio Frame Switching Points.............................................................................................. 2-18

Table 2-4 Layer Mapper Configuration ............................................................................................................ 2-22

Table 2-5 Codeword to Layer Mapping for Spatial Multiplexing .................................................................... 2-22

Table 2-6 Codeword to Layer Mapping for Transmit Diversity ....................................................................... 2-23

Table 2-7 Codebook for Transmission for Two Antenna Ports ......................................................................... 2-25

Table 2-8 Downlink PRB Parameters ............................................................................................................... 2-28

Table 2-9 Example of SSS Indices.................................................................................................................... 2-30

Table 2-10 CFI Mapping .................................................................................................................................. 2-36

Table 2-11 CFI Codewords ............................................................................................................................... 2-37

Table 2-12 DCI Formats ................................................................................................................................... 2-42

Table 2-13 DCI Ambiguous Sizes of Information Bits ..................................................................................... 2-43

Tables

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Table 2-14 Precoding Information Field for 4 Antenna Ports (Open Loop) ..................................................... 2-46

Table 2-15 Cell Selection Parameters ............................................................................................................... 2-57

Table 2-16 SC-FDMA verses OFDMA ............................................................................................................ 2-61

Table 2-17 Random Access Preamble Parameters ............................................................................................ 2-63

Table 2-18 PRACH Configuration Index ......................................................................................................... 2-65

Table 2-19 “K” Values for TDD Configurations............................................................................................... 2-69

Table 2-20 PUCCH Formats ............................................................................................................................. 2-75

Table 2-21 Parameters for Random Access ...................................................................................................... 2-80

Table 2-22 FDD Subframe Patterns .................................................................................................................. 2-87

Table 2-23 TDD Subframe Patterns .................................................................................................................. 2-88

Table 2-24 TDD HARQ Processes ................................................................................................................... 2-91

Table 2-25 UL HARQ Operation ...................................................................................................................... 2-92

Table 2-26 Codebook Precoding....................................................................................................................... 2-97

Table 3-1 Modulation and TBS index table for PDSCH..................................................................................... 3-7

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1 The Air Interface

Objectives

On completion of this section the participants will be able to:

1.1 Describe the evolution of cellular networks.

1.2 Summarize the evolution of 3GPP releases, from release 99 to release 8.

1.3 Describe radio interface techniques.

1.4 Explain the difference between FDD and TDD mode.

1.5 Describe flexible spectrum usage.

1.6 Explain the concepts of channel coding and FEC (Forward Error Correction).

1.7 Describe the principles for OFDM.

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1.1 Evolution of Cellular Networks

Cellular mobile networks have been evolving for many years. The initial networks are

referred to as “First Generation”. These have now been replaced with “Second Generation”

and “Third Generation” networks. It is only now that 4G or “Fourth Generation” systems are

being deployed.

Figure 1-1 Evolution of Cellular Networks

1G (First

Generation)

2G (Second

Generation)

3G (Third

Generation)

4G (Fourth

Generation)

1.1.1 First Generation Mobile Systems

The 1G (First Generation) mobile systems were not digital, i.e. they utilized analogue

modulation techniques. The main systems included:

AMPS (Advanced Mobile Telephone System) - This first appeared in 1976 in the United

States. It was mainly implemented in the Americas, Russia and Asia. Various issues

including weak security features made the system prone to hacking and handset cloning.

TACS (Total Access Communications System) - This was the European version of

AMPS with slight modifications, as well as operating in different frequency bands. It

was mainly used in the United Kingdom, as well as parts of Asia.

ETACS (Extended Total Access Communication System) - This provided an improved

version of TACS. It enabled a greater number of channels and therefore facilitated more

users.

These analogue systems were all proprietary based FM (Frequency Modulation) systems and

therefore they all lacked security, any meaningful data service and international roaming

capability.

1.1.2 Second Generation Mobile Systems

2G (Second Generation) systems utilize digital multiple access technology, such as TDMA

(Time Division Multiple Access) and CDMA (Code Division Multiple Access). Figure 1-2

illustrates some of the different 2G mobile systems, these include:

GSM (Global System for Mobile communications) - this is the most successful of all 2G

technologies. It was initially developed by ETSI (European Telecommunications

Standards Institute) for Europe and designed to operate in the 900MHz and 1800MHz

frequency bands. It now has world-wide support and is available for deployment on

many other frequency bands, such as 850MHz and 1900MHz. A mobile described as

tri-band or quad-band indicates support for multiple frequency bands on the same device.

GSM is TDMA, such that it employs 8 timeslots on a 200kHz radio carrier.

cdmaOne - this is a CDMA (Code Division Multiple Access) system based on IS-95

(Interim Standard 95). It uses a spread spectrum technique and utilizes a mixture of

codes and timing to identify cells and channels. The system bandwidth is 1.25MHz.

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D-AMPS (Digital - Advanced Mobile Phone System) - this is based on IS-136 (Interim

Standard 136) and is effectively an enhancement to AMPS which provides a TDMA

access technique. It has been primarily used on the North American continent, as well as

in New Zealand and parts of Asia-Pacific.

Figure 1-2 Second Generation Mobile Systems

2G (Second

Generation)

GSM

cdmaOne

(IS-95)

D-AMPS

(IS-136)

Other

In addition to being digital, as well as improving capacity and security, these 2G digital

systems also offer enhanced services such as SMS (Short Message Service) and circuit

switched data.

2.5G Systems

Most 2G systems are being evolved. For example, GSM was extended with GPRS (General

Packet Radio System) to support efficient packet data services, as well as increasing the data

rates.

As this feature does not meet 3G requirements, GRPS is often referred to as 2.5G. A

comparison between 2G and 2.5G systems is illustrated in Table 1-1.

2.75G Systems

GSM/GPRS systems also added EDGE (Enhanced Data Rates for Global Evolution). This

nearly quadruples the throughput of GPRS. The theoretical data rate of 473.6kbit/s enables

service providers to efficiently offer multimedia services. Like GPRS, since it does not

comply with all the features of a 3G system, EDGE is usually categorized as 2.75G.

Table 1-1 2.5G and 2.75G GSM/GPRS Systems

System Service Theoretical Data Rate

Typical Data Rate

2G GSM Circuit Switched

Data Service

9.6kbit/s or

14.4kbit/s

9.6kbit/s or

14.4kbit/s

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2.5G GPRS Packet Switched

Data

171.2kbit/s 4kbit/s to 50kbit/s

2.75G EDGE Packet Switched

Data

473.6kbit/s 120kbit/s

1.1.3 Third Generation Mobile Systems

3G (Third Generation) systems are defined by IMT2000 (International Mobile

Telecommunications - 2000). IMT2000 defines that a 3G system should provide higher

transmission rates, for example: 2Mbit/s for stationary or nomadic use and 348kbit/s in a

moving vehicle.

The main 3G technologies are illustrated in Figure 1-3.These include:

WCDMA (Wideband CDMA) - This was developed by the 3GPP (Third Generation

Partnership Project). There are numerous variations on this standard, including

TD-CDMA and TD-SCDMA. WCDMA is the main evolutionary path from GSM/GPRS

networks. It is a FDD (Frequency Division Duplex) based system and occupies a 5MHz

carrier. Current deployments are mainly at 2.1GHz, however deployments at lower

frequencies are also being seen, e.g. UMTS1900, UMTS850, UMTS900 etc. WCDMA

supports voice and multimedia services with an initial theoretical rate of 2Mbit/s, with

most service providers initially offering 384kbit/s per user. However, this technology is

continuing to evolve and later 3GPP releases have increased the rates to in excess of

40Mbit/s.

TD-CDMA (Time Division CDMA) - This is typically referred to as UMTS TDD (Time

Division Duplex) and is part of the UMTS specifications, however it has only limited

support. The system utilizes a combination of CDMA and TDMA to enable efficient

allocation of resources.

TD-SCDMA (Time Division Synchronous CDMA) - This was jointly developed by

Siemens and the CATT (China Academy of Telecommunications Technology).

TD-SCDMA has links to the UMTS specifications and is often identified as UMTS-TDD

LCR (Low Chip Rate). Like TD-CDMA, it is also best suited to low mobility scenarios

in micro or pico cells.

CDMA2000 - This is a multi-carrier technology standard which uses CDMA.

CDMA2000 is actually a set of standards including CDMA2000 EV-DO

(Evolution-Data Optimized) which has various “revisions”. It is worth noting that

CDMA2000 is backward compatible with cdmaOne.

WiMAX (Worldwide Interoperability for Microwave Access) - This is another wireless

technology which satisfies IMT2000 3G requirements. The air interface is part of the

IEEE (Institute of Electrical and Electronics Engineers) 802.16 standard which originally

defined PTP (Point-To-Point) and PTM (Point-To-Multipoint) systems. This was later

enhanced to provide mobility and greater flexibility. The success of WiMAX is mainly

down to the “WiMAX Forum”, which is an organization formed to promote conformity

and interoperability between vendors.

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Figure 1-3 Third Generation Mobile Systems

UMTS

WCDMA

TD-SCDMA

CDMA2000

WiMAX

UMTS

TD-CDMA

3G (Third

Generation)

1.1.4 Fourth Generation Mobile Systems

4G (Fourth Generation) cellular wireless systems need to meet the requirements set by the

ITU (International Telecommunication Union) as part of IMT Advanced (International Mobile

Telecommunications Advanced). These features are illustrated in Table 1-2 and enable IMT

Advanced to address evolving user needs.

Table 1-2 IMT Advanced Features

Key IMT Advanced Features

A high degree of commonality of functionality worldwide while retaining the flexibility to

support a wide range of services and applications in a cost efficient manner.

Compatibility of services within IMT and with fixed networks.

Capability of interworking with other radio access systems.

High quality mobile services.

User equipment suitable for worldwide use.

User-friendly applications, services and equipment.

Worldwide roaming capability.

Enhanced peak data rates to support advanced services and applications (100Mbit/s for high

and 1Gbit/s for low mobility were identified as targets).

The main three 4G systems include:

LTE Advanced - LTE (Long Term Evolution) is part of 3GPP, however it does not meet

all IMT Advanced features, as such it is sometimes referred to as 3.99G. In contrast, LTE

Advanced is part of a later 3GPP Release and has been designed specifically to meet 4G

requirements.

WiMAX 802.16m - The IEEE and the WiMAX Forum have identified 802.16m as their

offering for a 4G system.

UMB (Ultra Mobile Broadband) - This is identified as EV-DO Rev C. It is part of 3GPP2

however most vendors and service providers have decided to promote LTE instead.

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Figure 1-4 Forth Generation Mobile System

LTE

Advanced

UMB

(EV-DO Rev C)

WiMAX

802.16m

4G (Fourth

Generation)

1.2 3GPP Releases

The development of GSM, GPRS, EDGE, UMTS, HSPA and LTE is in stages known as 3GPP

releases. Hardware vendors and software developers use these releases as part of their

development roadmap. Figure 1-5 illustrates the main 3GPP Releases that enhance the radio

interface.

Figure 1-5 3GPP Releases

GSM

9.6kbit/s

GPRS

171.2kbit/s

EDGE

473.6kbit/s

UMTS

2Mbit/s

HSDPA

14.4Mbit/s

HSUPA

5.76Mbit/s

HSPA+

28.8Mbit/s

42Mbit/s

LTE

+300Mbit/s

Phase 1

Phase 2+

(Release 97)

Release 99

Release 99

Release 5

Release 6

Release 7/8

Release 8

Release 9/10

LTE Advanced

3GPP Releases enhance various aspects, not just the radio interface. For example, Release 5

started the introduction of the IMS (IP Multimedia Subsystem) in the core network.

1.2.1 Pre-Release 99

Pre-Release 99 saw the introduction of GSM, as well as the addition of GPRS. The main

GSM Phases and 3GPP Releases include:

GSM Phase 1.

GSM Phase 2.

GSM Phase 2+ (Release 96).


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1.2.2 Release 99

3GPP Release 99 saw the introduction of UMTS, as well as the EDGE enhancement to GPRS.

UMTS contains all features needed to meet the IMT-2000 requirements as defined by the ITU.

It is able to support both CS (Circuit Switched) voice and video services, as well PS (Packet

Switched) data services over common and dedicated bearers. Initial data rates for UMTS were

64kbit/s, 128kbit/s and 384kbit/s. Note that the theoretical maximum was 2Mbit/s.

1.2.3 Release 4

Release 4 included enhancements to the core network. The concept of “All IP Networks” was

included and service providers were able to deploy Soft Switch based networks, i.e. the MSC

(Mobile Switching Centre) was replaced by MSC Servers and MGW (Media Gateways).

1.2.4 Release 5

Release 5 is the first major addition to the UMTS air interface. It adds HSDPA (High Speed

Downlink Packet Access) which improves capacity and spectral efficiency. Figure 1-6

illustrates some of the main features which include:

Adaptive Modulation - In addition to the original UMTS modulation scheme, QPSK

(Quadrature Phase Shift Keying), HSDPA also includes support for 16 QAM

(Quadrature Amplitude Modulation).

Flexible Coding - Based on fast feedback from the mobile in the form of a CQI (Channel

Quality Indicator) the UMTS base station, i.e. the Node B, is able to modify the effective

coding rate and thus increase system efficiency.

Fast Scheduling - HSDPA includes a 2ms TTI (Time Transmission Interval), which

enables the Node B scheduler to quickly and efficiently allocate resources to mobiles.

HARQ (Hybrid Automatic Repeat Request) - In the event a packet does not get through

to the UE (User Equipment) successfully, the system employs HARQ (Hybrid Automatic

Repeat Request). This improves the retransmission timing, thus requiring less reliance on

the RNC (Radio Network Controller).

Figure 1-6 HSDPA

HSDPA

Adaptive Modulation

Flexible Coding

Fast Scheduling (2ms)

HARQ

UE

UTRAN

RNCNode B

Iub

1.2.5 Release 6

Release 6 adds various features, with HSUPA (High Speed Uplink Packet Data) being of most

interest to RAN development. Even though the term HSUPA is widespread, this 3GPP

enhancement also goes under the term “Enhanced Uplink”. It is also worth noting that

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HSDPA and HSUPA work in tandem and thus the term HSPA (High Speed Packet Access) is

used.

HSUPA, like HSDPA adds functionality to improve packet data. Figure 1-7 illustrates the

three main enhancements which include:

Flexible Coding - HSUPA has the ability to dynamically change the coding and therefore

improve the efficiency of the system.

Fast Power Scheduling - A key fact of HSUPA is that it provides a method to schedule

the power from different mobiles. This scheduling can use either a 2ms or 10ms TTI.

HARQ - Like HSDPA, HSUPA also utilizes HARQ. The main difference is the timing

relationship for the retransmission.

Figure 1-7 HSUPA

HSUPA

Flexible Coding

Fast Power Scheduling

HARQ

UE

UTRAN

RNCNode B

Iub

1.2.6 Release 7

The main RAN based feature of Release 7 is HSPA+. This, like HSDPA and HSUPA,

provides various enhancements to improve packet switched data delivery. Figure 1-8

illustrates the main features which include:

64 QAM - This is added to the DL (Downlink) and enables HSPA+ to operate at a

theoretical rate of 21.6Mbit/s.

16 QAM - This is added to the UL (Uplink) and enables the uplink to theoretically

achieve 11.76Mbit/s.

MIMO (Multiple Input Multiple Output) Operation - this is added to HSPA+ Release 7

and offers various benefits including the ability to offer a theoretical 28.8Mbits/s in the

downlink.

Power Enhancements -Various enhancements such as CPC (Continuous Packet

Connectivity) have been included. Thus enabling DTX (Discontinuous Transmission),

DRX (Discontinuous Reception) and HS-SCCH (High Speed - Shared Control Channel)

Less Operation. Collectively these improve the mobile’s battery consumption.

Less Overhead - The downlink includes an enhancement to the MAC (Medium Access

Control) layer which effectively means that fewer headers are required. This in turn

improves the system efficiency.

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Figure 1-8 HSPA+ (Release 7)

HSPA+

64 QAM (DL)

16 QAM (UL)

MIMO Operation (DL)

Power Enhancements (DL)

Less Overhead (DL)

UE

UTRAN

RNCNode B

Iub

1.2.7 Release 8

There are many additions to the RAN functionality in Release 8, such as enhancements to

HSPA+. However the main aspect is the inclusion of LTE (Long Term Evolution). Figure 1-9

illustrates some of the main features for Release 8 HSPA+ and LTE.

Release 8 HSPA+ enables various key enhancements, these include:

64 QAM and MIMO - Release 8 enables the combination of 64 QAM and MIMO, thus

quoting a theoretical rate of 42Mbit/s, i.e. 2 x 21.6Mbit/s.

Dual Cell Operation - DC-HSDPA (Dual Cell - HSDPA) is a Release 8 feature which is

further enhanced in Release 9 and Release 10. It enables a mobile to effectively utilize

two 5MHz UMTS carriers. Assuming both are using 64 QAM (21.6Mbit/s), the

theoretical maximum is 42Mbps. Note that in Release 8 a mobile is not able to combine

MIMO and DC-HSDPA.

Less Uplink Overhead - In a similar way to Release 7 in the downlink, the Release 8

uplink has been enhanced to reduce overhead.

Figure 1-9 Release 8 HSPA+ and LTE

HSPA+

64 QAM + MIMO (DL)

Dual Cell Operation

Less Overhead (UL)

UE

UTRAN

RNCNode B

Iub

eNB

E-UTRAN

LTE

Enhanced Techniques

Flexible Bandwidth

Flexible Spectrum Options

High Data Rates

Very Fast Scheduling

Improved Latency

LTE provides a new radio access technique, as well as enhancements in the E-UTRAN

(Evolved - Universal Terrestrial Radio Access Network). These enhancements are further

discussed as part of this course.

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1.2.8 Release 9 and Beyond

Even though LTE is a Release 8 system, it is further enhanced in Release 9. There are a huge

number of features in Release 9. One of the most important is the support of additional

frequency bands.

Figure 1-10 Release 9 and Beyond

LTE

Release 8

LTE

Release 9

LTE Advanced

Release 10

Release 10 includes the standardization of LTE Advanced, i.e. the 3GPP’s 4G offering. As

such it includes modification to the LTE system to facilitate 4G services.

1.3 Radio Interface Techniques

In wireless cellular systems, mobiles have to share a common medium for transmission. There

are various categories of assignment, the main four include: FDMA (Frequency Division

Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple

Access) and OFDMA (Orthogonal Frequency Division Multiple Access).

Figure 1-11 Radio Interface Techniques

FDMA

TDMA CDMA

OFDMA

Radio Interface

Techniques

1.3.1 Frequency Division Multiple Access

In order to accommodate various devices on the same wireless network, FDMA divides the

available spectrum into sub-bands or channels. The concept of FDMA is illustrated in Figure

1-12. Using this technique a dedicated channel can be allocated to a user, whilst other users

occupy other channels, i.e. frequencies.

In a cellular system mobiles typically occupy multiple channels; one for the downlink and one

for the uplink. This does however make FDMA less efficient since most data applications are

downlink intensive.

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Figure 1-12 Frequency Division Multiple Access

Frequency

Power Time

FDMA

Each user allocated a

different subband/

channel.

FDMA channels also suffer since they cannot be close together due to the energy from one

transmission affecting the adjacent/neighboring channels. To combat this, additional guard

bands between channels are required, which also reduces the system’s spectral efficiency.

1.3.2 Time Division Multiple Access

In TDMA systems the channel bandwidth is shared in the time domain. Figure 1-13 illustrates

the concept of TDMA. It shows how each device is allocated a time on the channel, known as

a “timeslot”. These are then grouped into a TDMA frame. The number of timeslots in a

TDMA frame is dependent on the system, for example GSM utilizes 8 timeslots.

Figure 1-13 Time Division Multiple Access

Frequency

PowerTime

TDMA


different time on the

channel.

Devices must be allocated a timeslot; therefore it is usual to have one or more timeslots

reserved for common control and system access.

TDMA systems are normally digital and therefore offer additional features such as ciphering

and integrity. In addition, they can employ enhanced error detection and correction schemes

including FEC (Forward Error Correction). This enables the system to be more resilient to

noise and interference and therefore they have a greater spectral efficiency when compared to

FDMA systems.

1.3.3 Code Division Multiple Access

The concept of CDMA is slightly different to that of FDMA and TDMA. Instead of sharing

resources in the time or frequency domain, the devices are able to use the system at the same

time and using the same frequency/bandwidth. This is possible due to the fact that each

transmission is separated using a unique code.

There are two main types of CDMA, FHSS (Frequency Hopping Spread Spectrum) and DSSS

(Direct Sequence Spread Spectrum), with all the current cellular systems utilizing DSSS.

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Figure 1-14 illustrates the basic concept of CDMA. The narrowband signals are spread with a

wideband code and then transmitted. The receivers are designed to extract the encoded signal

(with the correct code) and reject everything else as noise.

Figure 1-14 Code Division Multiple Access

Frequency

PowerTime

CDMA


different code on the

channel.

UMTS, cdmaOne and CDMA2000 all use CDMA. However the implementation of the codes

and the bandwidths used is different. For example UMTS utilizes a 5MHz channel bandwidth,

whereas cdmaOne uses only 1.25MHz.

1.3.4 Orthogonal Frequency Division Multiple Access

OFDMA (Orthogonal Frequency Division Multiple Access) is the latest addition to cellular

systems. It provides a multiple access technique based on OFDM (Orthogonal Frequency

Division Multiplexing). Figure 1-15 illustrates the basic view of OFDMA. It can be seen that

the bandwidth is broken down to smaller units known as “subcarriers”. These are grouped

together and allocated as a resource to a device. It can also be seen that a device can be

allocated different resources in both the time and frequency domain.

Additional detail on OFDM and OFDMA is provided in Section 1.7 and 2.6 .

Figure 1-15 Orthogonal Frequency Division Multiple Access

Frequency

PowerTime

OFDMA


different resource

which can vary in

time and frequency.

1.4 Transmission Modes Cellular systems can be designed to operate in two main transmission modes, namely FDD

(Frequency Division Duplex) and TDD (Time Division Duplex).

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1.4.1 Frequency Division Duplex

The concept of FDD is illustrated in Figure 1-16. A separate uplink and downlink channel are

utilized, enabling a device to transmit and receive data at the same time (assuming the device

incorporates a duplexer). The spacing between the uplink and downlink channel is referred to

as the duplex spacing.

Figure 1-16 Frequency Division Duplex

Uplink Downlink

Duplex Spacing

Frequency

Channel

Bandwidth

Channel

Bandwidth

Normally the uplink channel (mobile transmit) operates on the lower frequency. This is done

because higher frequencies suffer greater attenuation than lower frequencies and therefore it

enables the mobile to utilize lower transmit levels.

Some systems also offer half-duplex FDD mode, where two frequencies are utilized, however

the mobile can only transmit or receive, i.e. not transmit and receive at the same time. This

allows for reduced mobile complexity since no duplex filter is required.

1.4.2 Time Division Duplex

TDD mode enables full duplex operation using a single frequency band and time division

multiplexing the uplink and downlink signals. One advantage of TDD is its ability to provide

asymmetrical uplink and downlink allocation. Depending on the system, other advantages

include dynamic allocation, increased spectral efficiency, as well as the improved use of

beamforming techniques - this is due to having the same uplink and downlink frequency

characteristics.

Figure 1-17 Time Division Duplex

TDDFrequency

Downlink Uplink Downlink Uplink

TDD Frame TDD Frame

Time

Asymmetric

Allocation

Downlink

and Uplink

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1.5 Spectrum Usage

One of the main factors in any cellular system is the frequency of deployment. Most 2G, 3G

and 4G systems offer multiple options. For example, GSM can be deployed at various bands

including: 900MHz, 1800MHz, 1900MHz, 850MHz etc.

1.5.1 Frequency Bands

Each cellular system defines its own set of frequency bands it is able to operate on. In order to

identify possible LTE bands it is worth noting the bands used by other technologies such as

GSM, UMTS etc.

GSM Bands

Table 1-3 illustrates the main frequency bands defined for GSM. However, this does not

guarantee that the spectrum is available since there may be regulatory issues, as well as

limitations in some handsets and base stations.

The initial GSM band was referred to as P-GSM (Primary GSM). This was mainly defined to

replace the TACS system which was also in the 900MHz band. Other 900MHz bands which

were added include E-GSM (Extended GSM) and R-GSM (Railways GSM) bands, providing

more channels and support of a railway based variant. Finally, other bands away from the

900MHz band are also available; however the support for 450MHz and 480MHz is limited.

The terms DCS (Digital Cellular Service) and PCS (Personal Communications Service) are

typically used in Europe and North America respectively to identify the higher frequency

deployment options. It was expected that these frequencies would offer a better re-use in built

up areas and therefore provide additional capacity.

Table 1-3 GSM Frequency Bands

Operating Band Frequency Band

Uplink Frequency (MHz)

Downlink Frequency (MHz)

GSM 400 450 450.4 - 457.6 460.4 - 467.6

GSM 400 480 478.8 - 486.0 488.8 - 496.0

GSM 850 850 824.0 - 849.0 869.0 - 894.0

GSM 900 (P-GSM) 900 890.0 - 915.0 935.0 - 960.0

GSM 900 (E-GSM) 900 880.0 - 915.0 925.0 - 960.0

GSM-R (R-GSM) 900 876.0 - 880.0 921.0 - 925.0

DCS 1800 1800 1710.0 - 1785.0 1805.0 - 1880.0

PCS 1900 1900 1850.0 - 1910.0 1930.0 - 1990.0

UMTS Bands

UMTS, like GSM, has a number of frequency bands defined. These are identified by an

“Operating Band” number which is illustrated in Table 1-4, along with the associated Uplink

and downlink frequency ranges.

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Table 1-4 UMTS FDD Frequency Bands

Operating Band Frequency Band

Uplink Frequency (MHz)

Downlink Frequency (MHz)

I 2100 1920 - 1980 2110 - 2170

II 1900 1850 - 1910 1930 - 1990

III 1800 1710 - 1785 1805 - 1880

IV 1700 1710 - 1755 2110 - 2155

V 850 824 - 849 869 - 894

VI 800 830 - 840 875 - 885

VII 2600 2500 - 2570 2620 - 2690

VIII 900 880 - 915 925 - 960

IX 1700 1749.9 - 1784.9 1844.9 - 1879.9

X 1700 1710 - 1770 2110 - 2170

XI 1500 1427.9 - 1452.9 1475.9 - 1500.9

XII 700 698 - 716 728 - 746

XIII 700 777 - 787 746 - 756

XIV 700 788 - 798 758 - 768

In addition to the previous UMTS FDD bands, various UMTS TDD bands are also defined.

Table 1-5 illustrates the main TDD bands, however the majority of these have never been

implemented.

Table 1-5 UMTS TDD Frequency Bands

Frequency Band

1900 - 1920

2010 - 2025

1850 - 1910

1930 - 1990

1910 - 1930

2570 - 2620

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1.5.2 Existing Mobile Deployment

The list of current mobile service providers is constantly increasing. The latest list of

GSM/UMTS and LTE operators is maintained by the GSMA (GSM Association).

GSM Deployments

Figure 1-18 summarizes the main GSM deployment bands. It can be seen that GSM 900 and

GSM 1800 are used in most parts of the world, i.e. Europe, Middle East, Africa and most of

Asia/Pacific. In contrast, GSM 850 and GSM 1900 are mainly used in North America and

Canada, as well as many other locations. Finally, the lower frequency bands, i.e. GSM

400/450 has limited support.

Figure 1-18 GSM Deployments

GSM 400GSM 850

GSM 1900

GSM 900

GSM 1800

Europe, Middle

East, Africa,

and most of

Asia/Pacific.

United States,

Canada, and

many other

countries in the

Americas.

This has

limited

support.

Main UMTS Deployments

The main UMTS deployment bands are illustrated in Figure 1-19, these include:

Band I (WCDMA 2100) - This is mainly used in Europe, Africa, Asia, Australia, New

Zealand and Brazil.

Band II (WCDMA 1900) - This is used in North and South America.

Band IV (WCDMA 1700) - This is typically referred to as the AWS (Advanced Wireless

Services) band. Certain service providers in North America and Canada have access to

this band.

Band V (WCDMA 850) - This is found mainly in North and South America, as well as

Australia, New Zealand, Canada, Israel, Poland and Asia.

Band VIII (WCDMA 900) - This is now being found in Europe, Asia, Australia, New

Zealand and Venezuela.

This list and usage of bands is not exclusive. As such other countries, as well as other cellular systems

may exist.

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Figure 1-19 Key UMTS Deployment Bands

Band I

(WCDMA

2100)

Band IV

(WCDMA

1700)

Band II

(WCDMA

1900)

Band V

(WCDMA

850)

Band VIII

(WCDMA

900)

Main UMTS

Deployments

1.5.3 LTE Release 8 Bands

The LTE Radio interface, namely the E-UTRA (Evolved - Universal Terrestrial Radio Access),

is able to operate in many different radio bands. Table 1-6 illustrates the Release 8 frequency

bands as well as other parameters which are used to identify centre frequencies. FDD requires

two centre frequencies, one for the downlink and one for the uplink. These carrier frequencies

are each given an EARFCN (E-UTRA Absolute Radio Frequency Channel Number) which

ranges from 0 to 65535. In contrast, TDD only has one EARFCN. The parameters required to

calculate the EARFCN(s) include:

FDL_low - This is the lower frequency of the downlink band.

FDL_high - This is the higher frequency of the downlink band.

NOffs-DL - This is a parameter used as part of the downlink EARFCN calculation.

NDL - This is the actual downlink EARFCN number.

FUL_low - This is the lower frequency of the uplink band.

FUL_high - This is the higher frequency of the uplink band.

NOffs-UL - This is a parameter used as part of the uplink EARFCN calculation.

NUL - This is the actual uplink EARFCN number.

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Table 1-6 LTE Release 8 Frequency Bands

Band Duplex FDL_low

(MHz)

FDL_high

(MHz)

NOffs-DL NDL FUL_low

(MHz)

FUL_high

(MHz)

NOffs-UL NUL

1 FDD 2110 2170 0 0-599 1920 1980 18000 18000-18599

2 FDD 1930 1990 600 600-1199 1850 1910 18600 18600-19199

3 FDD 1805 1880 1200 1200-1949 1710 1785 19200 19200-19949

4 FDD 2110 2155 1950 1950-2399 1710 1755 19950 19950-20399

5 FDD 869 894 2400 2400-2649 824 849 20400 20400-20649

6 FDD 875 885 2650 2650-2749 830 840 20650 20650-20749

7 FDD 2620 2690 2750 2750-3449 2500 2570 20750 20750-21449

8 FDD 925 960 3450 3450-3799 880 915 21450 21450-21799

9 FDD 1844.9 1879.9 3800 3800-4149 1749.9 1784.9 21800 21800-22149

10 FDD 2110 2170 4150 4150-4749 1710 1770 22150 22150-22749

11 FDD 1475.9 1500.9 4750 4750-4999 1427.9 1452.9 22750 22750-22999

12 FDD 728 746 5000 5000-5179 698 716 23000 23000-23179

13 FDD 746 756 5180 5180-5279 777 787 23180 23180-23279

14 FDD 758 768 5280 5280-5379 788 798 23280 23280-23379

17 FDD 734 746 5730 5730-5849 704 716 23730 23730-23849

33 TDD 1900 1920 36000 36000-36199 1900 1920 36000 36000-36199

34 TDD 2010 2025 36200 36200-36349 2010 2025 36200 36200-36349

35 TDD 1850 1910 36350 36350-36949 1850 1910 36350 36350-36949

36 TDD 1930 1990 36950 36950-37549 1930 1990 36950 36950-37549

37 TDD 1910 1930 37550 37550-37749 1910 1930 37550 37550-37749

38 TDD 2570 2620 37750 37750-38249 2570 2620 37750 37750-38249

39 TDD 1880 1920 38250 38250-38649 1880 1920 38250 38250-38649

40 TDD 2300 2400 38650 38650-39649 2300 2400 38650 38650-39649

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Carrier Frequency EARFCN Calculation

The EARFCN is calculated using a combination of the equations in Figure 1-20 and the

values in Table 1-6.

The channel raster for LTE is 100kHz for all bands, i.e. the carrier centre frequency must be

an integer multiple of 100kHz. This is represented in the equation by the “0.1” value.

Figure 1-20 EARFCN Calculation

eNB

UE

FDL = FDL_low + 0.1(NDL - NOffs-DL)

FUL = FUL_low + 0.1(NUL - NOffs-UL)

The channel numbers that designate carrier frequencies close to the edges of the operating band are not

used. This implies that the first 7, 15, 25, 50, 75 and 100 channel numbers at the lower operating band

edge and the last 6, 14, 24, 49, 74 and 99 channel numbers at the upper operating band edge are not used for channel bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz respectively.

Example

It is possible to utilize the previous equations to calculate the frequency for a given EARFCN.

In addition, it is possible to calculate the EARFCN for a given frequency. Figure 1-21

illustrates an example with a defined uplink and downlink frequency. The calculation shown

in the figure translates a downlink frequency of 2127.4MHz to an EARFCN equal to 174.

Figure 1-21 Example Downlink EARFCN Calculation

Frequency

Uplink Downlink

100kHz Raster

2127.4MHz1937.4MHz

FDL = FDL_low + 0.1(NDL - NOffs-DL)

(FDL - FDL_low)

0.1+ NOffs-DL

(2127.4 - 2110)

0.1+ 0

NDL =

NDL = = 174

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1.6 Channel Coding in LTE

The term “channel coding” can be used to describe the overall coding for the LTE channel. It

can also be used to describe one of the individual stages.

LTE channel coding is typically focused on a TB (Transport Block). This is a block of

information which is provided by the upper layer, i.e. MAC (Medium Access Control). Figure

1-22 summarizes the typical processes performed by the PHY (Physical Layer), these include:

CRC (Cyclic Redundancy Check) attachment for the Transport Block.

Code block segmentation and CRC attachment.

Channel Coding.

Rate Matching.

Code Block Concatenation.

Figure 1-22 Summary of LTE Transport Channel Processing

Transport Block CRC Attachment

Code Block CRC Attachment and

Segmentation

Channel Coding

Rate Matching

Code Block Concatenation

Additional Layer 1 Processes

Transport Block MAC Layer

PHY Layer

The coding stages in Figure 1-22 are indicative of the LTE DL-SCH (Downlink Shared Channel) and the

PCH (Paging Channel). Other channels, such as the UL-SCH (Uplink Shared Channel), BCH (Broadcast

Channel) etc. are different but they can still utilize similar processes, e.g. they all have a “channel coding”

stage.

1.6.1 Transport Block CRC

The error detection method across the air interface is based on the addition of a CRC (Cyclic

Redundancy Check). Figure 1-23 illustrates the basic concept of attaching a CRC to the

Transport Block. The purpose of the CRC is to detect errors which may have occurred when

the data was being sent. In LTE the CRC is based on complex parity checking.

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Figure 1-23 Cyclic Redundancy Check Concept

CRC

Calculate

CRCTransport Block

Transport Block CRCTransport Block

Calculate

CRCCRC

Compare

Transmitter Receiver

Possible radio

interface errors

The LTE transport block is used to calculate the CRC parity bits. The size of the CRC is set to

24bits, 16bits or 8bits. This is typically indicated by higher layer signaling, i.e. RRC (Radio

Resource Control). Figure 1-24 illustrates the CRC parity bits, where A is the size of the

transport block and L is the number of parity bits. In addition, the lowest order information bit

a0 is mapped to the most significant bit of the transport block.

Figure 1-24 CRC Parity Bits

a0

Transport Block CRC Parity Bits

a1 a2 a3 aA-1 p0 p1 pL-1

A = Input Sequence L = Parity Length

The parity bits are generated by one of the following cyclic generator polynomials:

gCRC24A(D) = D24

+ D23

+ D18

+ D17

+ D14

+D11

+ D10

+ D7 + D

6 + D

5 + D

4 + D

3 + D + 1

gCRC16(D) = D16

+ D12

+ D5 + 1

gCRC8(D) = D8 + D

7 + D

4 + D

3 + D + 1

Parity Checking

The encoding is performed in a systematic form, which means that in GF(2) (Galois Field (2)),

the polynomial:

a0DA+23

+ a1DA+22

+…+ aA-1D24

+ p0D23

+ … + p1D22

+ p22D1 + p23

yields a remainder equal to 0 when divided by the corresponding 24bit CRC generator

polynomial. Note that the 16bit and 8bit CRC generators each have a different polynomial

which also yields a remainder equal to 0.

1.6.2 Code Block Segmentation and CRC Attachment

The next stage in the processing of the transport block is code block segmentation and CRC

attachment. Figure 1-25 illustrates the concept of code block segmentation. This process

ensures that the size of each block is compatible with later stages of processing, i.e. the turbo

interleaver. In addition, each code bock (segment) has a CRC included for the turbo coding.

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Figure 1-25 Code Block Segmentation and CRC Attachment

CRCTransport Block

Transport Block CRC

CRC

Code Block CRCFiller Bits

Code Block #1 Code Block #2 Code Block #3

The input bit sequence to the code block segmentation is denoted by b0 , b1 ,….bB−1.

Segmentation is performed if B is larger than the maximum code block size Z (6144bits).

Finally, an additional CRC sequence of 24bits is attached to each code block.

Note that if B < 40, filler bits are added to the beginning of the code block.

The code block CRC is different to the one used by the transport blocks. The polynomial is:

gCRC24B(D) = D24

+ D23

+ D6 + D

5 + D + 1

The verification polynomial is the same one used for the gCRC24A transport block which also

yields a remainder equal to 0.

Example

Figure 1-26 illustrates an example for segmentation when B=8000. In this instance the initial

segment size is 4200bits (including the 24bit transport block CRC) which gets a 24bit code

block CRC. The remaining 3800bits also get a 24bit code block CRC, however an additional

16bits of filler is required to ensure that the segments meet a valid turbo coding code block

size.

Figure 1-26 Example Calculation for Segmentation and Filler Bits.

8000bits

4200bits

4224bits

3800bits

24bit Code

Block CRC

16 Filler Bits

3840bits

24bit Code

Block CRC

In this example the total number of bits sent is 8064bits, thus an extra 64bits are sent (24bits

+24bits +16bits).

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1.6.3 Channel Coding

Channel coding in LTE facilitates FEC (Forward Error Correction) across the air interface.

There are four main types:

Repetition Coding

Block Coding.

Tail Biting Convolutional Coding.

Turbo Coding.

The actual method used is linked to the type of LTE transport channel (Table 1-7) or the

control information type (Table 1-8). Additional information on LTE channel types and

control information is discussed in Section 2.1 .

Table 1-7 Transport Channel Coding Options

Transport Channel Coding Method Rate

DL-SCH

Turbo Coding 1/3 UL-SCH

PCH

MCH

BCH Tail Biting Convolutional Coding 1/3

Table 1-8 Control Information Coding Options

Control Information Coding Method Rate

DCI Tail Biting Convolutional Coding 1/3

CFI Block Code 1/16

HI Repetition Code 1/3

UCI Block Code Variable

Tail Biting Convolutional Coding 1/3

Repetition Coding

Repetition coding is used for coding the HI (HARQ Indicator) bit. The HI bit set to “1” is

termed an ACK (Acknowledgement) and the HI bit set to “0” is a NACK (Negative

Acknowledgement). The process of repetition coding is applied to increase the channel

robustness. As such, for one initial bit, three bits are generated. These three bits are then map

to an orthogonal sequence. The use of the HI bit, as well as the orthogonal sequences, is

discussed in Section 2.21 .

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Figure 1-27 Repetition Coding of the HI

1

1 1 1 0 0 0

0

ACK NACK

Repetition

Coding

Orthogonal sequences Orthogonal sequences

Block Coding

The main utilization of block coding in LTE is for the CFI (Control Format Indicator). This

parameter is used to convey vital information about the size of the downlink control region.

Table 1-9 illustrates how the CFI values are encoded into a 32bit CFI codeword.

Table 1-9 CFI Encoding

CFI CFI Codeword < b0, b1, …, b31 >

1 <0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1>

2 <1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0>

3 <1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1>

4 (Reserved) <0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0>

The utilization of the CFI and the mapping to the Physical Channels is discussed in Section

2.9.2 .

Concept of Convolutional Coding

Prior to detailing the operation of tail biting convolutional coding and turbo coding in LTE it

is worth examining the basics of a CC (Convolutional Coder) and the decoding process.

Figure 1-28 illustrates a basic convolutional ½ rate coder, i.e. for 1bit input, 2bits are

generated. It also has a constraint value of 3, meaning that three consecutive bits are used to

calculate the output. For standard convolutional coders, before any information is sent, the

registers are set to zero. This ensures that the initial information sent in the channel is at a

known state at the receiver. For each subsequent input bit the previous input bit is used to load

the registers S1 and S2 in turn.

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Figure 1-28 Basic ½ Rate Convolutional Coder

Output

XOR Gate

G0

A

0

G1

B

1

C

1

D

0S1 S2Input

Shift

Registers

It can be seen in this simple coder that the output is dependent on the input and the state of the

registers at any given time. What is also important is to understand how the output will

change for any given input. For example, if the first input bit is “0” (bit A) and S1 and S2 are

both at “0”, both outputs will be “0”. As the next bit arrives (bit B) it affects the output, such

that G0 and G1 are both set to “1”. Table 1-10 illustrates bit B (in bold) clocking through the

shift registers, as well as the output for the given sequence.

Table 1-10 Convolutional Coding Example

Input S1 S2 G0 G1

0 0 0 0 0

1 0 0 1 1

1 1 0 0 1

0 1 1 0 1

Using the example coder from Figure 1-28 there are two possible outputs from each state.

Figure 1-29 illustrates these, as well as the relationship for an input of 0 or 1.

Figure 1-29 Convolutional Coding Trellis

00

10

01

11

00

10

01

11

Current

StateNext State

11

00

11

00

10

0101

10

Input 0

Input 1

Output

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Concept of Viterbi Decoding

The Viterbi algorithm is one of the main methods for decoding standard convolutional coded

signals and it provides a valuable insight to how similar encoded signals can be decoded. The

Viterbi method is based on a concept of maximum-likelihood decoding. Figure 1-30 illustrates

the concept of trellis decoding by mapping the encoded data and current state to one of two

outputs. The values shown on the input lines indicate the number of error(s) when comparing

the received signal with the encoding trellis in Figure 1-29. For example, when the first bit “0”

is encoded the output is “00”. If this is received without error then from the initiating state (00)

there are only two possibilities:

“0” was sent - This is the example shown, therefore there are “0” errors indicated on the

initial input=0 line.

“1” was sent - This is not the example shown, however the “2” on the input=1 line

illustrates 2 errors, i.e. if the original input sequence was a 1, i.e. coded as “11” two

errors must have happened on the air interface.

Figure 1-30 Example of Viterbi Decoding

00

10

01

11

00

10

01

11

00

10

01

11

00

10

01

11

Input

Sequence

00

10

01

11

0 1 1 0

Transmitted

/Received 00 11 01 01

0

2

2

0

1

1

1

1

1

2

1

00

2

1

1

1

2

1

00

2

Indicate possible

number of bits in

error.

Input 0 Input 1

In order for the Viterbi decoding trellis to work all possible states are considered for the

sequence of bits. If errors did occur, it is the “maximum-likelihood” path which is chosen, i.e.

the one with the least amount of errors.

Tail Biting Convolutional Coding

As previously mentioned, LTE utilizes tail biting convolutional coding for the downlink BCH

(Broadcast Channel) and DCI (Downlink Control Information), as well as possibly for the

UCI (Uplink Control Information).

Table 1-11 illustrates the main difference between the tail biting convolutional coding and

standard convolutional coding.

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Table 1-11 Standard Convolutional Coding Verses Tail Biting Convolutional Coding

Standard Convolutional Coding Tail Biting Convolutional Coding

Initializes the shift register with zeros. Initializes the shift register with the last bits

of the stream, i.e. zeros are not added for

initialization.

Padded with zeros. The shift register finishes, such that the last

bits of input are the same as what was used

to initialize the shift registers.

The initial value of the shift registers are set to the values corresponding to the last 6

information bits in the input stream as illustrated in Figure 1-31. This ensures that the initial

and final states of the shift registers are the same for the decoding process.

Figure 1-31 Initializing Tail Biting Convolutional Coding

Tail Biting

Convolutional CodingInput Bits

Last 6bits used to

initialize coder.

The actual LTE tail biting convolutional coder is shown in Figure 1-32. There are six shift

registers and hence 6bits are required to initialize the coder. The input bit stream is identified

by ck, dk

(0), dk

(1) and dk

(2) correspond to the first, second and third parity streams, respectively.

Figure 1-32 LTE 1/3 Rate Tail Biting Convolutional Coding

S1 S2 S3 S4 S5 S6ck

dk(1) G1

dk(0) G0

dk(2) G2

Turbo Coding

Turbo coding defines a high-performance FEC mechanism. The term “Turbo coding” can be

used to describe many different types of encoders. For example, in LTE the turbo encoder is

known as a PCCC (Parallel Concatenated Convolutional Code) and it has two 8 state

constituent encoders and one contention-free QPP (Quadratic Permutation Polynomial) turbo

code internal interleaver. As previously mentioned, the coding rate of the LTE turbo encoder

is 1/3, i.e. for each input bit, three bits are produced. The structure of a turbo encoder is

illustrated in Figure 1-33.

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Figure 1-33 LTE Turbo Coding

D D D

D D D

Turbo Code

Internal

Interleaver

ck

c’k

2nd Constituent Encoder

1st Constituent Encoderzk

xk

x’k

z’k

Systematic

Bits

Parity

Bits

Parity

Bits

The dotted lines

are part of the

trellis termination.

The LTE turbo encoder employs two recursive convolutional encoders connected in parallel,

with the QPP turbo interleaver preceding the second encoder. The outputs of the constituent

encoders are punctured and repeated to achieve the correct output. It can be seen that the turbo

coder encodes the input block twice, i.e. with and without interleaving, to generate two

distinct sets of parity bits.

1.6.4 Rate Matching

The rate matching for turbo coded transport channels is defined per coded block and consists

of interleaving the three information bit streams dk(0)

, dk(1)

and dk(2)

, followed by the collection

of bits and the generation of a circular buffer as illustrated in Figure 1-34.

Figure 1-34 LTE Rate Matching

dk(1)

dk(0)

dk(2)

vk(1)

vk(0)

vk(2)

wk

Virtual

Circular

Buffer

ek

Sub-block

Interleaver

Sub-block

Interleaver

Sub-block

Interleaver

Bit

Collection

Bit Selection

and Pruning

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The sub-block interleaver is a row-column interleaver with 32 columns. Table 1-12 illustrates

the column permutations.

Table 1-12 LTE Sub-block Interleaver

Number of Columns Inter-column Permutation Pattern

32 < 0, 16, 8, 24, 4, 20, 12, 28, 2, 18, 10, 26, 6,

22, 14, 30, 1, 17, 9, 25, 5, 21, 13, 29, 3, 19,

11, 27, 7, 23, 15, 31 >

The sub-block interlearver works by writing each stream of bits row-by-row into a matrix

with 32 columns. In so doing, the number of rows is based on the stream size. In addition,

padding is added to the front of each stream so that the matrix is complete.

The output of the sub-block interleaver consists of the columns read out in the permutation

order, i.e. 0, 16, 8 etc.

The bit collection block provides a circular buffer which can be read during “bit selection and

pruning”. The circular buffer is formed by concatenating the rearranged systematic bits with

the two rearranged/interlaced parity bit streams.

Finally, the bit selection and pruning block performs a very important function. It provides a

rate matching output, ek, of the correct length and utilizing the correct RV (Redundancy

Version). The redundancy version is identified by the parameter rvidx and can have the values

0, 1, 2 or 3. As such, this value impacts the HARQ (Hybrid ARQ) operation, enabling the

system to select and prune different sets of bits.

1.6.5 Code Block Concatenation

Code block concatenation effectively concatenates the previously segmented code blocks.

Figure 1-35 Code Block Concatenation

4200bits

4224bits

3800bits

3840bits

Channel Coding

Rate Matching

ek

Code Block CRC Attachment and Segmentation

Channel Coding

Rate Matching

Code Block Concatenation

ek

fk

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1.7 Principles of OFDM

The LTE air interface utilizes two different multiple access techniques both based on OFDM

(Orthogonal Frequency Division Multiplexing):

OFDMA (Orthogonal Frequency Division Multiple Access) used on the downlink.

SC-FDMA (Single Carrier - Frequency Division Multiple Access) used on the uplink.

Figure 1-36 Use of OFDM in LTE

eNB

UE

OFDM

(OFDMA)

OFDM

(SC-FDMA)

The concept of OFDM is not new and is currently being used on various systems such as

Wi-Fi and WiMAX. In addition, it was even considered for UMTS back in 1998. One of the

main reasons why it was not chosen at the time was the handset’s limited processing power

and poor battery capabilities.

LTE was able to choose OFDM based access due to the fact mobile handset processing

capabilities and battery performance have both improved. In addition, there is continual

pressure to produce more spectrally efficient systems.

1.7.2 Frequency Division Multiplexing

OFDM is based on FDM (Frequency Division Multiplexing) and is a method whereby

multiple frequencies are used to simultaneously transmit information. Figure 1-37 illustrates

an example of FDM with four subcarriers. These can be used to carry different information

and to ensure that each subcarrier does not interfere with the adjacent subcarrier, a guard band

is utilized. In addition, each subcarrier has slightly different radio characteristics and this may

be used to provide diversity.

Figure 1-37 FDM Carriers

Frequency

Guard Band

Channel

Bandwidth

Subcarrier

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FDM systems are not that spectrally efficient (when compared to other systems) since

multiple subcarrier guard bands are required.

1.7.1 OFDM Subcarriers

OFDM follows the same concept as FDM but it drastically increases spectral efficiency by

reducing the spacing between the subcarriers. Figure 1-38 illustrates how the subcarriers can

overlap due to their orthogonality with the other subcarriers, i.e. the subcarriers are

mathematically perpendicular to each other. As such, when a subcarrier is at its maximum the

two adjacent subcarriers are passing through zero. In addition, OFDM systems still employ

guard bands. These are located at the upper and lower parts of the channel and reduce

adjacent channel interference.

Figure 1-38 OFDM Subcarriers

Frequency

Channel

Bandwidth

Orthogonal

Subcarriers

Centre Subcarrier

Not Orthogonal

The centre subcarrier, known as the DC (Direct Current) subcarrier, is not typically used in OFDM

system due to its lack of orthogonality.

1.7.2 Fast Fourier Transforms

OFDM subcarriers are generated and decoded using mathematical functions called FFT (Fast

Fourier Transform) and IFFT (Inverse Fast Fourier Transform). The IFFT is used in the

transmitter to generate the waveform. Figure 1-39 illustrates how the coded data is first

mapped to parallel streams before being modulated and processed by the IFFT.

Figure 1-39 Inverse Fast Fourier Transform

Coded

BitsIFFT

Serial

to

Parallel

Subcarrier

Modulation

RF

Inverse Fast

Fourier

Transform

Complex

Waveform

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At the receiver side, this signal is passed to the FFT which analyses the complex/combined

waveform into the original streams. Figure 1-40 illustrates the FFT process.

Figure 1-40 Fast Fourier Transform

Coded

Bits

Parallel

to

Serial

FFT

Subcarrier

Demodulation

Receiver

Fast Fourier

Transform

1.7.3 LTE FFT Sizes

Fast Fourier Transforms and Inverse Fast Fourier Transforms both have a defining size. For

example, an FFT size of 512 indicates that there are 512 subcarriers. In reality, not all 512

subcarriers can be utilized due to the channel guard bands and the fact that a DC (Direct

Current) subcarrier is also required.

Table 1-13 illustrates the LTE channel bandwidth options, as well as the FFT size and

associated sampling rate. Using the sampling rate and the FFT size the subcarrier spacing can

be calculated, e.g. 7.68MHz/15kHz = 512.

Table 1-13 LTE Channel and FFT Sizes

Channel Bandwidth

FFT Size Subcarrier Bandwidth

Sampling Rate

1.4MHz 128

15kHz

1.92MHz

3MHz 256 3.84MHz

5MHz 512 7.68MHz

10MHz 1024 15.36MHz

15MHz 1536 23.04MHz

20MHz 2048 30.72MHz

The subcarrier spacing of 15kHz is also used in the calculation to identify the OFDM symbol duration.

1.7.4 OFDM Symbol Mapping

The mapping of OFDM symbols to subcarriers is dependent on the system design. Figure

1-41 illustrates an example of OFDM mapping. The first 12 modulated OFDM symbols are

mapped to 12 subcarriers, i.e. they are transmitted at the same time but using different

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subcarriers. The next 12 subcarriers are mapped to the next OFDM symbol period. In addition,

a CP (Cyclic Prefix) is added between the symbols.

Figure 1-41 OFDM Symbol Mapping

Time

Frequency

Amplitude

OFDM

Symbol

Cyclic

Prefix

Modulated

OFDM

Symbol

LTE allocates resources in groups of 12 subcarriers. This is known as a PRB (Physical Resource Block).

In the previous example 12 different modulated OFDM symbols are transmitted

simultaneously. Figure 1-42 illustrates how the combined energy from this will result in either

constructive peaks (when the symbols are the same) or destructive nulls (when the symbols

are different). This means that OFDM systems have a high PAPR (Peak to Average Power

Ratio).

Figure 1-42 OFDM PAPR (Peak to Average Power Ratio)

Amplitude

Time

OFDM

Symbol

PAPR (Peak to Average

Power Ratio) Issue

Peak

Average

1.7.5 Time Domain Interference

The OFDM signal provides some protection in the frequency domain due to the orthogonality

of the subcarriers. The main issue is with delay spread, i.e. multipath interference.

Figure 1-43 illustrates two of the main multipath effects, namely delay and attenuation. The

delayed signal can manifest itself as ISI (Inter Symbol Interference), whereby one symbol

impacts the next. This is illustrated in Figure 1-44.

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Figure 1-43 Delay Spread

Energy

Time

Delay Spread

ISI (Inter Symbol Interference) is typically reduced with “equalizers”. However, for the

equalizer to be effective a known bit pattern or “training sequence” is required. However, this

reduces the system capacity, as well as impacts processing on a device. Instead, OFDM

systems employ a CP (Cyclic Prefix).

Figure 1-44 Inter Symbol Interference

1st Received

SignalDelayed

Signal

Interference

Caused

Cyclic Prefix

A CP (Cyclic Prefix) is utilized in most OFDM systems to combat multipath delays. It

effectively provides a guard period for each OFDM symbol. Figure 1-45 illustrates the Cyclic

Prefix and its location in the OFDM Symbol. Notice that the Cyclic Prefix is effectively a

copy taken from the back of the original symbol which is then placed in front of the symbol to

make the OFDM symbol (Ts).

The size of the Cyclic Prefix relates to the maximum delay spread the system can tolerate. As

such, systems designed for macro coverage, i.e. large cells, should have a large CP. This does

however impact the system capacity since the number of symbols per second is reduced.

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Figure 1-45 Cyclic Prefix

Symbol Period T(s)T(g)

CP

CP

CP

CP

CP

CP

CP

CP

CP

CP

CP

CP

Frequency

TimeSymbol Period T(s)

Bit Period T(b)Cyclic Prefix

LTE has two defined Cyclic Prefix sizes, normal and extended. The extended Cyclic Prefix is designed

for larger cells.

1.7.6 OFDM Advantages and Disadvantages

OFDM Advantages

OFDM systems typically have a number of advantages:

OFDM is almost completely resistant to multi-path interference due to very long symbol

duration.

higher spectral efficiency for wideband channels.

flexible spectrum utilization.

relatively simple implementation using FFT and IFFT.

OFDM Disadvantages

OFDM also has some disadvantages:

frequency errors and phase noise can cause issues.

Doppler shift impacts subcarrier orthogonality.

some OFDM systems can suffer from high PAPR.

required accurate frequency and time synchronization.

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2-1

2 LTE Physical Layer

Objectives


2.1 Detail the channel structure of the radio interface.

2.2 Detail the radio interface protocols.

2.3 Describe the physical signals in the UL and DL.

2.4 Detail the time-domain structure in the radio interface in the UL and DL for both FDD and

TDD mode.

2.5 Have a good understanding of the OFDM principle, signal generation and processing.

2.6 Detail the DL transmission technique.

2.7 Detail the DL synchronization signals.

2.8 Detail the reference symbols in the DL.

2.9 Detail the DL physical Channels.

2.10 Detail the DL control signaling and formats.

2.11 Explain the cell search procedure.

2.12 Detail the UL transmission technique.

2.12 Have a good understanding of the SC-FDMA principle, signal generation and processing.

2.13 Explain the pros and cons with OFDM and SC-FDMA.

2.14 Detail the UL Physical Channels.

2.15 Explain the timing relationships between the UL and DL.

2.16 Detail the reference signals.

2.17 Detail the UL control signaling and formats.

2.18 Detail the random access procedure.

2.19 Describe the Power Control in the UL.

2.20 Detail the paging procedures.

2.21 Explain HARQ.


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2.22 Describe the concepts of layers, channel rank, spatial multiplexing, open and closed loop

spatial multiplexing, TX diversity, beamforming, SU-MIMO and MU-MIMO.

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2.1 The Uu Interface

The LTE air interface is identified as the E-UTRA (Evolved - Universal Terrestrial Radio

Access) and can support varying bandwidth options ranging from 1.4MHz to 20MHz. The

interface is identified as “Uu”, with the capital “U” indicating the “User to Network” interface

and the lower case “u” indicating Universal. The UE (User Equipment) will utilize a channel

bandwidth based on the configuration of the eNB (Evolved Node B). However, the eNB may

implement multiple channels to improve capacity or as part of a frequency reuse mechanism.

Figure 2-1 The LTE Air Interface

eNB

UE

E-UTRA

1.4MHz, 3MHz,

5MHz, 10MHz,

15MHz, 20MHz

Uu

2.2 LTE Radio Interface Protocols The E-UTRA interface provides connectivity between the User Equipment and the eNB. It

can be logically split into a control plane and a user plane. There are effectively two control

planes, the first is provided by RRC (Radio Resource Control) and carries signaling between

the User Equipment and the eNB. The second carries NAS (Non Access Stratum) signaling

messages to the MME (Mobility Management Entity), which are carried by RRC. Figure 2-2

illustrates the RRC and NAS control planes, as well as the user plane which focuses on the

delivery of IP datagrams to and from the EPC (Evolved Packet Core), namely the S-GW

(Serving Gateway) and PDN-GW (Packet Data Network - Gateway).

Figure 2-2 LTE Control Plane and User Plane

UE

PDN-GW

E-UTRAN EPC

MME

S-GW

eNB

S1-MME

S1-U

S5/S8

S11

NAS Control

Plane

User

Plane

RRC

Control

Plane


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2.2.1 Control and User Plane Protocols

The control and user plane lower layer protocols are the same. As such, they both utilize the

services of PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) and MAC

(Medium Access Control), as well as the PHY (Physical Layer). Figure 2-3 illustrates the

radio interface protocol stacks. It can be seen that the NAS signaling uses the services of RRC,

which is then mapped into PDCP. On the user plane IP datagrams are also mapped into PDCP.

Figure 2-3 E-UTRA Protocols

eNBUE

RLC

MAC

PHY

PDCP

RRC

RLC

MAC

PHY

PDCP

NAS User - IP

Control Plane

User PlaneNAS Signaling

2.2.2 Non Access Stratum

The term Non Access Stratum, or NAS, identifies the layer(s) above the AS (Access Stratum).

The access stratum defines the procedures and protocols associated with the RAN (Radio

Access Network), i.e. the E-UTRAN. There are two main aspects to NAS, namely higher

layer signaling and user data.

NAS Signaling

In terms of NAS signaling, messages pass between the User Equipment and the MME. This is

illustrated in Figure 2-4.

Figure 2-4 NAS Signaling

UE eNB

MME

EMM (EPS Mobility

Management)

ESM (EPS Session

Management)

Two categories of NAS signaling exist:

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EMM (EPS Mobility Management).

ESM (EPS Session Management).

Table 2-1 illustrates the main EMM and ESM LTE procedures.

Table 2-1 NAS EMM and ESM Procedures

EMM Procedures ESM Procedures

Attach Default EPS Bearer Context Activation

Detach Dedicated EPS Bearer Context Activation

Tracking Area Update EPS Bearer Context Modification

Service Request EPS Bearer Context Deactivation

Extended Service Request UE Requested PDN Connectivity

GUTI Reallocation UE Requested PDN Disconnect

Authentication UE Requested Bearer Resource Allocation

Identification UE Requested Bearer Resource Modification

Security Mode Control ESM Information Request

EMM Status ESM Status

EMM Information

NAS Transport

Paging

EMM Procedures

The key EMM procedures include:

Attach - this is used by the UE to attach to an EPC (Evolved Packet Core) for packet

services in the EPS (Evolved Packet System). Note that it can be also used to attach to

non-EPS services.

Detach - this is used by the UE to detach from EPS services. In addition, it can also be

used for other procedures such as disconnecting from non-EPS services.

Tracking Area Updating - this procedure is always initiated by the UE and is used for the

various purposes. The most common include normal and periodic tracking area updating.

Service Request - this is used by the UE to get connected and establish the radio and S1

bearers when uplink user data or signaling is to be sent.

Extended Service Request - this is used by the UE to initiate a Circuit Switched fallback

call or respond to a mobile terminated Circuit Switched fallback request from the

network.

GUTI Reallocation - This is used to allocate a GUTI (Globally Unique Temporary

Identifier) and optionally to provide a new TAI (Tracking Area Identity) list to a

particular UE.

Authentication - this is used for AKA (Authentication and Key Agreement) between the

user and the network.


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Identification - this is used by the network to request a particular UE to provide specific

identification parameters, e.g. the IMSI (International Mobile Subscriber Identity) or the

IMEI (International Mobile Equipment Identity).

Security mode control - this is used to take an EPS security context into use, and

initialize and start NAS signaling security between the UE and the MME with the

corresponding NAS keys and security algorithms.

EMM Status - this is sent by the UE or by the network at any time to report certain error

conditions.

EMM Information - this allows the network to provide information to the UE.

Transport of NAS messages - this is to carry SMS (Short Message Service) messages in

an encapsulated form between the MME and the UE.

Paging - this is used by the network to request the establishment of a NAS signaling

connection to the UE. Is also includes the Circuit Switched Service Notification

EMM Procedures

The key ESM procedures include:

Default EPS Bearer Context Activation - this is used to establish a default EPS bearer

context between the UE and the EPC.

Dedicated EPS Bearer Context Activation - this is to establish an EPS bearer context

with specific QoS (Quality of Service) and TFT (Traffic Flow Template) between the UE

and the EPC. The dedicated EPS bearer context activation procedure is initiated by the

network, but may be requested by the UE by means of the UE requested bearer resource

allocation procedure.

EPS Bearer Context Modification - this is used to modify an EPS bearer context with a

specific QoS and TFT.

EPS Bearer Context Deactivation - this is used to deactivate an EPS bearer context or

disconnect from a PDN by deactivating all EPS bearer contexts to the PDN.

UE Requested PDN Connectivity - this is used by the UE to request the setup of a

default EPS bearer to a PDN.

UE Requested PDN Disconnect - this is used by the UE to request disconnection from

one PDN. The UE can initiate this procedure to disconnect from any PDN as long as it is

connected to at least one other PDN.

UE Requested Bearer Resource Allocation - this is used by the UE to request an

allocation of bearer resources for a traffic flow aggregate.

UE Requested Bearer Resource Modification - this is used by the UE to request a

modification or release of bearer resources for a traffic flow aggregate or modification of

a traffic flow aggregate by replacing a packet filter.

ESM Information Request - this is used by the network to retrieve ESM information, i.e.

protocol configuration options, APN (access Point Name), or both from the UE during

the attach procedure.

ESM Status - this is used to report at any time certain error conditions detected upon

receipt of ESM protocol data.

NAS User

The NAS user plane is based on IP (Internet Protocol). As such, IP datagrams are passed to

the lower layers, i.e. PDCP, for processing.

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2.2.3 RRC

The main air interface control protocol is RRC (Radio Resource Control). For RRC messages

to be transferred between the UE and the eNB it uses the services of PDCP, RLC, MAC and

PHY. Figure 2-5 identifies the main RRC functions. In summary, RRC handles all the

signaling between the UE and the E-UTRAN, with signaling between the UE and Core

Network, i.e. NAS (Non Access Stratum) signaling, being carried by dedicated RRC

messages. When carrying NAS signaling, RRC does not alter the information but instead,

provides the delivery mechanism.

RRC provides the main configuration and parameters to the lower layers. As such, the PHY layer will

get information from RRC on how to configure certain aspects of the Physical Layer.

Figure 2-5 Main RRC Functions

eNB

RLC

MAC

PHY

PDCP

RRC

NAS Signaling

System Information

PLMN and Cell Selection

Admission Control

Security Management

Cell Reselection

Measurement Reports

Handovers and Mobility

NAS Transport

Radio Resource Management

2.2.4 PDCP

LTE implements PDCP in both the user plane and control plane. This is unlike UMTS, where

PDCP was only found in the user plane. The main reason for the difference is that PDCP in

LTE takes on the role of security, i.e. encryption and integrity. In addition, Figure 2-6

illustrates some of the other functions performed by PDCP.

Figure 2-6 PDCP Functions

eNB

RLC

MAC

PHY

PDCP

RRC

NAS Signaling

Control Plane

Encryption

Integrity Checking

User Plane

IP Header Compression

Encryption

Sequencing and Duplicate Detection


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In the control plane, PDCP facilitates encryption and integrity checking of signaling messages,

i.e. RRC and NAS. The user plane is slightly different since only encryption is performed. In

addition, the user plane IP datagrams can also be subjected to IP header compression

techniques in order to improve the system’s performance and efficiency. Finally, PDCP also

facilitates sequencing and duplication detection.

2.2.5 RLC

The RLC (Radio Link Control) protocol exists in the UE and the eNB. As its name suggests it

provides “radio link” control, if required. In essence, RLC supports three delivery services to

the higher layers:

TM (Transparent Mode) - This is utilized for some of the air interface channels, e.g.

broadcast and paging. It provides a connectionless service for signaling.

UM (Unacknowledged Mode) - This is like Transparent Mode, in that it is a

connectionless service; however it has the additional features of sequencing,

segmentation and concatenation.

AM (Acknowledged Mode) - This offers an ARQ (Automatic Repeat Request) service.

As such, retransmissions can be used.

These modes, as well as the other RLC features are illustrated in Figure 2-7. In addition to

ARQ, RLC offers segmentation, re-assembly and concatenation of information.

Figure 2-7 RLC Modes and Functions

eNB

RLC

MAC

PHY

PDCP

RRC

NAS Signaling

TM (Transparent Mode)

UM (Unacknowledged Mode)

AM (Acknowledged Mode)

Segmentation and Re-Assembly

Concatenation

Error Correction

2.2.6 MAC

MAC (Medium Access Control) provides the interface between the E-UTRA protocols and

the E-UTRA Physical Layer. In doing this it provides the following services:

Mapping - MAC maps the information received on the LTE Logical Channels into the

LTE transport channels. These channels and their mapping are discussed further in

Section 2.3 .

Multiplexing - The information provided to MAC will come from a RB (Radio Bearer)

or multiple Radio Bearers. The MAC layer is able to multiplex different bearers into the

same TB (Transport Block), thus increasing efficiency.

HARQ (Hybrid Automatic Repeat Request) - MAC utilizes HARQ to provide error

correction services across the air. HARQ is a feature which requires the MAC and

Physical Layers to work closely together. This is discussed further in Section 2.21 .

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Radio Resource Allocation - QoS (Quality of Service) based scheduling of traffic and

signaling to users is provided by MAC. There are various scheduling options, these are

described further in Section 3 .

In order to support these features the MAC and Physical layers need to pass various

indications on the radio link quality, as well as the feedback from HARQ operation.

Figure 2-8 Medium Access Control Functions

eNB

RLC

MAC

PHY

PDCP

RRC

NAS Signaling

Channel Mapping and Multiplexing

Error Correction - HARQ

QoS Based Scheduling

2.2.7 Physical

The PHY (Physical Layer) in LTE provides a new and flexible channel. It does however

utilize features and mechanisms defined in earlier systems, i.e. UMTS. Figure 2-9 illustrates

the main functions provided by the Physical Layer.

Figure 2-9 Physical Layer Functions

eNB

RLC

MAC

PHY

PDCP

RRC

NAS Signaling

Error Detection

FEC Encoding/Decoding

Rate Matching

Mapping of Physical Channels

Power Weighting

Modulation and Demodulation

Frequency and Time Synchronization

Radio Measurements

MIMO Processing

Transmit Diversity

Beamforming

RF Processing

2.3 LTE Channel Structure

The concept of “channels” is not new. Both GSM and UMTS defined various channel

categories, however LTE terminology is closer to UMTS. Broadly there are four categories of

channel.


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Figure 2-10 LTE Channels

Logical

Channels

Transport

Channels

Physical

Channels

Radio

Channels

2.3.1 Logical Channels

In order to describe Logical Channels it is best to identify where Logical Channels are located

in relation to the LTE protocols and the other channel types. Figure 2-11 shows Logical

Channels located between the RLC and the MAC layers.

Figure 2-11 Location of Channels

RLC

MAC

PHY

Logical

ChannelsTransport

Channels

Physical

Channels Radio

Channel

Logical channels are classified as either Control Logical Channels, which carry control data

such as RRC signaling, or traffic Logical Channels which carry user plane data.

Control Logical Channels

The various forms of these Control Logical Channels include:

BCCH (Broadcast Control Channel) - This is a downlink channel used to send SI

(System Information) messages from the eNB. These are defined by RRC.

PCCH (Paging Control Channel) - This downlink channel is used by the eNB to send

paging information.

Figure 2-12 BCCH and PCCH Logical Channels

BCCH

eNBUE

PCCH

System Information

Messages

Paging

Devices

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CCCH (Common Control Channel) - This is used to establish a RRC (Radio Resource

Control) connection, also known as a SRB (Signaling Radio Bearer). The SRB is

discussed further in Section 2.18 . The SRB is also used for re-establishment procedures.

SRB 0 maps to the CCCH.

DCCH (Dedicated Control Channel) - This provides a bidirectional channel for signaling.

Logically there are two DCCH activated:

− SRB 1 - This is used for RRC messages, as well as RRC messages carrying high

priority NAS signaling.

− SRB 2 - This is used for RRC carrying low priority NAS signaling. Prior to its

establishment low priority signaling is sent on SRB1.

Figure 2-13 CCCH and DCCH Signaling

CCCH

eNBUE

CCCH

DCCH

DCCH

SRB 0

SRB 0

SRB 1

SRB 2

Low Priority

NAS Signaling

Traffic Logical Channels

Release 8 LTE has one type of Logical Channel carrying traffic, namely the DTCH

(Dedicated Traffic Channel). This is used to carry DRB (Dedicated Radio Bearer) information,

i.e. IP datagrams.

Figure 2-14 Dedicated Traffic Channel

eNBUE

DTCHDRB

Carries AM or UM

RLC Traffic

The DTCH is a bidirectional channel that can operate in either RLC AM or UM mode. This is

configured by RRC and is based on the QoS (Quality of Service) of the E-RAB (EPS Radio

Access Bearer).

2.3.2 Transport Channels

Historically, Transport Channels were split between common and dedicated channels.

However, LTE has moved away from dedicated channels in favor of the common/shared

channels and the associated efficiencies provided. The main Release 8 Transport Channels

include:


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BCH (Broadcast Channel) - This is a fixed format channel which occurs once per frame

and carries the MIB (Master Information Block). Note that the majority of System

Information messages are carries on the DL-SCH (Downlink - Shared Channel).

PCH (Paging Channel) - This channel is used to carry the PCCH, i.e. paging messages. It

also utilizes DRX (Discontinuous Reception) to improve UE battery life.

DL-SCH (Downlink - Shared Channel) - This is the main downlink channel for data and

signaling. It supports dynamic scheduling, as well as dynamic link adaptation. In

addition, it supports HARQ (Hybrid Automatic Repeat Request) operation to improve

performance. As previously mentioned it also facilitates the sending of System

Information messages.

RACH (Random Access Channel) - This channel carries limited information and is used

in conjunction with Physical Channels and preambles to provide contention resolution

procedures.

UL-SCH (Uplink Shared Channel) - Similar to the DL-SCH, this channel supports

dynamic scheduling (eNB controlled) and dynamic link adaptation by varying the

modulation and coding. In addition, it too supports HARQ (Hybrid Automatic Repeat

Request) operation to improve performance.

Figure 2-15 LTE Release 8 Transport Channels

BCH

eNBUE

PCH

DL-SCH

RACH

UL-SCH

2.3.3 Physical Channels

The Physical Layer facilitates transportation of MAC Transport Channels, as well as

providing scheduling, formatting and control indicators. Sections 2.9 and 2.14 examines the

Physical Channels in greater detail.

Downlink Physical Channels

There are a number of downlink Physical Channels in LTE. These include:

PBCH (Physical Broadcast Channel) - This channel carries the BCH.

PCFICH (Physical Control Format Indicator Channel) - This is used to indicate the

number of OFDM symbols used for the PDCCH.

PDCCH (Physical Downlink Control Channel) - This channel is used for resource

allocation.

PHICH (Physical Hybrid ARQ Indicator Channel) - This channel is part of the HARQ

process.

PDSCH (Physical Downlink Shared Channel) - This channel carries the DL-SCH.

Uplink Physical Channels

There are a number of Uplink Physical Channels in LTE. These include:

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PRACH (Physical Random Access Channel) - This channel carries the Random Access

Preamble. The location of the PRACH is defined by higher layer signaling, i.e. RRC

signaling.

PUCCH (Physical Uplink Control Channel) - This channel carries uplink control and

feedback. It can also carry scheduling requests to the eNB.

PUSCH (Physical Uplink Shared Channel) - This is the main uplink channel and is used

to carry the UL-SCH (Uplink Shared Channel) Transport Channel. It carries both

signaling and user data, in addition to uplink control. It is worth noting that the UE is not

allowed to transmit the PUCCH and PUSCH at the same time.

2.3.4 Radio Channels

The term “Radio Channel” is typically used to describe the overall channel, i.e. the downlink

and uplink carrier for FDD or the single carrier for TDD.

Figure 2-16 Radio Channel

eNB

UE

Radio

Channel

Radio

Channel

UE

FDD

TDD

2.3.5 Channel Mapping

There are various options for multiplexing multiple bearers together, such that Logical

Channels may be mapped to one or more Transport Channels. These in turn are mapped into

Physical Channels. Figure 2-17 and Figure 2-18 illustrate the mapping options.


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Figure 2-17 Downlink Channel Mapping

DL-SCH

Physical Layer

MAC Layer

RLC Layer

PDCP Layer

RRC Layer

Physical

Channels

Transport

Channels

Logical

Channels

PDSCHPDCCHPHICHPCFICHPBCH

BCH PCH

BCCH PCCH CCCH DCCH DTCH

TM TM TM UM/AM UM/AM

Ciphering

Integrity

Ciphering

ROHC

RRC

ESM EMM IPNAS Layer

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Figure 2-18 Uplink Channel Mapping

Physical Layer

MAC Layer

RLC Layer

PDCP Layer

RRC Layer

Physical

Channels

Transport

Channels

Logical

Channels

PUSCHPUCCHPRACH

RACH

CCCH

TM UM/AM UM/AM

Ciphering

Integrity

Ciphering

ROHC

RRC

ESM EMM IPNAS Layer

UL-SCH

DCCH DTCH

In order to facilitate the multiplexing from Logical Channels to Transport Channels, the MAC

Layer typically adds a LCID (Logical Channel Identifier).

2.4 LTE Frame Structure

In LTE, devices are allocated blocks of subcarriers for a period of time. These are referred to

as a PRB (Physical Resource Block). The resource blocks are contained within the LTE

generic frame structure. Two types are defined, namely type 1 and type 2 radio frames.

2.4.1 Type 1 Radio Frames, Slots and Subframes

The type 1 radio frame structure is used for FDD and is 10ms in duration. It consists of 20

slots, each lasting 0.5ms. Two adjacent slots form one subframe of length 1ms. For FDD

operation 10 subframes are available for downlink transmission and 10 subframes are

available for uplink transmission, with each transmission separated in the frequency domain.

Figure 2-19 illustrates the FDD frame structure, as well as highlighting the slots and subframe

concept. In addition, it illustrates how the slots are numbered 0 to 19.


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Figure 2-19 LTE Frame Structure

Slot (0.5ms)

Radio Frame Tf = 307200 x Ts = 10ms

Subframe (1ms)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Ts = 1/(15000x2048)

= 32.552083ns

Tslot = 15360 x Ts

LTE Time Unit

The LTE time unit is identified as Ts and is calculated as 1/(15000x2048), which equates to

approximately 32.552083ns. Various aspects of LTE utilize this parameter, or multiple of it, to

identify timing and configuration.

Cyclic Prefix Options

Section 1.7.5 introduced the concept of a CP (Cyclic Prefix) in OFDM systems. In LTE, it

was chosen to have two different cyclic prefix sizes, namely “Normal” and “Extended”. In

order to facilitate these, two different slot formats are available. Figure 2-20 illustrates the 7

and 6 ODFM symbol options. Obviously, to facilitate a larger cyclic prefix one of the symbols

is sacrificed, thus the symbol rate is reduced.

Figure 2-20 Normal and Extended Cyclic Prefix

Radio Frame = 10ms

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

7 OFDM

Symbols (Normal

Cyclic Prefix)

6 OFDM Symbols

(Extended Cyclic

Prefix)

0 1 2 3 4 5 6

0 1 2 3 4 5

CP (Cyclic

Prefix)

Ts

Ts

The use of the extended cyclic prefix is intended for scenarios when the range of the cell

needs to be extended, e.g. for planning purposes.

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Table 2-2 illustrates the sizes of the cyclic prefix for different configurations. It can be seen

that the CP size can vary during a slot, such that the first CP is larger than the rest when the

normal CP size is chosen.

Table 2-2 Downlink CP Parameters

Configuration CP Length (Ts) Time Delay Spread

Normal Cyclic

Prefix

∆f = 15kHz 160 for slot 0 ~ 5.208µs ~ 1.562km

144 for slot 1, 2, …6 ~ 4.688µs ~ 1.406km

Extended Cyclic

Prefix

∆f = 15kHz 512 for slot 0, 1, …5 ~16.67µs ~ 5km

∆f = 7.5kHz 1024 for 0, 1, 2 ~ 33.33 µs ~ 10km

The 7.5kHz option is part of MBSFN (MBMS over Single Frequency Network) which is still in the Release 8 PHY specifications, however the MBMS feature which utilizes this has been delayed until

Release 9. In addition, this option (7.5kHz) is only available in the downlink.

The symbol (Ts) consists of a guard period, i.e. the cyclic prefix, and the Tb data duration

which is 2048 LTE time units for both the normal and extended 15kHz option. Figure 2-21

illustrates an example of the normal cyclic prefix configuration for a slot.

Figure 2-21 Normal CP Configuration

0

OFDM Symbols (= 7 for Normal CP)

21 3 4 5 6

NsymbDL

160 144 144 144 144 144 1442048 2048 2048 2048 2048 2048 2048

Larger first CP when

Normal CP is configured

E.g. NCP = 144,

TCP= 144 x Ts = 4.6875µs

2.4.2 Type 2 Radio Frames, Slots and Subframes

The type 2 radio frame structure is used for TDD. One key addition to the TDD frame

structure is the concept of “special subframes”. This includes a DwPTS (Downlink Pilot Time

Slot), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot). These have configurable

individual lengths and a combined total length of 1ms.

For TDD operation the 10 subframes are shared between the uplink and the downlink. A 5ms

and 10ms switch-point periodicity is supported however subframes 0 and 5 must be allocated

to the downlink as these contain the PSS (Primary Synchronization Signal) and SSS

(Secondary Synchronization Signal), as well as the broadcast information in subframe 0. The

PSS and SSS are discussed in Section 2.7


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Figure 2-22 Type 2 TDD Radio Frame

Type 2 Radio Frame Tf = 307200 x Ts = 10ms

0

Special

Subframe

2 3 4 5 7 8 9

DwPTS (Downlink

Pilot Time Slot)

GP (Guard Period)

UpPTS (Uplink

Pilot Time Slot)

There are various frame configuration options supported for TDD. Table 2-3 illustrates the

different options. Configuration options 0, 1, 2 and 6 have a 5ms switching point and

therefore require 2 special subframes, whereas the rest are based on a 10ms switching point.

In the table, the letter “D” is reserved for downlink transmissions, “U” denotes subframes

reserved for uplink transmissions and “S” denotes a special subframe with the three fields

DwPTS, GP and UpPTS.

Table 2-3 Type 2 Radio Frame Switching Points

Configuration Switching Point

Periodicity

Subframe Number

0 1 2 3 4 5 6 7 8 9

0 5ms D S U U U D S U U U

1 5ms D S U U D D S U U D

2 5ms D S U D D D S U D D

3 10ms D S U U U D D D D D

4 10ms D S U U D D D D D D

5 10ms D S U D D D D D D D

6 5ms D S U U U D S U U D

The DwPTS and UpPTS in a special frame may carry information. For example the DwPTS can include

scheduling information and the UpPTS can be configured to facilitate random access bursts.

2.5 OFDM Signal Generation

There are various Physical Layer stages involved in the generation of the downlink and uplink

signals. Figure 2-23 illustrates the possible stages for a PDSCH.

LTE Air Interface




2-19

Figure 2-23 Downlink Physical Layer Processing

Scrambling Modulation

Mapper

Layer

MapperPrecoding

Resource

Element

Mapper

OFDM

Signal

Generation

Resource

Element

Mapper

OFDM

Signal

Generation


Mapper

Codewords LayersAntenna

Ports

2.5.1 Codewords, Layers and Antenna Ports

Prior to identifying the various stages it is worth clarifying the concept of codewords, layers

and antenna ports. The use of layers and multiple antenna ports is related to diversity and

MIMO (Multiple Input Multiple Output). In addition, the term “rank” is typically applied to

the number of layers.

In LTE, when discussing the Physical Layer processing, a “codeword” corresponds to a TB

(Transport Block). One or two codewords can be used and these are mapped onto layers. The

number of layers can vary from one up to a maximum which is equal to the number of

antenna ports. When there is one codeword, i.e. one transport block, a single layer is used. In

contrast, two codewords, i.e. two transport blocks, can be used with two or more layers.

Figure 2-24 illustrates the mapping options.

Figure 2-24 Codeword, Layer and Antenna Port Mapping

1 Layer 2 Layers 3 Layers 4 Layers

1 1 2 1

Rank 1 Rank 2 Rank 3 Rank 4

2 2 2 21 1

Codeword

1, 2 or 4

Antenna

Ports

2 or 4

Antenna

Ports

4 Antenna

Ports

4 Antenna

Ports

It is important to note that the number of modulation symbols on each layer needs to be the

same. As such, when operating with three layers, the second codeword is twice as large as the

first. This can be achieved due to the supported TB sizes and the other Physical Layer stages.


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2.5.2 Scrambling

The initial stage of the Physical Layer processing is “scrambling”. This stage is applied to the

signal in order to provide interference rejection properties. Scrambling effectively randomizes

interfering signals using a pseudo-random scrambling process. Figure 2-25 illustrates the

concept of scrambling, showing a Physical Resource Block on each of the cells using the

same frequency. The scrambling feature statistically improves the interference by scrambling

the information with a scrambling code based on the physical cell ID and RNTI.

Figure 2-25 Scrambling in LTE

eNB eNB

PRB PRB

F1 F1

Interference

PRB PRB

Less

Interference

Cell RNTI

specific

scrambling

No

Scrambling

Figure 2-26 illustrates the generation of the scrambling code which is applied to most of the

Physical Channels. It is worth noting that scrambling is not used on the downlink PHICH and

on certain parts of the uplink.

Figure 2-26 LTE Scrambling Code Generation

cellcinit = nRNTI ∙ 214

+ q ∙ 213

+ ns / 2 ∙ 29 + NID For PDSCH

MSB LSB Scrambling

Code

Fixed Bit Pattern

cellcinit = nRNTI ∙ 214

+ ns / 2 ∙ 29 + NID For PUSCH

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2-21

2.5.3 Modulation Mapper

The modulation mapper converts the scrambled bits to complex-valued modulation symbols

(BPSK, QPSK, 16QAM or 64QAM).

Figure 2-27 BPSK, QPSK and 16QAM Modulation Mapper

I

Q

1-1

1

-1

0

1

I

Q

1-1

1

-1

00

01

10

11

I

Q

1 3-1-3

1

3

-1

-3

0000 0010

0001 0011

0100 0110

0101 0111

1000

1001

1100

1101

1010

1011

1110

1111

BPSK QPSK 16QAM

Figure 2-28 64QAM Modulation Mapper

I

Q

1 3 5 7-1-3-5-7

1

3

5

7

-1

-3

-5

-7

000011 000001 001001 001011

000010 000000 001000 001010

000110 000100 001100 001110

000111 000101 001101 001111

010011 010001 011001 011011

010010 010000 011000 011010

010110 010100 011100 011110

010111 010101 011101 011111

100011

100010

100110

100111

110011

110010

110110

110111

100001

100000

100100

100101

110001

110000

110100

110101

101001

101000

101100

101101

111001

111000

111100

111101

101011

101010

101110

101111

111011

111010

111110

111111

64QAM


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2.5.4 Layer Mapper

The layer mapper effectively maps the complex-valued modulation symbols onto one or

several transmission layers, thus splitting the data into a number of layers. Depending on the

transmission mode, various options are available.

Table 2-4 Layer Mapper Configuration

Mapper Configuration Layers ( ) Antenna Ports ( P )

Single Antenna 1 1P

Transmit Diversity P 1P (2 or 4)

Spatial Multiplexing P1 1P (2 or 4)

The complex-valued modulation symbols for each of the codewords to be transmitted are

mapped onto one or several layers. Complex-valued modulation symbols

)1(),...,0((q)symb

)()( Mdd qq for codeword q are mapped onto the layers

Tixixix )(...)()( )1()0( , 1,...,1,0layersymb Mi where is the number of layers and layer

symbM

is the number of modulation symbols per layer.

Single Antenna

For transmission on a single antenna port, a single layer is used, 1 , and the mapping is

defined by )()( )0()0( idix with (0)symb

layersymb MM .

Spatial Multiplexing

For spatial multiplexing, the layer mapping is illustrated in Table 2-5. The number of layers

is less than or equal to the number of antenna ports P used for transmission of the physical

channel. The case of a single codeword mapped to two layers is only applicable when the

number of antenna ports is 4.

Table 2-5 Codeword to Layer Mapping for Spatial Multiplexing

Number of Layers

Number of Codewords

Codeword to Layer Mapping 1,...,1,0layersymb Mi

1 1 )()( )0()0( idix )0(

symblayersymb MM

2 2 )()( )0()0( idix

)()( )1()1( idix

)1(symb

)0(symb

layersymb MMM

2 1

)12()(

)2()()0()1(

)0()0(

idix

idix

layer (0)

symb symb 2M M

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2-23

3 2 )()( )0()0( idix

)12()(

)2()()1()2(

)1()1(

idix

idix

2)1(

symb)0(

symblayersymb MMM

4 2

)12()(

)2()()0()1(

)0()0(

idix

idix

)12()(

)2()()1()3(

)1()2(

idix

idix

22)1(

symb)0(

symblayersymb MMM

Transmit Diversity

For transmit diversity there is only one codeword and the number of layers is equal to the

number of antenna ports used for transmission of the physical channel.

Table 2-6 Codeword to Layer Mapping for Transmit Diversity

Number

of Layers

Number

of Code words

Codeword to Layer Mapping 1,...,1,0layersymb Mi

2 1

)12()(

)2()(

)0()1(

)0()0(

idix

idix

2)0(

symblayersymb MM

4 1

)34()(

)24()(

)14()(

)4()(

)0()3(

)0()2(

)0()1(

)0()0(

idix

idix

idix

idix

04mod if

04mod if

42

4)0(

symb

)0(

symb

)0(

symb

)0(

symblayer

symbM

M

M

MM

If 04mod)0(

symb M two null symbols are

appended to )1( )0(

symb

)0( Md

2.5.5 Precoding

The next stage is precoding the complex-valued modulation symbols on each layer for

transmission. Figure 2-29 illustrates the different precoding options:

Single Antenna Port.

Transmit Diversity.

Spatial Multiplexing - This includes two options, i.e. with CDD (Cyclic Delay Diversity)

and without.

CDD (Cyclic Delay Diversity) is a method whereby a delayed version of the same OFDM symbol is

transmitted from multiple antennas. It provides a method for transforming spatial diversity into frequency diversity thus avoiding Inter Symbol Interference.


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Issue 01 (2010-05-01)

Figure 2-29 LTE Precoding Options

Transmit Diversity

Precoding

CDD (Cyclic

Delay

Diversity

LTE Spatial Multiplexing

LTE Precoding

Single Antenna Port

Precoding Concept

The precoder takes as input a block of vectors Tixixix )(...)()( )1()0( ,

1,...,1,0layersymb Mi from the layer mapping and generates a block of vectors

Tp iyiy ...)(...)( )( , 1,...,1,0apsymb Mi to be mapped onto resources on each of the antenna

ports, where )()( iy p represents the signal for antenna port .

Precoding for Single Antenna Port

For transmission on a single antenna port, precoding is defined by )()( )0()( ixiy p where

5,4,0p is the number of the single antenna port used for transmission of the physical

channel and 1,...,1,0apsymb Mi , layer

symbapsymb MM .

Precoding for Transmit Diversity

The precoding operation for transmit diversity is defined for two and four antenna ports. For

transmission on two antenna ports, 1,0p , the output Tiyiyiy )()()( )1()0( ,

1,...,1,0apsymb Mi of the precoding operation is defined by:

)(Im

)(Im

)(Re

)(Re

001

010

010

001

2

1

)12(

)12(

)2(

)2(

)1(

)0(

)1(

)0(

)1(

)0(

)1(

)0(

ix

ix

ix

ix

j

j

j

j

iy

iy

iy

iy

for 1,...,1,0layersymb Mi with layer

symbapsymb 2MM .

It is worth noting that any two columns of the coding matrix are orthogonal. In addition, the

precoding has facilitated space-frequency transmit diversity, i.e. coding in frequency domain.

The precoding for four antenna ports is similar, however typically layer

symb

ap

symb 4MM .

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2-25

Precoding for Spatial Multiplexing

Spatial multiplexing supports two or four antenna ports and the set of antenna ports used is

1,0p or 3,2,1,0p , respectively.

Without Cyclic Delay Diversity), precoding for spatial multiplexing is defined by:

)(

)(

)(

)(

)(

)1(

)0(

)1(

)0(

ix

ix

iW

iy

iy

P

where the precoding matrix )(iW is of size P and 1,...,1,0apsymb Mi , layer

symbapsymb MM .

Note that the values of )(iW are selected among the precoder elements in the codebook

configured in the eNodeB and the UE.

For large-delay CDD, precoding for spatial multiplexing is defined by

)(

)(

)()(

)(

)(

)1(

)0(

)1(

)0(

ix

ix

UiDiW

iy

iy

P

where the precoding matrix )(iW is of size P and 1,...,1,0apsymb Mi , layer

symbapsymb MM .

Compared to none CDD precoding, )(iD provides the CDD (Cyclic Delay Diversity)

diagonal matrix, whereas U uses a square matrix.

Spatial Multiplexing Codebook for Precoding

The size of the codebook varies for two and four antenna transmissions. The two antenna

ports, 1,0p , the precoding matrix )(iW is selected from Table 6.3.4.2.3-1 or a subset

thereof. For the closed-loop spatial multiplexing transmission mode, the codebook index 0 is

not used when the number layers is 2 .

Table 2-7 Codebook for Transmission for Two Antenna Ports

Codebook Index Number of layers

1 2

0

1

1

2

1

10

01

2

1

1

1

1

2

1

11

11

2

1

2

j

1

2

1

jj

11

2

1

3

j

1

2

1

-

Note that for transmission on four antenna ports there are 16 codebook indexes to choose

from.


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2.5.6 Resource Element Mapper

Following on from the precoding stage the resource element mapper maps the

complex-valued symbols to the allocated resources.

For each of the antenna ports used for transmission of the Physical Channel, the block of

complex-valued symbols is mapped in sequence to resource elements (see Section 2.6.2 .)

which meet all of the following criteria:

They are in the PRB (Physical Resource Blocks) assigned for transmission.

They are not used for transmission of PBCH, synchronization signals or reference

signals.

They are not in an OFDM symbol used for PDCCH.

Additional information on the physical resources is provided in Section 2.6.2 .

2.5.7 OFDM Signal Generation

The final Physical Layer processing stage is the actual OFDM signal generation, i.e. the

generation of time-domain signals for each antenna. This is a purely mathematical process

with various equations and parameters being used. Figure 2-30 illustrates the downlink

equation; however the detail is not discussed as part of this course.

Figure 2-30 Example of the Downlink Signal Generation Equation

TslCPNtfkj

RBscNDL

RBN

k

TslCPNtfkjp

lkRBscNDL

RBNk

eaeas p

lkt

p

l,2

2/

1

,2

,

1

2/,

2.6 Downlink OFDMA

2.6.1 General OFDMA Structure

The E-UTRA downlink is based on OFDMA. As such, it enables multiple devices to receive

information at the same time but on different parts of the radio channel. In most OFDMA

systems this is referred to as a “Subchannel”, i.e. a collection of subcarriers. However, in

E-UTRA, the term subchannel is replaced with the term PRB (Physical Resource Block).

Figure 2-31 illustrates the concept of OFDMA, whereby different users are allocated one or

more resource blocks in the time and frequency domain, thus enabling efficient scheduling of

the available resources.

LTE Air Interface




2-27

Figure 2-31 OFDMA in LTE

Frequency

Channel

Bandwidth

E.g. 3MHz

Time

Device is allocated one

or more PRB (Physical

Resource Blocks)

PRB consists of 12

subcarriers for 0.5ms

OFDMA

It is also worth noting that a device is typically allocated 1ms of time, i.e. a subframe, and not

an individual PRB.

2.6.2 Physical Resource Blocks and Resource Elements

A PRB (Physical Resource Block) consists of 12 consecutive subcarriers and lasts for one slot,

i.e. 0.5ms. Figure 2-32 illustrates the size of a PRB.

The NRBDL

parameter is used to define the number of RB (Resource Blocks) used in the DL

(Downlink). This is dependent on the channel bandwidth. In contrast, NRBUL

is used to

identify the number of resource blocks in the uplink. Each RB (Resource Block) consists of

NSCRB

subcarriers, which for standard operation is set to 12. In addition, another configuration

is available when using MBSFN and a 7.5kHz subcarrier spacing.

The PRB is used to identify an allocation. It typically includes 6 or 7 symbols, depending on

whether an extended or normal cyclic prefix is configured.

The term RE (Resource Element) is used to describe one subcarrier lasting one symbol. This

can then be assigned to carry modulated information, reference information or nothing.


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Issue 01 (2010-05-01)

Figure 2-32 Physical Resource Block and Resource Element

Radio Frame = 10ms

0 2 3 4 5 7 8 9

Slot 8 Slot 9

NRBDL

NSC

RB S

ub

carr

iers

= 1

2

61

Physical Resource

Block

Resource

Element

Subframe

NSymbDL

The different configurations for the downlink E-UTRA PRB are illustrated in Table 2-8.

Table 2-8 Downlink PRB Parameters

Configuration NSCRB NSymb

DL

Normal Cyclic Prefix ∆f = 15kHz 12

7

Extended Cyclic

Prefix

∆f = 15kHz 6

∆f = 7.5kHz 24 3

The uplink PRB configuration is similar; however the 7.5kHz option is not available.

2.7 LTE Physical Signals In order to acquire the system, the eNB must broadcast various downlink signals. In addition,

since the downlink is scalable from 1.4MHz to 20MHz and the device may not be aware of

the eNB configuration, the method of finding the system needs to be consistent. Consequently,

synchronization and cell identity information must appear on the downlink in a fixed place

irrespective of the radio spectrum configuration. Figure 2-33 illustrates the structure of the

NIDcell

(Physical Cell Identity).

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2-29

Figure 2-33 Downlink Cell ID

cell (1) (2)

(1)

(2)

Downlink Synchronization Signals

eNB

UEWhere:

NID = 3NID + NID

NID = 0,…..167

NID = 0, 1, or 2

In LTE there are two synchronization sequences, known as the PSS (Primary Synchronization

Signal) and the SSS (Secondary Synchronization Signal). The location of these is dependent

on the transmission mode, i.e. FDD or TDD, as well as the use of the normal or extended

cyclic prefix.

Figure 2-34 PSS and SSS Location for FDD

Radio Frame

Slots

0 1 2 3 4 5 6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Bandwidth

72

Subcarriers

PSS (Primary

Synchronization

Sequence)

Repeated in

slots 0 and 10

SSS (Secondary

Synchronization

Sequence)

0 1 2 3 4 5

Bandwidth

Normal CP

Extended CP

62

Subcarriers


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Issue 01 (2010-05-01)

Figure 2-35 PSS and SSS Location for TDD

Radio Frame

Slots

0 1 2 3 4 5 6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Bandwidth

0 1 2 3 4 5

Bandwidth

Normal CP

Extended CP

0 1 2 3 4 5 6

0 1 2 3 4 5

Primary Synchronization Signal

The PSS provides downlink synchronization information for the device. The signal sent is one

of three ZC (Zadoff-Chu) sequences. This provides a pseudo noise pattern which devices can

correlate, whilst at the same time enabling adjacent cells/sectors on the eNB to utilize

different synchronization signals. The NID(2)

(0,1 or 2) is mapped to a Zadoff-Chu root index

which is used in the sequence generation process.

Secondary Synchronization Signal

The SSS is generated from the interleaved concatenation of two length-31 binary sequences

which are cyclic shifted based on the value of N ID(1)

. Table 2-9 illustrates the indices

generated from NID(1)

. It is worth noting that additional algorithms are used, as well as a

different combination for subframe 0 and subframe 10.

Table 2-9 Example of SSS Indices

N 1

ID m0 m1 N 1

ID m0 m1 N 1

ID m0 m1 N 1

ID m0 m1 N 1

ID m0 m1

0 0 1 34 4 6 68 9 12 102 15 19 136 22 27

1 1 2 35 5 7 69 10 13 103 16 20 137 23 28

2 2 3 36 6 8 70 11 14 104 17 21 138 24 29

3 3 4 37 7 9 71 12 15 105 18 22 139 25 30

. . . . .

. . . . 167 2 9

33 3 5 67 8 11 101 14 18 135 21 26

LTE Air Interface




2-31

The secondary synchronization sequence is an interleaving of two length-31 sequences s0(m0)

and s1(m1)

scrambled with sequences c0 and c1, which are based on NID(2)

, as well as scrambled

with a z sequence. Figure 2-36 illustrates the concept mapping the sequences to the 62

subcarriers in subframes 0 and 5.

Figure 2-36 SSS Scrambling

s0 s1

c0 c1 and z1

Length 31 Sequence

Cyclic Shift based on

NID(1)

s0s1

c1 and z1c0

(m0) (m1)

(m0)

(m1) (m0)

(m1)

Scrambling sequence c0

and c1 based on NID(2)

Scrambling

sequence z

Subframe 0 Subframe 5

62 interleaved

bits

The concatenated sequence is scrambled with a scrambling sequence given by the primary

synchronization signal.

2.8 Downlink Reference Signals

Unlike other systems, the LTE air interface does not employ a frame preamble. Instead it

utilizes various RS (Reference Signals) to facilitate coherent demodulation, channel

estimation, channel quality measurements and timing synchronization etc. Fundamentally

there are three types of downlink reference signals:

Cell Specific (non-MBSFN).

MBSFN (MBMS over Single Frequency Network).

UE Specific.

2.8.1 Cell Specific Reference Signals

In LTE, the cell specific reference signals are arranged in a two dimensional lattice of time

and frequency. This has been done so that they are equidistant and therefore provides a

minimum mean squared error estimate for the channel. In addition, the spacing in time

between the Reference Symbols is an important factor for channel estimation and relates to

the maximum Doppler spread supported, i.e. speed. In LTE, this works out at 2 Reference

symbols per slot.

The spacing in the frequency domain is also an important factor, as this relates to the expected

coherent bandwidth and delay spread of the channel. In LTE there is a 6 subcarrier separation

of reference signals, however these are staggered in time such that they appear every 3

subcarriers.


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Issue 01 (2010-05-01)

One Antenna Port Configuration

The location of the RSs is dependent on the number of antennas and use of a Normal CP or

Extended CP. Figure 2-37 illustrates the two options.

Figure 2-37 Reference Signals - One Antenna Port

R

R

R

R

R

R

R

R

Antenna Port 0

R

R

R

R

R

R

Antenna Port 0

R

R

Normal CP Extended CP

This is used for a single TX (Transmit) antenna. The reference signals are transmitted during

the first and fifth OFDM symbols of each slot when the normal CP is used and during the first

and fourth OFDM symbols when the extended CP is used.

Cell ID Offset

It is worth noting that the position of the reference signals is dependent on the value of the

Physical Cell ID. As such, the system performs a calculation (Physical Cell ID mod 6) to

determine the correct offset. Figure 2-38 illustrates two cells, each producing a different

offset.

Figure 2-38 Reference Signal Physical Cell ID Offset

R

R

R

R

R

R

R

R

Physical Cell ID = 0R

R

R

R

R

R

R

R

Physical Cell ID = 8RS position is

based on Physical

Cell ID (Physical

Cell ID mod 6)eNB eNB

Two Antenna Port Configuration

LTE is designed to operate with multiple transmit antennas for MIMO, or Transmit Diversity.

The concept of reference signals is used to define different patterns for multiple antenna ports.

Figure 2-39 illustrates the concept for two antennas. The RS pattern corresponding to a given

antenna port enables the device to derive channel estimation.

Figure 2-39 Reference Signals - Two Antenna Ports (Normal CP)

x R

R x

x R

R x

x R

R x

x R

R x

R x

x R

R x

x R

R x

x R

R x

x R

R RS symbol for antenna port 0


Antenna Port 0 Antenna Port 1

LTE Air Interface




2-33

Whilst Reference Symbols are transmitted on one antenna, the other antennas resource element is null. In addition, like the single antenna port configuration the location of the reference signals is offset based

on the Physical Cell ID.

Four Antenna Port Configuration

LTE supports up to four cell-specific antenna ports (0 to 3). As such, the device is required to

derive up to four separate channel estimates. Figure 2-40 illustrates the configuration for four

antenna ports.

Figure 2-40 Reference Signals - Four Antenna Ports (Normal CP)

x R

R x

x R

R x

x R

R x

x R

R x

R x

x R

R x

x R

R x

x R

R x

x R

x

R

x

R

R

x

R

x

x

R

x

R

Antenna Port 0 Antenna Port 1 Antenna Port 2 Antenna Port 3

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

xR

R

x

x

x

x

x

x

x

x

x

x

x

x

x

x





Antenna port “2” and antenna port “3” both have a reduced number of reference symbols.

This is to reduce the reference signal overhead. It does also have a negative impact on the

system since the lack of reference signals will mean that in high mobility, i.e. fast channel

variations, the channel estimation will not be as accurate. This however can be offset by the

fact that spatial multiplexing MIMO with 4 antennas will mostly be performed in low

mobility scenarios. In addition, like the single antenna port configuration the location of the

reference signals is offset based on the Physical Cell ID.

2.8.2 MBSFN Reference Signals

The LTE system also defines a set of reference signal for MBSFN. This is referred to as

“antenna port 4”. Figure 2-41 illustrates the two MBSFN reference signal configurations, one

for 15kHz and one for 7.5kHz.


LTE Air Interface

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Issue 01 (2010-05-01)

Figure 2-41 MBSFN Reference Signals

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

Extended CP

15kHz

Slot

Subframe

R R

R

R R

R

R R

R

Extended CP

7.5kHz

Subframe

R R

R

R R

R

R R

R

R

R

2.8.3 UE Specific Reference Signals

UE specific reference signals are supported for single antenna port transmission on the

PDSCH and are transmitted on antenna port 5. It is typically used for beamforming when

non-codebook based precoding is applied.

Figure 2-42 UE Specific Reference Signals

R

R

R

R

R

R

R

R

R

R

R

R

Antenna Port 5

R

R

R

R

R

R

Antenna Port 5

R

R

R

R

R

R

Normal CP Extended CP

Since the device has no information on the beamforming attributes applied by the eNB it

needs to estimate these as part of the channel estimation process.

2.9 Downlink LTE Physical Channels

In Release 8 there are five downlink Physical Channels.

2.9.1 PBCH (Physical Broadcast Channel)

Along with synchronization information the eNB also schedules a MIB (Master Information

Block) over the logical BCCH (Broadcast Control Channel). This is mapped into the transport

BCH (Broadcast Channel) and ultimately into the PBCH (Physical Broadcast Channel).

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Figure 2-43 Broadcast Signaling

BCCH (Broadcast Information)

eNB

UE

The coded BCH TB (Transport Block) is mapped into four subframes within a 40ms interval.

This 40ms timing is blindly detected by the UE and the information within the subframe is

assumed to be self decodable. This means that it is not dependent on information in

subsequent transmissions of Transport Blocks on the PBCH. The PBCH is located in 4

symbols of slot 1 only (symbols 0, 1, 2 and 3).

Figure 2-44 MIB to PBCH Mapping (FDD and Normal CP)

MIB

Sys

tem

Band

wid

th

CRC

Channel Coding

Rate Matching

Scrambling

Modulation

Layer Mapping

Precoding

Mapping to REs

10ms Frame

PBCH

Only the MIB is carried in the PBCH, other SIB (System Information Blocks) are sent using the

PDSCH.

Section 2.11.4 discusses the LTE SI (System Information) messages and scheduling options.

2.9.2 PCFICH (Physical Control Format Indicator Channel)

The PCFICH (Physical Control Format Indicator Channel) is used to inform the UE about the

number of OFDM symbols used for the PDCCH in a subframe. This channel consists of

32bits which are cell-specific and scrambled prior to modulation and mapping.


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Table 2-10 CFI Mapping

CFI Value Number of OFDM Symbols Assigned to PDCCH

N DL

RB 10 N DL

RB 10

1 1 2

2 2 3

3 3 4

The control area within a PRB is grouped into multiple REG (Resource Element Group), with

one REG containing four Resource Elements. It is worth noting that the REG does not use

Resource Elements assigned to Reference Signals.

Figure 2-45 CFI to PCFICH Mapping

NRBDL

CFI OFDM Symbols

allocated to

PDCCHChannel Coding

(Block1/16)

Scrambling

Modulation

Layer Mapping

Precoding

Mapping to REs

k

PCFICH

Reserved RSsk = (Nsc /2)∙(NID mod 2NRB)

k = k

k = k + NRB)/2 ∙ Nsc /2

k = k + 2NRB)/2 ∙ Nsc /2

k = k + 3NRB)/2 ∙ Nsc /2

RB DL

DL RB

DL

DL

RB

RB

Cell

The PCFICH requires four REGs, i.e. 16 Resource Elements, which are distributed over the

channel bandwidth. The location of these varies depending on the system bandwidth (NSCRB

)

and the NIDcell

. Figure 2-45 illustrates the processes involved in mapping the CFI (Control

Format Indicator) to the correct REGs. In addition, the calculations required are also

illustrated. Table 2-11 illustrates the CFI codewords which are mapped to the PCFICH. These

can change every subframe, i.e. 1ms.

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Table 2-11 CFI Codewords

CFI CFI Codeword < b0, b1, …, b31 >

1 <0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1>

2 <1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0>

3 <1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1,0,1,1>

4 (Reserved) <0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0>

Since there are 2bits, i.e. four combinations, coded to 32bits the result is 1/16 Block Coding.

2.9.3 PDCCH (Physical Downlink Control Channel)

The PDCCH control area size is defined by the PCFICH, i.e. 1, 2 or 3 OFDM symbols. The

PDCCH carries scheduling assignments and other control information. Figure 2-46 illustrates

the downlink control region. In addition, it shows how the size of the region can vary per

subframe.

Figure 2-46 FDD Downlink Control Region

0 1 2 3 4 5 6 7 8 9

Frame - 10ms

5M

Hz

(25

Reso

urc

e B

lock

s)

Downlink

Control

Region

In TDD the control regions are only available on the downlink subframes and the DwPTS.

The PDCCH is transmitted on an aggregation of one or several consecutive CCE (Control

Channel Element), where a CCE corresponds to nine REGs. The number of REGs not

assigned to PCFICH or PHICH (Physical Hybrid ARQ Indicator Channel) is NREG. The CCEs

available in the system are numbered from 0 and NCCE -1, where NCCE = NREG / 9. The

PDCCH supports multiple formats, these include:

PDCCH Format 0 - This consist of one CCE.

PDCCH Format 1 - This consist of two CCE.

PDCCH Format 2 - This consist of four CCE.


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PDCCH Format 3 - This consist of eight CCE.

Figure 2-47 illustrates the PDCCH mapping process.

Figure 2-47 REG to CCE and PDCCH Mapping

PDCCH

REG

CCE (9 x REG)

PDCCH PDCCH

Resource Element

1, 2, 4 or

8 CCE’s

PDCCH Mapping

Figure 2-48 illustrates the concept of mapping the PDCCH to REGs. It assumes that the

PCFICH indicated 2 symbols, as well as two antennas and one PHICH. The numbers in the

control region relate to the grouping of REs into a REG.

Figure 2-48 PDCCH to Control Region Mapping

0

0

x 0 R

0

1

R 1 x

1

1

x 2 R

2

2

R 2 x

x R

R x

x R

R x

3 4

3 4

x 4 R

3 4

5

R 5 x

6 5

6 5

x 7 R

6 7

6 7

R 7 x

x R

R x

x R

R x

3

PDCCH #0 PDCCH #N

REG

Interleaving and Cyclic

Shift based on NID

REG

cell

RB

RB

PCFICH

PHICH

Each control channel carries downlink or uplink scheduling information for one MAC identity,

namely a C-RNTI (Cell - Radio Network Temporary Identifier). This is implicitly encoded in

the CRC.

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There are various rules governing when a PDCCH can start in a subframe. Effectively there is

a tree based method to the aggregation of CCE, these include:

1 - CCE - these start on any CCE position (0, 1, 2, 3, 4, ...).

2 - CCE - these start every second location (0, 2, 4, 6, ...).

4 - CCE - these start on every fourth (0, 4, 8, ...).

8 - CCE - these start on every eighth position (0, 8, ...).

Figure 2-49 illustrates how CCEs could be mapped.

Figure 2-49 CCE Allocation Levels

1 CCE Level

2 CCE Level

4 CCE Level

8 CCE Level

Utilization

Search Spaces

The set of PDCCH candidates to monitor are defined in terms of search spaces. The diagram

illustrates the concept of search spaces and the relationship to the CCEs.

Figure 2-50 Common and UE-Specific Search Spaces

0 1 2 3 4 5 976 8

Common Search Space UE-specific Search Space

1 - CCE

2 - CCE

4 - CCE

8 - CCE

CCE

Candidate

Aggregation Set

for Common

Control

Candidate

Aggregation Set

for UE-specific

Control

There are two types of search spaces, namely common and UE specific. The common search

space corresponds to CCEs 0-15 at two levels:

4-CCE - CCEs 0-3, 4-7, 8-11, 12-15.

8-CCE - CCEs 0-7, 8-15.

These are monitored by all UEs in the cell and can be used for any PDCCH signaling. In

addition, a UE must monitor one UE specific search space at each of the aggregation levels 1,

2, 4 and 8. This may overlap with the common control search space. The location of the

UE-specific search space is based on the C-RNTI (Cell - Radio Network Temporary Identity).


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The number of available CCEs in a cell is dependent on a number of attributes including:

Bandwidth.

Number of antenna ports.

PHICH configuration.

PCFICH value (1, 2 or 3).

2.9.4 PHICH (Physical Hybrid ARQ Indicator Channel)

The PHICH carries HARQ (Hybrid ARQ) ACK/NAKs and is transmitted in PHICH groups. A

PHICH group consists of up to eight ACK/NACK processes and requires three REGs for

transmission. Each PHICH within the same PHICH group is separated through different

orthogonal sequences.

There are two PHICH frame formats:

Frame structure type 1 - the number of PHICH groups remains constant.

Frame structure type 2 (TDD) - the number of PHICH groups may vary between

downlink subframes; this is achieved through different configuration formats.

The amount of PHICH resources (Ng) is signaled on the PBCH, as part of the MIB. Figure

2-51 illustrates how the number of PHICH groups is calculated using this parameter.

Figure 2-51 PHICH Mapping

DL

groupNPHICH

Ng (NRB /8)

DLNg (NRB /8)2

For normal CP

For extended CP

Where: Ng = 1/6, ½ , 1 or 2

ACK/

NACK

Repetition 1/3

Modulation

Orthogonal Sequence

Scrambling

Layer Mapping

Precoding

Group 0

PHICH

Mapping

Equation

Up to eight

ACK/NACK per

PHICH Group

PCFICH

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Normal and Extended PHICH

It is worth noting that the different REGs belonging to a PHICH group may be transmitted on

different symbols.

Figure 2-52 Extended PHICH Example

Subframe

5M

Hz

(25 R

eso

urc

e B

lock

s)

Extended

PHICH

Normal

PHICH

2.9.5 PDSCH (Physical Downlink Shared Channel)

The PDSCH is used to send various Transport Channels, such as the PCH and DL-SCH.

Figure 2-53 illustrates PDSCH mapping for one subframe. In this example the PDSCH

symbols are mapped, avoiding the control region and symbols reserved for reference signals.

Figure 2-53 Generic PDSCH Mapping

x

R

x

R

x R

R x

x R

R x

x

R

x

R x

x R

R x

x R

R x

R

x

R

x

R

x

R

PDSCH

Symbol

Mapping

PDSCH

Symbols

Subframe

Reserved for

Control


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2.10 Downlink Control Signaling

The LTE system uses a set of DCI (Downlink Control Information) messages to convey

control and scheduling information to devices. The set of Downlink Control Information

messages is defined LTE Release 8. Note that future releases could include additional formats.

Table 2-12 illustrates the DCI Formats.

Table 2-12 DCI Formats

DCI Format Usage

0 Scheduling of PUSCH

1 Scheduling of one PDSCH codeword

1A Compact scheduling of one PDSCH codeword and random access

procedure initiated by a PDCCH order

1B Compact scheduling of one PDSCH codeword with precoding

information (Rank-1 transmission)

1C Very compact scheduling of one PDSCH codeword

1D Compact scheduling of one PDSCH codeword with precoding and

power offset information (multi-user MIMO)

2 Scheduling PDSCH to UEs configured in closed-loop spatial

multiplexing MIMO

2A Scheduling PDSCH to UEs configured in open-loop spatial

multiplexing MIMO

3 Transmission of TPC (Transmit Power Control) commands for

PUCCH and PUSCH with 2-bit power adjustments

3A Transmission of TPC (Transmit Power Control) commands for

PUCCH and PUSCH with 1-bit power adjustments

DCI formats 0, 1A, 3, and 3A have the same payload size.

The size of the DCI format depends on its function, as well as the system bandwidth. There

are various rules associated with the formatting of the DCI messages. As such, padding is

typically added to ensure the rules are met.

2.10.1 DCI Format 0

This is used when scheduling the PUSCH. The following information is sent:

Flag for format0/format1A differentiation - 1 bit, where value 0 indicates format 0 and

value 1 indicates format 1A.

Hopping flag.

Resource block assignment and hopping resource allocation.

Modulation and coding scheme and redundancy version.

New data indicator.

TPC command for scheduled PUSCH.

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Cyclic shift for DM RS.

UL index - This field is present only for TDD operation with uplink-downlink

configuration 0.

DAI (Downlink Assignment Index) - This field is present only for TDD operation with

uplink-downlink configurations 1-6.

CQI Request.

2.10.2 DCI Format 1

This is used when scheduling one PDSCH codeword. The following information is sent:

Resource allocation header (resource allocation type 0 / type 1).

Resource block assignment.

Modulation and coding scheme.

HARQ process number.

New data indicator.

Redundancy version.

TPC command for PUCCH.

Downlink Assignment Index - This field is present in TDD.

It is important that the size of a DCI format 1 message does not match other DCI messages. If

the number of information bits in DCI format 1 is equal to that for format 0/1A, one zero is

added. In addition, if the number of information bits in DCI format 1 belongs to one of the

sizes in Table 2-13, one or more zeros can be added.

Table 2-13 DCI Ambiguous Sizes of Information Bits

Ambiguous Sizes of Information Bits

12, 14, 16 ,20, 24, 26, 32, 40, 44, 56

2.10.3 DCI Format 1A

This is used for compact scheduling of one PDSCH codeword and random access procedure

initiated by a PDCCH order.

When used for the random access procedure initiated by a PDCCH order the CRC is

scrambled with C-RNTI and the following information is sent:



Localized/Distributed VRB assignment flag - This is 1 bit and set to 0.

Resource block assignment - all bits are set to 1.

Preamble Index.

PRACH Mask Index.

All the remaining bits are set to zero.


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Otherwise, when used for compact scheduling of one PDSCH codeword the following

information is sent:



Localized/distributed VRB (Virtual Resource Block) assignment flag.

Resource block assignment (localized VRB /distributed VRB).



New data indicator.

Redundancy version.


Downlink Assignment Index - This is present in TDD and is applicable to TDD

configurations 1-6.

Like format 0, various rules apply to the size of the message, such that zeros may need to be

inserted. In addition, depending on the channel usage, i.e. the CRC is scrambled with random

access, paging or system information RNTIs, certain fields may be reserved.

2.10.4 DCI Format 1B

This is used for compact scheduling of one PDSCH codeword with precoding information

(Rank-1 transmission). The message contains the following information:

Localized/Distributed VRB assignment flag.

Resource block assignment - different for localized and distributed VRB.



New data indicator.

Redundancy version.



configurations 1-6.

TPMI information for precoding - The TPMI (Transmitted Precoding Matrix Indicator)

information indicates which codebook index is used corresponding to the single-layer

transmission.

PMI (Precoding Matrix Indicator) confirmation for precoding - This indicates whether

precoding is based on the indicated TPMI or on the latest PMI report sent on the

PUSCH.

If the number of information bits in format 1B belongs to one of the sizes in Table 2-13, one

zero bit is added.

2.10.5 DCI Format 1C

This is used for very compact scheduling of one PDSCH codeword. The messages include:

Gap value - This indicates if gap,1N or gap,2N is to be utilized.


Transport block size index.

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2.10.6 DCI Format 1D

This is used for compact scheduling of one PDSCH codeword with precoding and power

offset information. The following information is sent:

Localized/Distributed VRB assignment flag.



HARQ process number - the size of this varies depending on FDD or TDD mode.

New data indicator.

Redundancy version.



configurations 1-6.

TPMI information for precoding.

Downlink power offset - This is required for multi-user MIMO scheduling in the

downlink.

If the number of information bits in format 1D belongs to one of the sizes in Table 2-13, one

zero bit is added.

2.10.7 DCI Format 2

This is used for scheduling PDSCH to UEs configured in closed-loop SM (Spatial

Multiplexing). The concept of MIMO and SM is discussed in Section 2.22 .

The following information is sent as part of DCI format 2:

Resource allocation header - This indicates resource allocation type 0 or type 1.

Resource block assignment - This is for type 0 or 1 information.



configurations 1-6.

HARQ process number - the size of this varies depending on FDD or TDD mode.

Transport block to codeword swap flag - This determines the transport block to

codeword mapping. However, if one of the transport blocks is disabled the mapping is

different.

For the first Transport Block:

− Modulation and coding scheme.

− New data indicator.

− Redundancy version.

For the second Transport Block:




Precoding information - This is either 3bits or 6bits depending on the number of antenna

ports.


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This is for scheduling PDSCH to UEs configured in open-loop spatial multiplexing MIMO,

i.e. without PMI feedback. The format of DCI format 2A is the same as format 2, except that

the precoding information parameter is not used for 2 antenna ports (0 bits) and carries

transmission rank information (2bits) if 4 antenna ports are used.

Table 2-14 Precoding Information Field for 4 Antenna Ports (Open Loop)

One codeword: Codeword 0 enabled, Codeword 1 disabled

Two codewords: Codeword 0 enabled, Codeword 1 enabled

Bit field

mapped to index

Message Bit field

mapped to index

Message

0 4 layers: Transmit diversity 0 2 layers: precoder cycling with

large delay CDD

1 2 layers: precoder cycling

with large delay CDD

1 3 layers: precoder cycling with

large delay CDD

2 Reserved 2 4 layers: precoder cycling with

large delay CDD

3 Reserved 3 Reserved

2.10.9 DCI Format 3

DCI format 3 is for the transmission of TPC (Transmit Power Control) commands for

PUCCH and PUSCH with 2-bit power adjustments. The following information is transmitted:

TPC command number 1, TPC command number 2,…, TPC command number N,

where:

2

0format LN ,

The parameter 0format L is equal to the payload size of format 0 before CRC attachment.

A power control parameter, namely tpc-Index, is provided by higher layers. This is utilized by

the mobile to determine the index to the TPC command for a given UE. Power control is

discussed in Section 2.19 .


Transmission of TPC (Transmit Power Control) commands for PUCCH and PUSCH with

1-bit power adjustments. The following information is transmitted by means of the DCI

format 3A:

TPC command number 1, TPC command number 2,…, TPC command number M -

where 0format LM , and where 0format L is equal to the payload size of format 0

before CRC attachment.

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2.11 LTE Cell Search Procedure

The LTE device needs to perform an LTE Attach procedure, i.e. transition from the LTE

Detached to LTE Active State, to connect to the EPC (Evolved Packet Core) and ultimately

services.

Figure 2-54 Initial Procedures

Power On Cell SearchPLMN/Cell

Selection

RACH

Process

Downlink Synchronization

Complete

Uplink Synchronization

Complete

In order to access a cell the device must find and synchronize to the cell. It is then able to

decode the System Information messages and perform PLMN (Public Land Mobile Network)

and Cell Selection. Once this has been completed, the device is in a position to access the cell

and establish a RRC connection, i.e. a SRB (Signaling Radio Bearer).

2.11.1 Cell Search

The downlink in LTE is based on scalable OFDMA with channels ranging from 1.4MHz to

20MHz (Note that not all bandwidths are available at the different frequency bands). Initially

the UE is unaware of the downlink configuration of the cell, unless it has stored information

from when it was previously attached. Assuming no information, the synchronization process

must be quick and concise. Figure 2-55 illustrates the location of the PSS and SSS.

Figure 2-55 PSS and SSS for Cell Search (FDD Mode)

0 1 2 3 4 5 6 7 8 9

Frame - 10ms

5MHz (25

Resource

Blocks)

PSS

SSS

PBCH


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In order for the UE to identify the cell and synchronize with the downlink transmission, the

eNB sends synchronization signals over the centre 72 sub-carriers. For FDD mode (using a

normal CP) this is in the first and sixth subframes of each downlink frame.

These synchronization signals comprise of the PSS (Primary Synchronization Signal) and

SSS (Secondary Synchronization Signal). Together they enable the UE to become downlink

synchronized and identify the Physical Cell Identity. There are 504 unique physical cell

identities, divided into 168 cell identity groups each containing three cell identities (sectors).

Figure 2-56 Physical Cell Identities

eNB

eNB

eNB

PSS - One of 3 Identities

SSS - One of 168

Group Identities

504 Unique Cell

Identities

The Physical Cell ID is able to be reused based on the cell and frequency reuse mechanism employed.

2.11.2 PSS Correlation

The device cross correlates 3 possible PSSs with the received signal. Figure 2-57 illustrates

the cross correlation results. In this example PSS1 is found.

Figure 2-57 PSS Correlation

Subframe

Correlation

PSS0

PSS1

PSS2

At this stage the cell identity within the group is known. In addition, the location of the SSS is

also known because it occupies the previous OFDM symbol (FDD mode). However, at this

stage the frame synchronization is not known since subframe 0 and 5 both utilize the same

PSS sequence.

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2.11.3 SSS Correlation

As previously discussed in Section 2.7 the sequence used for the second synchronization

signal is an interleaved concatenation of two length-31 binary sequences. The concatenated

sequence is scrambled with a scrambling sequence given by the primary synchronization

signal.

The combination of two length-31 sequences defining the secondary synchronization signal

differs between subframe 0 and subframe 5 according to:

5 subframein )(

0 subframein )()12(

5 subframein )(

0 subframein )()2(

)(11

)(0

)(11

)(1

0)(

1

0)(

0

10

01

1

0

nzncns

nzncnsnd

ncns

ncnsnd

mm

mm

m

m

where 300 n .

The indices 0m and 1m are derived from the Physical Layer cell identity group (1)IDN and

are shown in Table 2-9.

The references to the m-sequences include:

The two sequences )()(

00 ns

m and )(

)(1

1 nsm

are defined as two different cyclic shifts of

the m-sequence )(~ ns .

The two scrambling sequences )(0 nc and )(1 nc depend on the primary

synchronization signal and are defined by two different cyclic shifts of the m-sequence

)(~ nc .

The scrambling sequences )()(

10 nz

m and )(

)(

11 nz

m are defined by a cyclic shift of the

m-sequence )(~ nz .

Figure 2-58 illustrates the correlation of the SSS. Note that the device is

monitoring/processing a number of different SSS possibilities, i.e. more than the two shown.

Figure 2-58 SSS Correlation Example

Subframe

SSS

SSS

Cyclic Shift based

on Cell ID and

Subframe (0 or 5)

Device can

identify Cell ID

and frame timing


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2.11.4 Master Information Block

Once the device has decoded the PSS and SSS it is able to:

Decode cell specific Reference Signals (since their location is based on the Physical Cell

ID).

Perform channel estimation procedures.

Decode the PBCH which carries the MIB (Master Information Block).

The MIB repeats every 40ms and uses a 40ms TTI (Time Transmission Interval), i.e. the

message is interleaved over 4 frames. The MIB transmission is aligned to the SFN (System

Frame Number) such that it starts when SFN mod 4 = 0.

Figure 2-59 PBCH and the Master Information Block

0 1 2 3 4 5 6 7 8 9

NRB

Frame

MIB (Master Information Block)

DL-Bandwidth (6, 15, 25, 50, 75, 100)

PHICH Configuration (Ng and Normal/Extended)

System Frame Number

The MIB is always transmitted in subframe 0. The MIB carries three very important bits of

information. It indicates the downlink bandwidth, i.e. 6, 15, 25, 50, 75 or 100 Resource

Blocks. This enables the device to know where it should be looking (subcarriers) for the

downlink control information. In addition, the PHICH configuration parameter is included.

This indicates that Ng is equal to 1/6, 1/2, 1 or 2 and whether “Normal” or “Extended”

PHICH mode is being used. These are used by the device to determine the number of PHICH

groups configured on the cell and their location. Finally, the SFN is also included.

In addition, the PBCH is layer mapped and precoded. As such, the PBCH can employ transmit

diversity over multiple antennas ports.

Based on the MIB the UE is able to decode the PCFICH. This identifies the number of OFDM

symbols assigned to the downlink control region in the subframe.

2.11.5 System Information Messages

Limited system information is sent on the MIB. As such additional SIB (System Information

Block) messages are required. SIBs, other than SIB 1 (System Information Block Type1), are

carried in System Information messages which are then transmitted on the DL-SCH

(Downlink - Shared Channel) based on various system parameters. SIB 1 is slightly different

in that it has predefined rules on how it may be sent.

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System Information Block Type 1

System Information Block Type 1 contains key information about the cell and network. In

addition, it defines the scheduling window for the other System Information messages. SIB1

is transmitted on subframe 5 when SFN mod 8=0. It is also repeated in subframe 5 when SFN

mod 2=0. This is illustrated in Figure 2-60.

Figure 2-60 System Information Block Type 1

0 1 2 3 4 5 6 7 8 9

NRB

Frame

SIB1 (System Information Block Type 1)

PLMN Identity List

Tracking Area Code

E-CGI (Evolved Cell Global Identity)

Cell Barred Indication

Intra Frequency Reselection

CSG Indication

CSG Identity

Qrxlevminoffset

P-Max

Frequency Band Indicator

Scheduling Info List

SIB Window Length (1, 2, 5, 10, 15, 20, 40ms)

System Info Value Tag

Repetitions are scheduled in subframe #5 of all

other radio frames for which SFN mod 2 = 0

The main information in SIB1 includes:

PLMN Identity List - This is a list of PLMN identities. The first listed PLMN-Identity is

the primary PLMN.

Tracking Area Code - This is a TAC (Tracking Area Code) that is common for all the

PLMNs listed.

E-CGI - This is a 28bit cell identifier.

Cell Barred Indication.

Intra Frequency Reselection - This is used to control cell reselection to intra-frequency

cells when the highest ranked cell is barred, or treated as barred by the UE.

CSG Indication - if set to “TRUE”, the UE CSG (Closed Subscriber Group) identity

needs to match.

CSG Identity - This is the identity of the Closed Subscriber Group within the primary

PLMN the cell belongs to.

Qrxlevminoffset - This affects the minimum required Rx level in the cell.

P-Max - This is part of the cell selection process.


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Frequency Band Indicator.

SI Periodicity Mapping Information - This denotes a value in radio frames: rf8, rf16, rf32,

rf64, rf128, rf256, rf512 and is used to calculate the occurrence of messages.

SIB Window Length - This is a common SI scheduling window for all SIB and indicates

1, 2, 5, 10, 15, 20 or 40ms.

System Info Value Tag - Common for all SIBs other than MIB, SIB1, SIB10 and SIB11.

Acquisition of an SI Message

When acquiring an SI message, the UE performs various calculations to determine the start of

the SI-window for the concerned SI message:

For the concerned SI message, determine the number n which corresponds to the order of

entry in the list of SI messages configured by “schedulingInfoList” in

SystemInformationBlockType1.

Determine the integer value x = (n – 1)*w, where w is the si-WindowLength.

The SI-window starts at the subframe #a, where a = x mod 10, in the radio frame for

which SFN mod T = FLOOR(x/10), where T is the si-Periodicity of the concerned SI

message.

In order to identify the scheduling of SI messages the UE looks for the SI-RNTI (System

Information - Radio Network Temporary Identifier) on the PDCCH.

Figure 2-61 Example of SI Mapping

0 1 2 3 4 5 6 7 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 258


.

SI-Window=5ms

Scheduling Info List

- SI1 {rf8, SIB3, SIB4, SIB5}

- SI2 {rf16, SIB6, SIB7, SIB8, SIB9}

SI1 {rf8, SIB3, SIB4, SIB5}

SI2 {rf16, SIB6, SIB7, SIB8, SIB9}

SFN

E-UTRAN should configure an SI-window of 1 ms only if all SIs are scheduled before subframe #5 in

radio frames for which SFN mod 2 = 0.


System Information Block Type 2 contains radio resource configuration information that is

common for all UEs. This includes detailed information on the access channels and paging

channels.

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Access Class Information

Uplink Carrier Frequency

UL Bandwidth

MBSFN Configuration Information


System Information Block Type 3 contains cell reselection information common for

intra-frequency, inter-frequency and/or inter-RAT cell reselection (i.e. applicable for more

than one type of cell reselection but not necessarily all), as well as intra-frequency cell

reselection information other than that which is neighbor cell related.



Cell Reselection Information

Q-Hyst

Speed State Reselection Parameters

Q-Hyst Speed SF (Scaling Factor)

Treselection EUTRA

Treselection EUTRA SF

S Intra Search

Cell Reselection Serving Freq Info

S-Non-Intra Search Info

Threshold Serving Low Value

Intra Freq Cell Reselection Info

p-Max

Allowed Measurement Bandwidth


System Information Block Type 4 contains neighboring cell related information relevant only

for intra-frequency cell reselection. It includes cells with specific reselection parameters and

blacklisted cells.



Intra Freq Neighbour Cell List

q-OffsetCell

Intra Freq Black Cell List

CSG Physical Cell Id Range


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System Information Block Type 5 contains information relevant only for inter-frequency cell

reselection i.e. information about other E-UTRA frequencies and inter-frequency neighboring

cells relevant for cell reselection. It includes cell reselection parameters common for a

frequency as well as cell specific reselection parameters.



Inter Frequency Carrier Freq List

Inter Frequency Carrier Freq Info

Inter Frequency Neighbour Cell List

Inter Frequency Neighbour Cell Info

Inter Frequency Black Cell List

Inter Frequency Black Cell Info


System Information Block Type 6 contains information relevant only for inter-RAT cell

reselection i.e. information about UTRA frequencies and UTRA neighboring cells relevant for

cell reselection. It includes cell reselection parameters common for a frequency as well as cell

specific reselection parameters.



Carrier Frequency List UTRA

UTRA Reselection Information


The System Information Block Type 7 contains information relevant only for inter-RAT cell

reselection i.e. information about GERAN frequencies relevant for cell reselection. It includes

cell reselection parameters for each frequency.



Carrier Frequency List GERAN

GERAN Reselection Information


The System Information Block Type 8 contains information relevant only for inter-RAT cell

reselection i.e. information about CDMA2000 frequencies and CDMA2000 neighboring cells

relevant for cell reselection. It includes cell reselection parameters common for a frequency as

well as cell specific reselection parameters.

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CDMA2000 Information


The System Information Block Type 9 contains a HeNB (Home eNB) name.



Home eNB Name

System Information Block Type 10 and 11

SIB 10 and SIB 11 are used to send ETWS (Earthquake and Tsunami Warning System)

primary notification and ETWS secondary notification respectfully.

2.11.6 PLMN Selection

The transition from LTE Detached to LTE Active can be used to describe the processes

through which the UE must progress in order to establish a point of attachment within the

Evolved Packet Core and ultimately connect to services. The initial processes including

scanning for downlink and uplink channels and synchronization are passive in that the

information required to achieve this is broadcast from the eNB within the relevant E-UTRAN.

Before the UE can access the network it must first select a suitable PLMN (Public Land

Mobile Network) and then a suitable cell. Services may be available to the user through a

choice of several serving networks in a given location, possibly using different types of Radio

Access Network.

Figure 2-70 PLMN Selection

3G Visited

PLMN

LTE Visited

PLMN

LTE Home

PLMN

eNB

eNB

Node B

PLMN selection

may be initiated

automatically or

manually

eNB may contain

upto 6 PLMN

Identities

UE


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E-UTRA PLMN Selection

In the UE, the Access Stratum reports available PLMNs to the NAS on request from the NAS

or autonomously. During PLMN selection, based on the list of PLMN identities in priority

order, the particular PLMN may be selected either automatically or manually. Each PLMN in

the list of PLMN identities is identified by a 'PLMN identity'. In the system information on

the broadcast channel, the UE can receive one or multiple 'PLMN identity' in a given cell.

The UE scans all RF channels in the E-UTRA bands according to its capabilities to find

available PLMNs. On each carrier, the UE searches for the strongest cell and read its system

information, in order to find out which PLMN(s) the cell belongs to. If the UE can read one or

several PLMN identities in the strongest cell, each found PLMN is reported to the NAS as a

high quality PLMN (but without the RSRP value), provided that the following high quality

criterion is fulfilled:

For an E-UTRAN cell, the measured RSRP value is greater than or equal to -110 dBm.

Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been

able to read the PLMN identities are reported to the NAS together with the RSRP value. The

quality measure reported by the UE to NAS is the same for each PLMN found in one cell.

Note that the UE may optimize the PLMN search by using stored information e.g. carrier

frequencies and optionally also information on cell parameters from previously received

measurement control information elements.

NAS PLMN Selection

The UE utilizes all the information stored in the USIM (Universal Subscriber Identity Module)

related to the PLMN selection; e.g. "HPLMN (Home PLMN) Selector with Access

Technology", "Operator controlled PLMN Selector with Access Technology", "User

Controlled PLMN Selector with Access Technology", "Forbidden PLMNs", "Equivalent

HPLMN". Note that these are the same for UMTS PLMN selection.

The PLMN/access technology combinations are listed in priority order. If no particular access

technology is indicated in an entry, the UE assumes that all access technologies supported by

the UE apply. In addition, the UE stores a list of EHPLMN (Equivalent HPLMN). This list is

replaced or deleted as part of various EMM Procedures. The stored list consists of a list of

equivalent PLMNs as downloaded by the network plus the PLMN code of the registered

PLMN that downloaded the list. All PLMNs in the stored list, in all access technologies

supported by the PLMN, are regarded as equivalent to each other for PLMN selection, cell

selection/re-selection and handover.

The UE selects and attempts registration on other PLMN/access technology combinations, if

available and allowable, in the following order:

Either the HPLMN (if the EHPLMN list is not present or is empty) or the highest priority

EHPLMN that is available (if the EHPLMN list is present).

Each PLMN/access technology combination in the "User Controlled PLMN Selector

with Access Technology" data file in the SIM (in priority order).

Each PLMN/access technology combination in the "Operator Controlled PLMN Selector

with Access Technology" data file in the SIM (in priority order).

Other PLMN/access technology combinations with received high quality signal in

random order.

Other PLMN/access technology combinations in order of decreasing signal quality.

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Once the UE NAS has selected a PLMN, the cell selection procedure can be performed in

order to select a suitable cell of that PLMN to camp on.

2.11.7 Cell Selection

For LTE there are two cell selection procedures supported:

Initial Cell Selection - This is when the UE has no prior knowledge of the cell.

Stored Information Cell Selection - This is when the UE has stored information which is

used to optimize the selection process, i.e. it stored information before it was switched

off.

Once a UE has synchronized with the cell and decoded the necessary System Information

messages, it must camp on it; or one of the surrounding cells. This is achieved through the cell

selection process. The UE is aiming to find the cell which will provide the best quality radio

link between it and the network. Figure 2-71 illustrates the S (Cell Selection) calculation.

Figure 2-71 LTE Cell Selection

Qrxlevmeas

Qrxlevmeas

Qrxlevmeas

UE eNB

eNB

eNB

Srxlev > 0

Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevminoffset) - Pcompensation

Table 2-15 identifies the parameters used as part of the Cell Selection process.

Table 2-15 Cell Selection Parameters

Parameter Description

Srxlev Cell Selection RX level value (dB).

Qrxlevmeas Measured cell RX level value (RSRP),

where RSRP is defined as the linear average

over the power contributions of the resource

elements that carry cell specific reference

signals within the considered measurement

frequency bandwidth.

Qrxlevmin Minimum required RX level in the cell

(dBm).


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Qrxlevminoffset Offset to the signaled Qrxlevmin taken into

account in the Srxlev evaluation as a result

of a periodic search for a higher priority

PLMN while camped normally in a visited

PLMN.

Pcompensation max (PEMAX - PUMAX, 0), where PEMAX is the

maximum allowed power configured by

higher layers.

PUMAX RF output power of the UE (dBm) according

to the UE power class (this may vary

depending on allowed tolerances).

In terms of the radio channel, the UE measures the RSRP (Reference Signal Received Power).

The LTE downlink contains cell specific RS (Reference Signals) which are used for channel

equalization and determining the RSRP (Reference Signal Received Power).

The device calculates the Qrxlevmeas for each cell. It then gathers the related Qrxlevmin and

other parameters from the SI messages (each cell may provide different parameters). Once it

has gathered all the information it is able to calculate Srxlev for each cell.

All cells that return a value of Srxlev greater than zero are considered candidates for selection.

The cell with the most positive value is selected and becomes the camped on cell.

Cell Random Access

Once a UE has selected a cell it performs a random access procedure on the PRACH/RACH.

Section 2.18 details this procedure.

2.12 Uplink Transmission Technique The uplink in LTE, as previously mentioned, is based on SC-FDMA (Single Carrier -

Frequency Division Multiple Access). This was chosen for its low PAPR (Peak to Average

Power Ratio) and flexibility which reduced complexity in the handset and improved power

performance and battery life. SC-FDMA tries to combine the best characteristics of single

carrier systems like low peak-to-average power ratio, with the advantages of multi carrier

OFDM and as such, is well suited to the LTE uplink requirements.

2.12.1 SC-FDMA Signal Generation

The basic transmitter and receiver architecture is very similar (nearly identical) to OFDM, and

it offers the same degree of multipath protection. Importantly, because the underlying

waveform is essentially single carrier, the PAPR is lower. It is quite difficult to visually

represent SC-FDMA in the time and frequency domain. This section aims to illustrate the

concept. Figure 2-72 illustrates the basic structure of the SC-FDMA process.

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Figure 2-72 SC-FDMA Subcarrier Mapping Concept

Subcarrier

MappingDFT IDFT

Symbols

Time Domain Frequency Domain Time Domain

0

0

0

0

0

0

0

CP

Insertion

In Figure 2-72 the SC-FDMA signal generation process starts by creating a time domain

waveform of the data symbols to be transmitted. This is then converted into the frequency

domain, using a DFT (Discrete Fourier Transform). DFT length and sampling rate are chosen

so that the signal is fully represented, as well as being spaced 15kHz apart. Each bin

(subcarrier) will have its own fixed amplitude and phase for the duration of the SC-FDMA

symbol. Next the signal is shifted to the desired place in the channel bandwidth using the zero

insertion concept, i.e. subcarrier mapping. Finally, the signal is converted to a single carrier

waveform using an IDFT (Inverse Discrete Fourier Transform) and other functions. Finally a

cyclic prefix can be added. Note that additional functions such as S-P (Serial to Parallel) and

P-S (Parallel to Serial) converters are also required as part of a detailed functional description.

Figure 2-73 illustrates the concept of the DFT, such that a group of N symbols map to N

subcarriers. However depending on the combination of N symbols into the DFT the output

will vary. As such, the actual amplitude and phase of the N subcarriers is like a “code word”.

For example the first combination represents the first set of symbols. Since the second set of

symbols is different the amplitude and phase of the N subcarriers would then be different.


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Figure 2-73 SC-FDMA Signal Generation

DFT

N symbols sequence

produces N subcarriers

Different input sequence

produces different output

First N Symbols

DFT Output

Modulated and

Coded Symbols

DFT

Second N Symbols

The process at the eNB receiver takes the N subcarriers and reverses the process. This is

achieved using an IDFT (Inverse Discrete Fourier Transform) which effectively reproduces

the original N symbols.

Figure 2-74 illustrates the basic view of how the subcarriers received at the eNB are converted

back into the original signals.

Note that the SC-FDMA symbols have a constant amplitude and phase and like ODFMA, a

CP (Cyclic Prefix) is still required.

Figure 2-74 SC-FDMA and the eNB

N Subcarriers

Time

Power

IDFT

IDFT

Cyclic

Prefix First N Symbols

Second N Symbols

SC-FDMA Signal Generation Equation

The previous diagrams go some way to visualizing the concept of SC-FDMA. However the

true time-continuous signal tsl in SC-FDMA symbol l in an uplink slot is defined by the

equation in Figure 2-75.

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Figure 2-75 Example of the Uplink Signal Generation Equation

12/

2/

212

,

RBsc

ULRB

RBsc

ULRB

s,CP

)(

NN

NNk

TNtfkj

lklleats

for s,CP0 TNNt l where 2)( RBsc

ULRB NNkk , 2048N , kHz 15f and lka ,

is the content of resource element lk, .

The SC-FDMA symbols in a slot are transmitted in increasing order of l , starting with 0l ,

where SC-FDMA symbol 0l starts at time

1

0s,CP )(

l

ll TNN within the slot.

2.13 OFDMA Verses SC-FDMA The main reason SC-FDMA was specified for the uplink was because of its PA (Power

Amplifier) characteristics. Typically, the SC-FDMA signal will operate with a 2-3dB lower

PAPR (Peak-to-Average Power Ratio). This makes the system more efficient, thus increasing

the battery life for mobile users. SC-FDMA is also better when it comes to larger cell

coverage.

It must be noted that OFDMA is better in a number of areas, such as Inter-symbol

orthogonality and the ability to provide a more flexible frequency domain scheduling

mechanism. This increases the system performance. In addition, OFDMA is more suitable for

uplink MIMO operation and associated high date rate services.

Table 2-16 highlights three main features and indicates which technology is best suited.

Table 2-16 SC-FDMA verses OFDMA

Feature SC-FDMA OFDMA

Low PAPR Y X

Performance X Y

Uplink MIMO X Y

2.14 Uplink LTE Physical Channels

There are a number of Uplink Physical Channels in LTE. These include:

PRACH (Physical Random Access Channel) - This channel carries the Random Access

Preamble. The location of the PRACH is defined by higher layer signaling.

PUCCH (Physical Uplink Control Channel) - This channel carries UCI (Uplink Control

Information) such as ACK/NAKs in response to downlink transmission, as well as CQI

(Channel Quality Indicator) reports. It also carries scheduling request indicators and

MIMO codeword feedback.


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PUSCH (Physical Uplink Shared Channel) - This is the main uplink channel and is used

to carry the UL-SCH (Uplink Shared Channel) Transport Channel. It carries both

signaling and user data, in addition to UCI.

Figure 2-76 Release 8 Uplink Physical Channels

PRACH PUSCH PUCCH

2.14.1 PRACH (Physical Random Access Channel)

The random access procedure is used in various scenarios, including initial access, handover,

or re-establishment. Like other 3GPP systems the random access procedure provides a method

for contention and non-contention based access. The PRACH (Physical Random Access

Channel) includes RA (Random Access) preambles generated from ZC (Zadoff-Chu)

sequences. Figure 2-77 illustrates the basic structure of the PRACH preamble. This is

effectively an OFDM symbol.

Figure 2-77 PRACH Preamble

SequenceCP

TCP TSEQ Guard Period

Preamble

The Guard Period is required since the eNB does not know when the preambles will arrive.

Figure 2-78 illustrates an example with two UEs. The first is next to the eNB therefore there

is very little delay. In contrast UE “B” is some distance from the eNB, as such the initial

access preamble is delayed, i.e. there is a round trip delay. The eNB must allocate a large

enough window such that the preambles from UE at the edge of the cell don’t arrive outside

of this window.

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Figure 2-78 PRACH Guard Period

SequenceCP

eNB

UE - A

SequenceCP

UE - B

UE - A

UE - B

eNB Access Window UE “B” delay

due to distance

PRACH Frame Formats

As well as the position of the PRACH, four PRACH frame formats for FDD are also defined.

These contain a CP (Cyclic Prefix) and Zadoff Chu sequence. The formats are designed to

enable efficient operation in different scenarios. For example, the varying length of CP can be

employed to counter either small or large delay spread effects due to the mobile’s position.

Table 2-17 illustrates the different PRACH formats.

Table 2-17 Random Access Preamble Parameters

Preamble Format

Allocated Subframes

TSEQ

(Ts)

TCP

(Ts)

TCP (µs) TGT (Ts)

TGT (µs)

Max. Delay Spread

(µs)

Max Cell Radius

(km)

0 1 24576 3168 103.125 2976 96.875 5.208 14.531

1 2 24576 21024 684.375 15840 515.625 16.666 77.344

2 2 49152 6240 203.125 6048 196.875 5.208 29.531

3 3 49152 21024 684.375 21984 715.625 16.666 102.65

4 (TDD) Special

Frame

4096 448 14.583 576 18.75 16.666 4.375

Format 4 is only available for frame structure type 2 and special subframe configurations with UpPTS

lengths 4384⋅Ts and 5120⋅Ts only.

For FDD format 0, 1 2 or 3 can be configured. Figure 2-79 visualizes the different formats. It

is worth noting that they can occupy more than a subframe and in addition the guard period is

not specified.


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Figure 2-79 PRACH FDD Formats

Format 0

Format 1

Format 2

CP Zadoff Chu Sequence

6

PRB

Subframe 1ms Subframe 1ms

Format 3

The actual PRACH channel utilizes 6 PRBs, i.e. it occupies 1.4MHz of uplink channel

capacity.

For FDD the subcarrier spacing is 1.25kHz and there are 839 subcarriers, whilst TDD utilizes

a 7.5kHz subcarrier spacing and 139 carriers. As such for FDD the duration is 1/T =

1/1.25kHz = 0.8ms.

Figure 2-80 PRACH Configuration

C

P

Subframe

0

Subframe

1

Subframe

2RB 24

RB 0

ZC

839

Subcarriers

1.25kHz

(6RBs) PRACH Frequency

Offset (0 to 104

Resource Blocks)

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The exact position of the PRACH is defined in the SI (System Information) messages by

using the PRACH Configuration Index. This is based on a table and can vary from 0 to 63.

Table 2-18 illustrates the first part of the table.

Table 2-18 PRACH Configuration Index

PRACH Configuration Index

Preamble Format

System Frame Number

Subframe Number

0 0 Even 1

1 0 Even 4

2 0 Even 7

3 0 Any 1

4 0 Any 4

5 0 Any 7

6 0 Any 1, 6

7 0 Any 2 ,7

8 0 Any 3, 8

9 0 Any 1, 4, 7

10 0 Any 2, 5, 8

11 0 Any 3, 6, 9

12 0 Any 0, 2, 4, 6, 8

13 0 Any 1, 3, 5, 7, 9

14 0 Any 0, 1, 2, 3, 4,

5, 6, 7, 8, 9

15 0 Even 9

. . . .

. . . .

63 3 Even 9

PRACH Sequence Generation

The network configures the set of preamble sequences the UE is allowed to use. There are 64

preamble sequences per cell.


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Figure 2-81 PRACH Configuration and Preamble Sequences Per Cell

eNB

Cell has 64

Preamble

SequencesPRACH-Configuration

Root Sequence Index (0 to 837)

PRACH Configuration Index (0 to 63)

High Speed Flag

Zero Correlation Zone Configuration (0 to 15)

PRACH Frequency Offset (0 to 104)

The random access preamble is generated from Zadoff-Chu sequences. These have key

properties:

Constant Amplitude - This improves the PARP and increases the amplifier efficiency.

Autocorrelation - This enables the eNB to provide accurate timing.

Cross Correlation - This enables different base sequence cyclic shifts to be used.

Additional mechanisms are required when the cyclic shift is greater than the time

expected for round trip propagation and signal delay spread.

The set of 64 preamble sequences in a cell is found by including first, in the order of

increasing cyclic shift, all the available cyclic shifts of a root Zadoff-Chu sequence with the

logical index RACH_ROOT_SEQUENCE, where RACH_ROOT_SEQUENCE is

broadcasted as part of the System Information.

Additional preamble sequences, in case 64 preambles cannot be generated from a single root

Zadoff-Chu sequence, are obtained from the root sequences with the consecutive logical

indexes until all the 64 sequences are found.

The relation between a logical root sequence index and physical root sequence index “u” is

defined by various tables and calculations in the 3GPP 36.211 specification - Physical

Channels and Modulation.

The thu root Zadoff-Chu sequence is defined by:

10, ZC

)1(

ZC

NnenxN

nunj

u

where the length ZCN of the Zadoff-Chu sequence, e.g. 829 for Format 0. Various rules

apply to identify the chosen set. In addition, the parameter “Highspeed-flag” is provided by

higher layers and determines if “unrestricted set” or “restricted set” is used. The restricted set

adds additional rules on the cyclic shifts that can be used as preambles, i.e. taking Doppler

spread into account.

2.14.2 PUSCH (Physical Uplink Shared Channel)

Uplink resource scheduling is performed by the eNB. Note that Section 3 provides more

information on resource allocation and scheduling. The eNB utilizes information, e.g. QoS

parameters, buffer status, UE capabilities, CQI (Channel Quality Indicator) measurements, to

identify the best scheduling of resources. Like the downlink, the uplink allocation is multiples

of Resource Blocks, each consisting of 12 subcarriers.

The Physical Uplink Shared Channel is the main delivery mechanism for higher layer

Transport Channels. Figure 2-82 illustrates an example of the mapping of PUSCH symbols to

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the Resource Elements. Like the downlink, the uplink also has resource elements reserved for

Reference Signals and control.

Figure 2-82 PUSCH Mapping

PDSCH

Symbol

Mapping

PUSCH

Symbols

Subframe

Reference

Signals

Additional Resource Elements are typically required to carry extra control signaling, e.g. CQI (Channel

Quality Information), ACK/NACK, etc.

Multiplexing of Control Signaling and UL-SCH Data

There are various types of control signaling which may need to be sent in the same subframe

as the allocated PUSCH. A device is not allowed to transmit the PUCCH and PUSCH in the

same subframe; therefore the control information needs to be multiplexed with the UL-SCH

Transport Channel before the DFT process.

Figure 2-83 Multiplexing Control Signaling

Subframe

PUSCH Data

PUSCH Reference Signals

CQI/PMI

ACK/NACK

RI (Rank)


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Figure 2-83 illustrates an example of control signaling to the PUSCH. In this example, three

additional types of signaling are added:

ACK/NACK - These are part of the HARQ process and are located next to the RS. This

ensures that they benefit from the best possible channel estimation. The information is

punctured to make way for the ACK/NACK information.

CQI/PMI - The CQI (Channel Quality Information) and PMI (Precoding Matrix

Indicator) can also be multiplexed onto the PUSCH. These are rate matched with the

UL-SCH. The mapping of these is sequential on one subcarrier before continuing on the

next.

RI - RI (Rank Indication) - These are placed next to the ACK/NACK.

Various rules on the mapping and coding of control information exist. In addition, it is also

possible to send control information on the PUSCH without data, i.e. not the UL-SCH.

2.14.3 PUCCH (Physical Uplink Control Channel)

The PUCCH carries UCI (Uplink Control Information); examples include: ACK/NAKs in

response to downlink transmission, CQI (Channel Quality Indicator) reports, SR (Scheduling

Requests) and MIMO feedback such as PMI (Precoding Matrix Indicator) and RI (Rank

Indication).

The PUCCH is transmitted on a reserved frequency region. This is configured by the higher

layer. Figure 2-84 illustrates an example of this mapping. It is worth noting that the number of

control regions is variable.

Figure 2-84 Mapping to Physical Resource Blocks for PUCCH

Subframe

Control Region 0

Control Region 1

Up

link C

arr

ier

Band

wid

th

Slot n Slot n+1

PRB=0

PRB=n

Control Region 2

The PUCCH resource blocks are located at both edges of the uplink bandwidth. It uses

inter-slot hopping to improve frequency diversity. Note that a UE only uses the PUCCH when

it does not have any data to transmit on the PUSCH, i.e. no allocated resources.

There are various types of PUCCH formats associated with uplink control. Section 2.17

discusses these in detail.

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2.15 Timing Relationships

FDD Timing

In LTE there are various rules associated with timing between the downlink and uplink

transmissions. The timing for FDD is illustrated in Figure 2-85. If a UE detects a PDCCH

with DCI format 0 and/or a PHICH transmission in subframe n intended for the UE, it will

adjust the corresponding PUSCH transmission in subframe n+4 according to the PDCCH and

PHICH information. This offset is identified as K, such that K=4 for FDD.

Figure 2-85 FDD Timing

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

4 Subframe Delay

FDD: K=4

PDCCH

PUSCH

Downlink

Uplink

TDD Timing

For TDD the timing relationship is more complex. As such, it now depends on the UL/DL

TDD configurations, namely 0 to 6. Table 2-19 illustrates the different K values for TDD.

Table 2-19 “K” Values for TDD Configurations

TDD UL/DL Configuration

K value for DL Subframe Number

0 1 2 3 4 5 6 7 8 9

0 4* 6* 4* 6*

1 6 4 6 4

2 4 4

3 4 4 4

4 4 4

5 4

6 7 7 7 7 5


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The UE, upon detection of a PDCCH with DCI format 0 and/or a PHICH transmission in

subframe n intended for the UE, adjusts the corresponding PUSCH transmission in subframe

n+k, with k given in Table 2-19.

Figure 2-86 illustrates an example of frame configuration 2. In this configuration, K=4 in

subframes 3 and 8. This relates to transmission being scheduled for subframes 7 and 2

respectively.

Figure 2-86 Example of TDD Configuration 2

K=4 Subframe Delay

0

Special

Subframe

2 3 4 5 7 8 9

Switch to

Uplink

TDD Configuration 2

(DSUDDDSUDD)

Switch to

Downlink

2.16 Uplink Reference Signals In addition to the higher layer control and data being sent on the uplink, lower layer Reference

Signals are also required. Like other Reference Signals these require good auto correlation

and cross correlation properties. In addition, there needs to be a sufficient number of

sequences to minimize interference.

There are two variants of uplink Reference Signal supported:

DRS (Demodulation Reference Signal) - This is associated with transmission of PUSCH

or PUCCH.

SRS (Sounding Reference Signal) - This is not associated with transmission of PUSCH

or PUCCH.

Figure 2-87 Uplink Reference Signals

DRS (Demodulation

Reference Signal)

SRS (Sounding

Reference Signal)

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Base Sequences

Reference Signals are generated using “Base Sequences”, with the same set of base sequences

used for demodulation and sounding Reference Signals. These sequences need to support

different bandwidth options whilst at the same time having auto correlation and cross

correlation properties. In addition, they need to have acceptable cubic metric values.

2.16.1 Demodulation Reference Signal

The DRS (Demodulation Reference Signal) is used for channel estimation to help the

demodulation of the control and data channels in the eNB. There are two different

demodulation Reference Signals; these are used for the PUSCH and PUCCH respectively.

There are various RS sequences defined, as well as different lengths. As a result, the DRS is

defined using four parameters:

Sequence length - This is part of the uplink allocation.

Sequence Groups (0-29) - This is cell specific.

Sequence - Each group contains one sequence for each length up to 5PRB, and two

sequences for each length from 6PRB.

12 Cyclic Shift options.

Sequence Group Selection

In any given slot, the reference sequences used within a cell are from the same group.

However the group assignment may change. There are two group assignment methods.

Figure 2-88 DRS Sequence Group Selection

Sequence Group Selection

Fixed Group Group Hopping

When using a fixed group, i.e. not group hopping, the same group is used for all slots.

However, the group number to use is dependent on the channel type. As such, the PUCCH

group number is based on the cell identity and the PUSCH group number is influenced by a

higher layer parameter.

If using group hopping, the group number changes with slots based on an equation. There are

17 different hopping patterns and 30 different sequence-shift patterns. As such, the PUCCH

and PUSCH have the same hopping pattern but may have different sequence-shift patterns.

PUSCH DRS

The DRS varies in its location depending on a number of attributes, such as the use of a

normal or extended cyclic prefix. Figure 2-89 illustrates the DRS location for the PUSCH and

a normal CP. In this case the DRS is located on the 4th symbol in each slot and uses the same

transmission bandwidth allocated to the UEs in the uplink. Reference Signals for different

UEs are derived by different cyclic shifts from the same base sequence.


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Figure 2-89 Uplink Demodulation Reference Signal (Normal CP)

Slot Slot

Subframe

RRRRRRRRRRRR

12

Sub

carr

iers

RRRRRRRRRRRR

LTE DRS (Demodulation

Reference Signals) transmitted

across all subcarriers assigned

to a UE

In contrast, if the system is utilizing an extended CP then the DRS is located in a different

OFDM symbol.

Figure 2-90 Uplink Demodulation Reference Signal (Extended CP)

Slot Slot

Subframe

12

Sub

carr

iers

RRRRRRRRRRRR

RRRRRRRRRRRR

Extended CP

DRS Location

2.16.2 Sounding Reference Signal

The SRS (Sounding Reference Signal) provides the eNB with uplink channel quality

information which can be used for scheduling. The UE sends a Sounding Reference Signal in

different parts of the allocated bandwidth where no uplink data transmission is available.

Figure 2-91 illustrates an example whereby a UE has been allocated resources in the uplink.

The eNB is able to use the DRS to provide channel estimation in this sub-band. However the

eNB does not know how the UE will perform in the other bands. As such, if the eNB was to

allocate resources in these other bands, the conditions may not be “favorable” and additional

errors could be introduced.

Effectively there are two modes for transmitting SRS, either wideband mode or frequency

hopping mode. In wideband mode, the SRS occupies the bandwidth required. This could

however lead to poor channel quality estimates. In contrast, frequency hopping mode sends

multiple SRS signals using a narrowband transmission. This will, over time, cover the same

bandwidth.

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Figure 2-91 Requirement for SRS

eNB

Subframe

5M

Hz

(25 R

eso

urc

e B

lock

s)

Assigned

Resources

UE

Subframe

No Channel

Information

No Channel

Information

The configuration of the sounding signal, e.g. bandwidth, duration and periodicity, are given

by higher layers. The SRS is transmitted in the last symbol of the subframe. Figure 2-92

illustrates an example, whereby the eNB has configured the mobile to send SRS over a

desired portion of the band.

Figure 2-92 Example of SRS Frequency Hopping

eNB

Subframe

5M

Hz

(25

Reso

urc

e B

lock

s)

UE

0 1 2 3 4

SRS

Since the SRS can be sent when the UE has no current PUSCH or PUCCH assignment,

mechanisms must exist to stop the UE interfering with other users’ PUSCHs. This is done by

making sure all UEs know when the SRS are transmitted, such that the last symbol of the

subframe where SRS is transmitted is not used by any mobiles for their PUSCH.


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SRS Transmission

There are various Sounding Reference Symbol parameters defined. Most are UE

semi-statically configurable by higher layers:

Transmission comb.

Starting physical resource block assignment.

Duration of SRS transmission: single or indefinite (until disabled).

SRS configuration index ISRS for SRS periodicity and SRS subframe offset offsetT .

SRS bandwidth SRSB .

Frequency hopping bandwidth, hopb .

Cyclic shift cs

SRSn .

In addition, “cell specific” parameters, SRS transmission bandwidths ( SRSC ) and subframe

transmission are configured by higher layers.

Figure 2-93 illustrates an example of multiplexing the SRS from different users. Notice that

multiple UEs can send the SRS at the same time, using different resources as well as a

different cyclic shift.

Figure 2-93 Example SRS Allocation

Subframe

12 S

ub

carr

iers

SRS Symbol

UE 1 and 2 (Using

different cyclic

shifts)

UE 3 and 4 (Using

different cyclic

shifts)

Note that the SRS may need to interact with ACK/NACK, CQI or SR information. If

interacting with ACK/NACK the SRS may be dropped or the ACK/NACK punctured. In

contrast, when interacting with the CQI and SR information, the SRS is dropped.

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2.17 Uplink Control Signaling

The PUCCH supports multiple formats; these are illustrated in Table 2-20.

Table 2-20 PUCCH Formats

PUCCH

Format

Description Modulation

Type

Bits per

subframe

1 Scheduling Request N/A N/A

1a ACK/NACK BPSK 1

ACK/NACK+SR

1b ACK/NACK QPSK 2

ACK/NACK+SR

2 CQI/PMI or RI QPSK 20

(CQI/PMI or RI)+ACK/NACK

(Extended CP only)

2a (CQI/PMI or RI)+ACK/NACK

(normal CP only)

QPSK+BPSK 21

2b (CQI/PMI or RI)+ACK/NACK

(normal CP only)

QPSK+QPSK 22

Demodulation Reference Signal on the PUCCH

The PUCCH formats include DRS (Demodulation Reference Signal). The location of these is

dependent on the format type and the use of normal or extended CP. In summary these are:

Format 1, 1a and 1b (Normal CP) - DRS is symbols 2, 3 and 4.

Format 1, 1a and 1b (Extended CP) - DRS is symbols 2 and 3.

Format 2, 2a and 2b (Normal CP) - DRS is symbols 1 and 5.

Format 2 (Extended CP) - DRS is symbol 3.

If a UE has a scheduling request or CQI to send, higher layer signaling configures the

resource.

2.17.1 PUCCH Format 1

For PUCCH format 1, information is carried by the presence/absence of transmission of the

PUCCH from the UE.

The UE is assigned a resource index which indicates a resource every nth

frame that can be

used to transmit a SR (Scheduling Request). The size of PUCCH format 1 is 0bits. However,

the eNB knows when to expect a scheduling request from a UE. As such, if the eNB detects

energy on the PUCCH it can assume it came from the “scheduled” UE.


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Note that various rules apply to the sending of scheduling requests, especially if the UE is

multiplexing it with CQI and/or ACK/NAK on PUCCH. In this case:

CQI: Drop CQI when SR is transmitted.

ACK/NAK: Support multiplexing of SR and ACK/NAK.

2.17.2 PUCCH Format 1a and 1b

The PUCCH Format 1a and 1b carry 1 or 2 HARQ bits. Figure 2-94 illustrates the process for

one of the slots. The BPSK/QPSK symbol are applied to a cyclically shifted length-12

sequence )()(, nr vu . Finally, an orthogonal cover code (Walsh Code) is applied. The example

shows a Normal CP option with 3 DRS included. A length 3 code is applied to these, enabling

the eNB to perform channel estimations for devices sharing the same resource.

Figure 2-94 PUCCH Format 1a and 1b (Normal CP)

UL RS UL RS UL RS

1 or 2 bit ACK/NACK

IFFT IFFT IFFT IFFT

W0 W1 W2 W3

Length 4 Sequence

Slot

BPSK/QPSK

Cyclically

shifted

length-12

sequence

To Next

Slot

For an extended CP, there are six symbols and only two UL RS (Reference Signals).

Interference Issues

There should be no intra cell interference in a RB since the system is using the same base

reference sequence with different cyclic shifts and orthogonal codes. However there may be

inter cell interference. This is improved with the use of different cyclic shifts and orthogonal

codes, as well as applying different hopping patterns (since these are cell specific too).

PUCCH Format 2

Format 2 is used when CQI/PMI is transmitted without ACK/NACK or when CQI/PMI and

ACK/NACK are jointly coded for the case of the extended cyclic prefix. Format 2 is

characterized as follows:

It is bit scrambled by a UE specific scrambling sequence.

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The initialization of the scrambling sequence generator is the same as that of the

PUSCH.

It contains CS (Cyclic Shift) based sequences.

CS hopping is performed on a symbol basis.

Figure 2-95 PUCCH Format 2 (Normal CP)

IFFT IFFT IFFT

Slot (Normal CP)

IFFT IFFT

Cyclically

shifted

length-12

sequence

To Next

Slot

No Orthogonal

Code AppliedCQI/PMI or RI

Figure 2-96 PUCCH Format 2 (Extended CP)

IFFT IFFT IFFT

Slot (Extended CP)

IFFT IFFT

CQI/PMI or RI + ACK/NACK

Cyclically

shifted

length-12

sequence

To Next

Slot

PUCCH Format 2a and 2b (ACK/NACK and CQI)

These formats are only supported when using the normal CP. They are characterized as

follows:

They are bit scrambled by a UE specific scrambling sequence.

The initialization of the scrambling sequence generator is the same as that of the

PUSCH.

BPSK (2a) or QPSK (2b) modulation for the 2nd RS symbol in each slot is used. This

carries ACK/NACK.

Format 2a: QPSK CQI + BPSK ACK/NACK

Format 2b: QPSK CQI + QPSK ACK/NACK


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Figure 2-97 PUCCH Format 2a and 2b ACK/NACK Coding

2nd RS

Slot (Normal CP)

IFFT

1st RS

Cyclically

shifted

length-12

sequence

1 or 2 bit

ACK/NACKTo Next

Slot

2.18 LTE Random Access Procedure

Prior to registering on the network the UE must first establish a SRB (Signaling Radio Bearer)

to the eNB that it has “camped on” during the cell selection process. Figure 2-98 illustrates

the overall processes required, typically termed the RACH (Random Access Channel)

process.

Figure 2-98 Overall Random Access Procedure

Identify RACH

Preambles

Identify

PRACH

Format

Send

Preamble

Receive

Response

Decode

Response

Send RRC

Connection

Request

MAC

Connection

Resolution

SRB

Established

Yes

No

2.18.1 RRC Connection

The SRB is also termed the “RRC Connection”, i.e. the UE has moved into the

RRC-Connected State. In order to achieve this signaling between the eNB and the UE is

required. Figure 2-99 illustrates the main signaling messages to establish a SRB. Note: some

of these are messages or indicators at the PHY or MAC layer.

The sequence starts with the probing of the network on the PRACH. Once the UE has

successfully probed for uplink resources and has been allocated these on the UL-SCH, the

RRC Connection is established through a three way signaling handshake on the UL-SCH and

the DL-SCH respectively.

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Figure 2-99 Random Access RRC Signaling Procedure

PRACH Preamble Sequence

MAC Scheduling Grant

RACH

RRC Connection RequestUL-SCH

RRC Connection Setup

RRC Connection Setup CompleteUL-SCH

DL-SCH

UE eNB

Signalling Radio Bearer

(RRC Connected)

MAC

Contention

Resolution

2.18.2 PRACH Preambles

Figure 2-100 illustrates the probing process. The UE send an initial probe based on the

PRACH configuration parameter, discussed in Section 2.14.1 and open loop power control.

This is discussed in Section 2.19.3 .

Figure 2-100 PRACH Probing

Scheduled Message

E.g. RRC Connection

Request

PRACH PRACH PRACH PDCCH

DL-SCHPUSCH

Noise/

Interference

PRACH

Power

Control

eNB indicates the

preamble/ZC sequence

was received and

includes initial UL grant

In this example the initial probe is below the noise/interference level and thus is not heard.

The UE increases its power based on a step size until a response is heard on the PDCCH.


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2.18.3 Random Access Procedure Initialization

The Random Access procedure is initiated by the MAC sublayer or by a PDCCH Order. The

UE is required to gather various parameters before it can initiate the random access procedure.

Table 2-21 lists the main parameters.

Table 2-21 Parameters for Random Access

Parameter Description

PRACH-ConfigInfo This contains: prach-ConfigIndex, highSpeedFlag,

zeroCorrelationZoneConfig and prach-FreqOffset.

ra-ResponseWindowSize Random access response window size in subframes

(sf2, sf3, sf4, sf5, sf6, sf7, sf8 or sf10).

powerRampingStep Power ramping factor (dB0, dB2,dB4 or dB6).

preambleTransMax Maximum number of preamble transmission (n3, n4,

n5, n6, n7, n8, n10, n20, n50, n100 or n200).

preambleInitialReceivedTargetPower Initial preamble power (-120, -118, -116, -114, -112,

-110, -108, -106, -104, -102, -100, -98, -96, -94, -92

or -90 dBm).

DELTA_PREAMBLE Preamble format based offset.

maxHARQ-Msg3Tx Maximum number of Msg3 HARQ transmissions (1

to 8).

mac-ContentionResolutionTimer Contention Resolution Timer (sf8, sf16, sf24, sf32,

sf40, sf48, sf56 or sf64).

numberOfRA-Preambles Number of preambles used (n4, n8, n12, n16 ,n20,

n24, n28, n32, n36, n40, n44, n48, n52, n56, n60 or

n64).

sizeOfRA-PreamblesGroupA Number of preambles assigned to group A (n4, n8,

n12, n16 ,n20, n24, n28, n32, n36, n40, n44, n48,

n52, n56 or n60).

messagePowerOffsetGroupB Part of the power equation to identify which group

to use (minusinfinity, dB0, dB5, dB8, dB10, dB12,

dB15, or dB18).

messageSizeGroupA Part of the size equation to identify which group to

use (b56, b144, b208, b256}.

ra-PreambleIndex The preamble to use as part of dedicated

configuration (0 to 63).

ra-PRACH-MaskIndex The resource to use as part of dedicated

configuration (0 to 15).

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Allocation of Preamble Groups

The LTE random access procedure can group the access preambles into one of two groups. In

so doing, it enables the UE to indicate power or payload size requirements to the eNB for the

initial UL-SCH allocation.

Figure 2-101 illustrates how the random access preambles are grouped into group A and group

B. Two key parameters are required to make the groups: numberOfRA-Preambles and

sizeOfRA-PreamblesGroupA. The preambles in random access preamble group A are the

preambles 0 to sizeOfRA-PreamblesGroupA - 1 and, if it exists, the preambles in random

access preamble group B are the preambles sizeOfRA-PreamblesGroupA to

numberOfRA-Preambles - 1 from the set of 64 preambles.

Figure 2-101 Allocating Preambles to Group A and Group B

eNB

UE

numberOfRA-Preambles

0 1 2 3 4 63

sizeOfRA-PreamblesGroupA

0 1 2 3 4 Preambles Group B

is used dependent

on messages size

and pathloss

If sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no Random Access Preambles group B.

Group Utilization

For the first Msg3 (Higher Layer Message) the selection of group B is based on message size

and pathloss attributes:

Data size plus MAC and control is greater than messageSizeGroupA.

Pathloss is less than (PCMAX – preambleInitialReceivedTargetPower –

deltaPreambleMsg3 – messagePowerOffsetGroupB).

For retransmissions the UE uses the same group as was used for the initial preamble

transmission attempt.

PDCCH Access Order

If a UE receives a PDCCH transmission consistent with a PDCCH order masked with its

C-RNTI, it initiates a Random Access procedure.


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2.18.4 Random Access Response Window

Once the UE has transmitted the randomly selected preamble from the appropriate group, it

monitors the PDCCH for Random Access Response(s) identified by the RA-RNTI (Random

Access - RNTI) in the RA Response window. This starts at the subframe that contains the end

of the preamble transmission plus three subframes and has length ra-ResponseWindowSize

subframes.

Figure 2-102 Random Access Response Window

eNB

Subframes

UE

Random

Access

sf2, sf3, sf4, sf5,

sf6, sf7, sf8, sf10

RA Response

Window Size

+3 Subframes

The RA-RNTI is calculated using the formula: 1 + t_id+10*f_id, where t_id is the index of

the first subframe of the specified PRACH (Physical Random Access Channel) resource and

f_id is the index of the specified PRACH resource within that subframe.

2.18.5 Random Access Response

On receiving the preamble, the eNB sends a Random Access Response on the DL-SCH. This

is addressed to the RA-RNTI on the PDCCH (Physical Downlink Control Channel). It

includes the RAPID (Random Access Preamble Identifier), TA (Timing Alignment)

information, initial UL (Uplink) grant and assignment of a Temporary C-RNTI.

Figure 2-103 MAC Random Access Response

PRACH Preamble Sequence

MAC Scheduling Grant

UE eNB

RAPID (Random Access Preamble ID)

TA

UL Grant

Temporary C-RNTI

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The UL grant contains 20bits of information, including:

Hopping flag - 1bit.

Fixed size resource block assignment - 10bits.

Truncated modulation and coding scheme - 4bits.

TPC command for scheduled PUSCH - 3bits.

UL delay - 1bit.

CQI request - 1bit.

The UE utilizes these parameters to access the resource.

2.18.6 Uplink Transmission

If the UE decodes a PDCCH with the correct RA-RNTI identified, it decodes the DL-SCH

transport block to check if the RAPID is included. If so, it transmits an UL-SCH transport

block in the first subframe n+k1, where k1 ≥ 6.

Figure 2-104 Random Access - Assigned UL-SCH

eNB

n

Subframes

UE

Random

Access

RA

Response

Window

RAPID

Response

+3 n+k1 (k1 ≥ 6)

Assigned

UL-SCH

The UE would postpone the PUSCH transmission to the next available UL subframe if the UL Delay

field is set to 1.

If no random access response is received in the RA response window, the UE is able to

transmit a new preamble sequence. This should happen no later than 4 subframes after the end

of the RA response window.

Figure 2-105 illustrates the MAC contention resolution process. This is achieved by the UE

sending its identity to the eNB in the first UL-SCH message. Granted, this resource could be

contention based, i.e. another UE sent the same access preambles in the same subframe.

Consequently, each would include their own higher layer identity.

The eNB then adds the UE identity in the MAC header. Other UEs with different identifiers

realize that a collision has taken place and then re-access the system, i.e. they send a new

preamble.


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Figure 2-105 MAC Contention Resolution

RRC Connection RequestUL-SCH

MAC Contention Resolution

UE eNBIncludes UE

Identity

MAC Responds

with UE Identity

2.19 Uplink Power Control

The E-UTRA, like most cellular systems, requires power control to be implemented. This

reduces interference and enables it to be managed/optimized by the eNB. Uplink power

control determines the average power over a SC-FDMA symbol in which the Physical

Channel is transmitted.

Figure 2-106 Uplink Power Control

eNB

UE

Uplink Power Control

PUSCH

PUCCH

PRACH

SRS

2.19.1 PUSCH Power Control

The setting of the UE Transmit power PUSCHP (dBm) for the Physical Uplink Shared Channel

transmission in subframe i is defined by:

)}()()()())((log10,min{)( TFO_PUSCHPUSCH10CMAXPUSCH ifiPLjjPiMPiP

Where:

CMAXP - This is the configured UE transmitter power. It relates to either the maximum

allowed by the eNB or the UE power class.

)(PUSCH iM - This is related to the bandwidth of the PUSCH resource assignment

expressed in number of resource blocks.

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)(O_PUSCH jP - This parameter is the sum of various cell and UE specific parameters. It is

also based on retransmission and scheduling options.

- This is a 3bit cell specific parameter provided by higher layers (0, 0.4, 0.5,

0.6,….1).

PL (Pathloss) - This is the downlink pathloss estimate calculated in the UE. Note

pathloss is calculated based on the reference signal power and other higher layer filter

configurations.

ΔTF - This is a UE specific parameter which relates to the MCS (Modulation and Coding

Scheme) and TF (Transport Format), i.e. TBS (Transport Blok Size).

F - This enables UE specific power control, i.e. TPC (Transmit Power Control). Different

options can be configured, e.g. accumulation or current absolute power.

Power headroom

The LTE System also defines UE PH (Power Headroom) as:

)()()()())((log10)( TFO_PUSCHPUSCH10CMAX ifiPLjjPiMPiPH db

A PHR (Power Headroom Report) is typically sent by the UE when the “prohibitPHR-Timer”

expires, or when the power headroom reporting functionality is configured or re-configured.

2.19.2 PUCCH Power Control

The UE power calculation whilst on the PUCCH (Physical Uplink Control Channel) is

defined as:

igFnnhPLPPiP HARQCQI F_PUCCH0_PUCCHCMAXPUCCH ,,min dBm

Where:



O_PUCCHP - This is a parameter is the sum of cell specific and UE specific parameters.


pathloss is calculated based on the reference signal power and other higher layer filter

configurations.

nh - This is a PUCCH format dependent value, where CQIn relates to the number of

CQI bits and HARQn is the number of HARQ bits.

F_PUCCH ( )F - This is provided by higher layers and provides a frame format dB offset.

)(ig - This is the current PUCCH power control and enables UE specific power control,

i.e. TPC (Transmit Power Control).


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2.19.3 PRACH Power Control

The UE power calculation whilst on the PRACH (Physical Random Access Channel), i.e. for

preambles, is determined as:

PPRACH = min{ CMAXP , PREAMBLE_RECEIVED_TARGET_POWER + PL} dBm

Where:



PREAMBLE_RECEIVED_TARGET_POWER - This is set to the

preambleInitialReceivedTargetPower + DELTA_PREAMBLE +

(PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep.


pathloss is calculated based on the Reference Signal Power and other higher layer filter

configurations.

2.20 Paging Procedures

2.20.1 Discontinuous Reception for Paging

A UE in the Idle State is required to listen for paging messages. However, if left unmanaged

the UE would potentially have to look at every subframe for a possible paging message.

Figure 2-107 illustrates the issue this would cause, i.e. a reduction in battery performance.

Figure 2-107 Paging Issues

eNB

Subframes

UE

Decoding every subframe would

impact battery performance

Paging Message

for this UE

To combat this, LTE supports DRX (Discontinuous Reception) of paging messages. Figure

2-108 illustrates the concept, whereby a UE looks at pre-determined times.

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Figure 2-108 System with DRX Reception of Paging

eNB

Subframes

UE

DRX improves battery

performance

Buffered in eNB Paging Message

for this UE

The eNB may have to buffer the paging message until a UE’s paging occasion occurs. The UE

is given various parameters which enable it to identify a time when it should listen. This is

termed a PO (Paging Occasion) and relates to a subframe. In addition, the DRX parameters

also define a PF (Paging Frame), i.e. Radio Frame, which may contain one or multiple Paging

Occasion(s). The system information messages provide the necessary DRX parameters to

enable a UE to calculate listening times. Alternatively they can be sent to a specific UE as part

of higher layer signaling.

2.20.2 Paging Frame

The PF is given by the following equation: SFN mod T= (T div N)*(UE_ID mod N).

This indicates the frames in which the PO (Paging Occasion) could occur. In addition, to

derive the PO, a subframe pattern table and calculation is used to derive the i_s (Index). The

calculation is defined as: i_s = floor(UE_ID/N) mod Ns.

The following Parameters are used for the calculation of the PF and i_s:

T - This is a range of DRX values: 32, 64, 128, 256 radio frames. Note that shorter UE

specific values override T.

N - This is calculated as: min(T,nB).

nB -This is defined as: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32.

Ns - This is calculated as: max(1,nB/T).

UE_ID - This is calculated as: IMSI mod 1024.

The i_s and Ns parameters are used to identify the PO pattern from the pattern tables. Table

2-22 illustrates the subframe patterns for FDD.

Table 2-22 FDD Subframe Patterns

Ns PO when i_s=0

PO when i_s=1

PO when i_s=2

PO when i_s=3

1 9 N/A N/A N/A

2 4 9 N/A N/A

4 0 4 5 9


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Table 2-23 illustrates the subframe patterns for TDD.

Table 2-23 TDD Subframe Patterns

Ns PO when i_s=0

PO when i_s=1

PO when i_s=2

PO when i_s=3

0 0 N/A N/A N/A

2 0 5 N/A N/A

4 0 1 5 6

2.21 HARQ Operation

2.21.1 Retransmission Types

There are two types of retransmissions, namely ARQ (Automatic Repeat Request) and HARQ

(Hybrid Automatic Repeat Request). The ARQ is performed by RLC (Radio Link Control),

whereas the HARQ is part of the MAC (Medium Access Control) and Physical Layer. Figure

2-109 illustrates some of the features/issues of ARQ, as well the benefits of HARQ.

Figure 2-109 ARQ Verses HARQ

eNBUE

ARQ

Implemented at RLC Layer

Slow Retransmission

Not optimized for Radio Interference

HARQ

Not New – used in HSPA and HSPA+

Implemented at MAC and PHY Layers

Fast Retransmission

Optimized for Radio Interference

Improved system efficiency

2.21.2 HARQ Methods

HARQ provides a Physical Layer retransmission function that significantly improves

performance and adds robustness. The retransmission protocol selected in LTE is SAW (Stop

And Wait) due to the simplicity of this form of ARQ. In SAW, the transmitter persists on the

transmission of the current transport block until it has been successfully received, before

initiating the transmission of the next one. Figure 2-110 illustrates the basic concept of SAW.

It also highlights a possible issue associated with sending more packets between each

transmission.

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Figure 2-110 Basic Concept of SAW

eNB

1 2

ACK

UE

SAW (Stop

and Wait) What is sent here?

UE acknowledges and next

transmission can be sent

The mechanism for sending more packets between each transmission is relatively simple;

have a number of HARQ processes that can run in parallel. Figure 2-111 illustrates the

concept of the HARQ processes. In LTE there are various rules and options for how many

HARQ processes are configured, i.e. it depends on downlink, uplink, FDD or TDD. This

example illustrates the downlink FDD frame where 8 HARQ processes are used. It also

highlights one of these processes, namely process “3”, being sent by the eNB and initially

acknowledged by the UE. Whilst the eNB is awaiting the ACK (Acknowledgement) for this,

the additional processes can be utilized to ensure the UE can receive a stream of packets.

Figure 2-111 HARQ Parallel Processes

eNB

1 2

UE

3 4 7 85 6 1 2 3 4 7 85 6 1 2 3

AAA AAA AAA NA A AAA

HARQ with 8

parallel processesNew

data Retransmission

If the mobile identified an error in the transmission it is able to send a NACK (Negative

Acknowledgement) to the eNB. The eNB is then able to quickly re-schedule the data.

There are two main concepts of HARQ, namely CC (Chase Combining) and IR (Incremental

Redundancy).

Figure 2-112 HARQ Methods

Chase

Combing

Incremental

Redundancy


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Chase Combining

Chase Combining ensures that each retransmission is simply a replica of the data first

transmitted. The decoder at the receiver combines these multiple copies (of the same

information). This type of combining provides time diversity and soft combining gain at a low

complexity cost and imposes the least demanding UE memory requirements of all H-ARQ

methods.

Incremental Redundancy

The IR (Incremental Redundancy) method ensures that retransmissions include additional

redundant information that is incrementally transmitted if the decoding fails on the first

attempt. This causes the effective coding rate to increase based on the number of

retransmissions sent. Incremental Redundancy can be further classified in Partial IR and Full

IR. Partial IR includes the systematic bits in every coded word, which implies that every

retransmission is self-decodable, whereas Full IR only includes parity bits, and therefore its

retransmissions are not self-decodable.

Figure 2-113 illustrates an example showing how rate matching and redundancy versions are

used for retransmission. In addition, it highlights the concept of the “effective” code rate.

Figure 2-113 Example of Redundancy Versions and Soft Bits

Original Data

1/3 Rate Turbo Coding

1st TX

2nd TX

Reff. = 4/5

Reff. = 4/5

IR Buffer Size = 10bits

Reff.=4/5

Reff.=2/5

NACK

ACK

Rate Matching Redundancy

Version

2.21.3 HARQ in LTE

The HARQ within the MAC sublayer is designed to transmit and retransmit transport blocks.

For FDD, there are 8 HARQ processes in the downlink. In contrast the uplink has 8 HARQ

processes for non-subframe bundling operation, i.e. normal HARQ operation, and 4 HARQ

processes in the uplink for subframe bundling operation. The concept of subframe bundling is

discussed in Section 3 0as part of LTE scheduling options.

Various HARQ scheduling parameters are required, such as NDI (New Data Indicator) and

TB (Transport Block) size. In addition, the DL-DSCH HARQ information also includes the

HARQ process ID. For UL-SCH transmission the HARQ info also includes RV (Redundancy

Version). In case of spatial multiplexing, i.e. MIMO, on the DL-SCH the HARQ information

comprises a set of NDI and TB size for each transport block.

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Figure 2-114 FDD HARQ Processes

eNB

UE

8 HARQ Processes

8 HARQ Processes - Normal Scheduling

4 HARQ Processes - Subframe Bundling Scheduling

The number of HARQ processes for TDD is related to the frame configuration and varies

between 4 and 15.

Table 2-24 illustrates the different TDD HARQ configurations.

Table 2-24 TDD HARQ Processes

TDD UL/DL Configuration

Maximum Number of HARQ Processes

0 4

1 7

2 10

3 9

4 12

5 15

2.21.4 HARQ In the Downlink

The downlink HARQ is summarized by:

Asynchronous adaptive HARQ.

Uplink ACK/NAKs in response to downlink (re)transmissions are sent on PUCCH or

PUSCH.

PDCCH signals the HARQ process number, indicating transmission or retransmission.

Retransmissions are always scheduled through PDCCH.

2.21.5 HARQ In the Uplink

The uplink HARQ is summarized by:

Synchronous HARQ.

Maximum number of retransmissions configured per UE (as opposed to per Radio

Bearer).

Downlink ACK/NAKs in response to uplink (re)transmissions are sent on PHICH.


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HARQ operation in uplink is governed by the following principles:

Regardless of the content of the HARQ feedback (ACK or NACK), when a PDCCH for

the UE is correctly received, the UE follows what the PDCCH asks the UE to do i.e.

perform a transmission or a retransmission (referred to as adaptive retransmission).

When no PDCCH addressed to the C-RNTI of the UE is detected, the HARQ feedback

dictates how the UE performs retransmissions:

− NACK - the UE performs a non-adaptive retransmission i.e. a retransmission on the

same uplink resource as previously used by the same process.

− ACK - the UE does not perform any uplink (re)transmission and keeps the data in the

HARQ buffer. A PDCCH is then required to perform a retransmission i.e. a

non-adaptive retransmission cannot follow.

Measurement gaps (part of the measurements for mobility) are of higher priority than

HARQ retransmissions: whenever an HARQ retransmission collides with a measurement

gap, the HARQ retransmission does not take place.

The sequence of redundancy versions is 0, 2, 3, 1.

Table 2-25 illustrates the UE behavior in various situations.

Table 2-25 UL HARQ Operation

HARQ feedback seen by the UE

PDCCH seen by the UE

UE behaviour

ACK or NACK New Transmission New transmission according to PDCCH.

ACK or NACK Retransmission Retransmission according to PDCCH (adaptive

retransmission).

ACK None No (re)transmission, keep data in HARQ

buffer and a PDCCH is required to resume

retransmissions.

NACK None Non-adaptive retransmission.

2.21.6 ACK NACK Timing

FDD Mode

In FDD mode, when data is sent on the PDSCH for a UE, the DCI scheduling messages

provide the UE with the necessary information to decode the message. Based on the

validation of a CRC the UE then sends an ACK or NACK to the eNB. Figure 2-115

illustrates the ACK/NACK in the transmission in subframe i+4, where subframe i is

associated with the PDSCH.

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Figure 2-115 Downlink FDD HARQ Timing

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

FDD: K=4

PDCCH+PDPSCH

Data

ACK on PUCCH

or PUSCH

Downlink

Uplink

Figure 2-116 illustrates an ACK/NACK received on the PHICH assigned to a UE in subframe

i, where the associated PUSCH was in transmission subframe i-4.

Figure 2-116 Uplink FDD HARQ Timing

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

Subframe

3

Subframe

4

Subframe

5

Subframe

6

Subframe

7

Subframe

8

FDD: K=4

PHICH

PUSCH

Downlink

Uplink

TDD ACK Modes

In TDD, an ACK/NACK received on the PHICH assigned to a UE in subframe i is associated

with the PUSCH transmission in the subframe i-k, where k is dependent on the TDD

configuration mode table. In addition, TDD has two ACK/NACK feedback modes defined:

ACK/NACK bundling feedback mode - This is used when the associated HARQ

ACK/NACK from multiple PDSCH subframes map into the same uplink subframe. It

utilizes a logical “AND” operation across the downlink subframes.

ACK/NACK multiplexing feedback mode - This uses spatial ACK/NACK bundling

across multiple codewords within a downlink subframe and is performed by a logical

“AND” operation of all the corresponding individual ACK/NACKs.


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2.22 Diversity Options

Cellular systems are continually improving the performance and spectral efficiency achieved

on the radio interface. One method of providing this is through the inclusion of diversity

techniques. This may be through schemes like SFBC (Space Frequency Block Coding) and

FSTD (Frequency Shift Time Diversity), as well as various types of MIMO (Multiple Input

Multiple Output).

2.22.1 SU-MIMO and MU-MIMO

MIMO relates to the use of multiple antennas at both the transmitter (multiple input) and

receiver (multiple output). The terminology and methods used in MIMO can differ from

system to system, however most fall into one of two categories:

SU-MIMO (Single User - MIMO) - this utilizes MIMO technology to improve the

performance towards a single user.

MU-MIMO (Multi User - MIMO) - this enables multiple users to be served through the

use of spatial multiplexing techniques.

Figure 2-117 SU-MIMO and MU-MIMO

SU-MIMO

MU-MIMO

eNB

UE

Increases capacity since a

single user benefits from

multiple data streams.

eNBUE

UE

Increases sector

capacity by allowing

users to share streams.

2.22.2 MIMO and Transmission Options

The LTE system supports various “modes” of transmission, some of which include TD

(Transmit Diversity) techniques. Some techniques are “open-loop”, i.e. no feedback, which

are mainly used for common downlink channels that are not able to benefit from channel

selective scheduling.

Transmission Modes

In the downlink, the method of transmission is sent when a mobile is semi-statically

configured via higher layer signaling to receive PDSCH data. LTE includes the following

Transmission Modes:

Mode 1 - Single-Antenna transmission, port 0, no MIMO.

Mode 2 - Transmit diversity.

Mode 3 - Transmit diversity or with Large Delays CDD (Cyclic Delay Diversity) is used.

Mode 4 - Transmit diversity or Closed-loop spatial multiplexing.

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Mode 5 - Transmit diversity or multi user MIMO (more than one UE is assigned to the

same resource block).

Mode 6 - Transmit diversity or closed loop precoding for rank=1 (i.e. no spatial

multiplexing, but precoding is used).

Mode 7 - Single-antenna port, port 5 (beamforming).

2.22.3 MIMO Modes

LTE supports MIMO (Multiple Input Multiple Output), or multi-antenna transmission, with 2

or 4 transmit antennas. The maximum number of codewords is two, irrespective of the

number of antennas with fixed mapping between code words to layers.

Spatial Multiplexing

The most common MIMO category is referred to as SM (Spatial Multiplexing). This allocates

multiple modulation symbol streams to a single UE using the same time/frequency. The

differentiation of signals is achieved by the different Reference Signals which were sent as

part of the PRB (Physical Resource Block). Figure 2-118 illustrates the concept of Spatial

Multiplexing using a 2x2 MIMO system.

Figure 2-118 Spatial Multiplexing MIMO

eNB

UE

Port 0

Port 1TB

TB

MIMO

TB

TB

2x2 SM (Spatial

Multiplexing)

The main issue with Spatial Multiplexing in a cellular system is associated with high levels of

interference, especially at the cell edge. Unfortunately, this can affect both spatial streams and,

as such, twice as many errors could be introduced. Hence, SM is typically used close to the

eNB, i.e. not at the cell edge.

Figure 2-119 Spatial Multiplexing Interference Issues

eNB

UE

Port 0

Port 1TB

TB

MIMO

TB

TB

Interference

causes twice

as may errors

Interference

If a UE was at the cell edge it could still benefit from MIMO. However it would rely on

different implementations, such as using a single stream precoding. Figure 2-120 illustrates

the basic concept of precoding using STC (Space Time Coding) as a visual example. Note that

precoding is more involved.


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Figure 2-120 MIMO Single Stream

eNB

UE

Port 0

Port 1

MIMO TB

Interference

TB

1 2 3 4 5 6

1 2 3 4 5 6

123 456

Form of

STC

TB Still

Recoverable

Increased

Robustness

AMS (Adaptive MIMO Switching)

To truly optimize the channel efficiency, some systems offer the ability to support AMS

(Adaptive MIMO Switching). Figure 2-121 illustrates how a system could utilize a mixture of

Spatial Multiplexing and other methods, such as Space Time Coding, to optimize the eNB

performance.

Figure 2-121 AMS Concept

Other

Methods

Spatial

Multiplexing

High SNRLow SNR

Effic

ien

cy

UE

eNB

AMS Point

Other Techniques

In addition, the following techniques are supported in LTE:

Code-book-based pre-coding.

Rank adaptation with single rank feedback. Note: the eNB can override a rank report.

2.22.4 Spatial Multiplexing in LTE

LTE allows up to two code words to be mapped onto different layers. The system uses

precoding to enable spatial multiplexing. Figure 2-122 illustrates the processing undertaken

by the PDSCH. This was previously introduced in Section 2.5 with the concept of rank

transmission and layers.

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Figure 2-122 PDSCH Processing


Mapper

Layer

MapperPrecoding

Resource

Element

Mapper

OFDM

Signal

Generation

Resource

Element

Mapper

OFDM

Signal

Generation


Mapper

Codewords LayersAntenna

Ports

In order for the signal to be spatially multiplexed onto the different antenna ports various

mathematical processes are required. In addition, variances occur for 2 and 4 antenna

configurations, as well as open and closed loop spatial multiplexing.

Codebook Based Precoding

A key part of the system is the codebook based coding mechanism. It uses a:

7 element codebook for 2 antenna ports.

16 element codebook for 4 antenna ports.

Table 2-26 illustrates the mapping of codebook indexs onto layers for a 2 transmit antenna

configuration. Note that the 3GPP 36.211 specification includes the detail of precoding and

layer mapping equations for the different techniques and also for 4 antenna configurations.

Table 2-26 Codebook Precoding

Codebook Index Number of Layers

1 2

0

1

1

2

1

10

01

2

1

1

1

1

2

1

11

11

2

1

2

j

1

2

1

jj

11

2

1

3

j

1

2

1

-

For the closed-loop spatial multiplexing transmission mode, the codebook index 0 is not used when the number of layers is equal to 2.


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2.22.5 Feedback Reporting

In order to optimize the system’s performance, the UE can provide various feedback

information about the radio channel environment. LTE has various feedback reporting options

which depend on the MIMO and eNB configuration. The reporting may consist of the

following elements.

Figure 2-123 Feedback Reporting

CQI PMI RI

CQI (Channel Quality Indicator)

This provides an indication of the downlink channel quality and effectively identifies an

optimum modulation and coding scheme for the eNB to use. There are various coding options

for the CQI; Figure 2-124 illustrates the main CQI index.

Figure 2-124 4-bit CQI Table

CQI Index Modulation Code Rate x 1024 Efficiency

0 out of range

1 QPSK 78 0.1523

2 QPSK 120 0.2344

3 QPSK 193 0.3770

4 QPSK 308 0.6016

5 QPSK 449 0.8770

6 QPSK 602 1.1758

7 16QAM 378 1.4766

8 16QAM 490 1.9141

9 16QAM 616 2.4063

10 64QAM 466 2.7305

11 64QAM 567 3.3223

12 64QAM 666 3.9023

13 64QAM 772 4.5234

14 64QAM 873 5.1152

15 64QAM 948 5.5547

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The system defines multiple types of CQI, whereby the term “wideband CQI” relates to the

entire system bandwidth. In contrast, “sub-band CQI” relates to a value per sub-band. This is

defined and configured by the higher layers and relates to the number of resource blocks. It is

also worth noting that a CQI per codeword is reported for MIMO spatial multiplexing.

Depending on the scheduling mode, Periodic and Aperiodic CQI reporting can be used. In

“Frequency Non-selective” and “Frequency selective” mode the PUCCH is used to carry

periodic CQI reports. In contrast, for “Frequency selective” mode, the PUSCH is used to carry

aperiodic CQI reports.

PMI (Precoding Matrix Indicator)

This enables the mobile to select an optimal precoding matrix. The PMI value relates to a

codebook table within the specifications. Like sub-band CQI, the eNB defines which resource

blocks are related to a PMI report. The PMI reports are used in various mode, including:

closed loop spatial multiplexing, multi-user MIMO and closed-loop rank 1 precoding.

RI (Rank Indication)

This indicates the number of useful transmission layers when spatial multiplexing is used.

Thus, in case of transmit diversity, rank is equal to 1 (RI=1).


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Training Manual 3 Dynamic Resource Allocation



3-1

3 Dynamic Resource Allocation

Objectives


3.1 Describe UL and DL Scheduling principles and signaling

3.2 Explain how the scheduler interactions with other functions

3.3 Explain the concepts of dynamic and semi-persistent scheduling


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3.1 Scheduling Principles and Signaling

LTE air interface scheduling is the responsibility of the eNB, however additional scheduling

and QoS (Quality of Service) handling could take place in the EPC (Evolved Packet Core).

Typically, the main goal of scheduling is to meet the different users’ expectations. Historically

the radio interface is the “weak link” or “bottle neck” in the overall end-to-end service. This is

typically due to limited physical resources, i.e. limited bandwidth or channels. The scheduling

in previous systems, such as GSM and UMTS, was easier. This was due to the fact that voice

was the main service and required a dedicated channel. As such, the number of channels (or

elements) on the base station limited the number of simultaneous calls.

Systems are now evolving, e.g. UMTS has evolved into HSPA and HSPA+, towards packet

based services. LTE is the same, such that it is a pure packet based system. In so doing, all

services utilize IP (Internet Protocol).

Figure 3-1 IP Scheduling

eNBBTS

Node B

Historically voice is

delivered on dedicated

channels

Services including

voice are packetized

LTE is purely

IP based

Since LTE is 100% packet based it makes the system design easier. This is because the eNB

does not have to “interwork” its scheduling algorithms with dedicated functions.

Figure 3-2 illustrates the basic scheduling concept. In this example three users, each with a

defined QoS, have data to send.

Figure 3-2 Basic Scheduling in a Cell

Available Cell

ResourcesTime

eNB

A CB

UsersIdeal Resource (based

on QoS) for this

subframe (1ms TTI)

Loading and scheduling

issues need managing

This is a simple example but it does highlight some of the fundamental concepts:

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Multiple users can have different amounts of data in the eNB buffers, as well as in their

uplink buffers.

UEs could be in different locations and hence features such as MIMO may or may not be

available.

Users and specifically the services (QoS) supported could have different priorities, thus

requiring the eNB to prioritize traffic. In the previous example, User B’s data was

scheduled, however User A’s data was delayed until the next subframe. This could have

been based on the service, e.g. a guaranteed service.

The eNB only has a finite amount of resources. This can vary based on a number of

factors. One such factor is the location of users, whereby if they were all close to the

eNB, the scheduler could allocate SM MIMO resources.

3.1.1 QoS in Packet Switched Networks

Packet switched technologies are designed to provide enhance network utilization and

converge multiple data types (multimedia). Unfortunately, services such as voice and

multimedia have various issues associated with delay and jitter. To combat this, the LTE

packet switches / bearer managers are QoS aware, in that they are able to classify packets, as

well as enforce forwarding characteristics. The eNB (Evolved Node B), S-GW (Serving

Gateway) and PDN-GW (Packet Data Network - Gateway) all get involved in the managing

of QoS. Figure 3-3 illustrates the concept of packet classifiers and packet schedulers. Note

that most of the packets have already been classified by the time they reach the eNB.

Figure 3-3 Packet Classifier and Packet Scheduler

VoIP

FTPPacket

Classifier

Packet

Scheduler

BFTP

AVoIP

BFTP

AVoIP x5

X2

eNB

UE

UE

PDN-GW

MME

S-GW

EPCE-UTRAN


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3.1.2 Key Factors Influencing Scheduling

Figure 3-4 illustrates a number of factors which influence the scheduling process. This is not a

complete list and some of the factors may contain a lot of other aspects. For example, “eNB

configuration” could relate to:

Frequency planning.

Cell size.

Power limitations.

MIMO feature support.

Etc.

Figure 3-4 Key Factors Influencing Scheduling

eNB

UE

UE

Neighbor Cell

InterferenceFeedback

e.g. CQI

UE

Retransmissions

eNB

UE Category

Uplink

Interference

Buffer

Status

Guaranteed

Bearers

Bandwidth

Configuration

eNB

Configuration

Scheduling

Mode

3.1.3 Scheduling Methods

One of the other big influences in the performance of the eNB and the scheduler is the actual

algorithm used and its associated efficiency for the type(s) of traffic being scheduled. Broadly

speaking, there is a handful of basic scheduling methods, which are then customized into

proprietary scheduling algorithms. Most schedulers use QoS classes of the services for radio

resource allocation.

Figure 3-5 Possible Scheduling Method

Minimum throughput

demands for

guaranteed service in

order of priority

Maximum throughput

demands (various

methods)

Proportional Fair

MAX C/I

Biased

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Proportional Fair

This is a very common scheduling method. It effectively allocates the same amount of

resources to all the users. In so doing, each user will get the resources they require

(throughput demand) or they will get an equal share. This is effectively the total amount of

resources divided by the total number of users.

MAX C/I

In order to achieve the “best” eNB throughput rates it makes sense to allocate resources to

those users with the best signal, i.e. C/I (Carrier to Interference). In this way features such as

MIMO SM and high order modulation schemes (64 QAM) can be used. In so doing this

increases the system’s spectral efficiency.

Unfortunately, this means that users closer to the eNB continually get resources allocated up

to their maximum demanded rate. At the same time, users at the cell edge will be limited to

their minimum guaranteed rate. This could be detrimental to the “marketing plan”, since users

of LTE will expect higher data rates.

Biased (QoS Based)

The biased scheduling method relates to the user’s services and their QoS class, such that

users with high QoS service attributes are allocated the resources first. If multiple users shared

the same QoS, e.g. they are both performing a VoIP session, then the system revertes back to

another method (usually Proportional Fair).

3.1.4 Downlink Scheduling

The signaling required for scheduling downlink resources is firstly dependent on the type of

resources being scheduled. The LTE system defines various DCI (Downlink Control

Information) messages which were introduced in Section 2.10 . These enable both downlink

and uplink scheduling, as well as linking to different MIMO and diversity options.

For the purpose of this section, DCI Format 2 for FDD is re-visited.

DCI Format 2

This is used for scheduling PDSCH to UEs configured in closed-loop SM (Spatial

Multiplexing). The following information is sent as part of DCI format 2:

Resource allocation header - This indicates resource allocation type 0 or type 1. These

are detailed in Section 3.1.5 .

Resource block assignment - This is for type 0 or 1 information.

TPC command for PUCCH - Previous discussed under power control.


Transport block to codeword swap flag - This determines the transport block to

codeword mapping. However, if one of the transport blocks is disabled the mapping is

different.

For the first Transport Block:




For the second Transport Block:


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Precoding information - This is either 3bits or 6bits depending on the number of antenna

ports.

3.1.5 PDSCH Resource Allocation

The UE interprets the DCI resource allocation field depending on the PDCCH DCI format

message. A resource allocation field in each PDCCH includes two parts. These are:

A resource allocation header field.

Information consisting of the actual resource block assignment.

There are three types of resource allocation.

Type 0 Resource Allocation

The resource block assignment information includes a bitmap indicating the RBG (Resource

Block Groups) that are allocated to the scheduled UE, where a RBG is a set of consecutive

PRBs. Resource Block Group size (P) is a function of the system bandwidth, examples

include: 5MHz P=2, 10MHz P=3 and for 15MHz and 20MHz P=4.

Figure 3-6 Type 0 Resource Allocation

0 1 2 3 4 5 6

P P P

Type 0

1 Bit


Type 1 - resource block assignment information of size NRBG indicates to a scheduled UE

the PRBs from the set of PRBs from one of P RBG subsets. A RBG subset“p”, where 0 ≤

p < P , consists of every Pth RBG starting from RBG p . The resource block assignment

information consists of three fields:

The first field is used to indicate the selected RBG subset among P RBG subsets.

The second field with one bit is used to indicate a shift of the resource allocation span

within a subset. A bit value of 1 indicates a shift is triggered. Otherwise a shift is not

triggered.

The third field includes a bitmap, where each bit of the bitmap addresses a single PRB in

the selected RBG subset in such a way that MSB to LSB of the bitmap are mapped to the

PRBs in the increasing frequency order.

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0 1 2 3 4 5 6

P

P

P

P

P

P

Subset 0

Subset p

Type 1

p Bits


Type 2 - resource block assignment information indicates to a scheduled UE a set of

contiguously allocated localized VRB (Virtual Resource Block) or distributed VRB, which are

then mapped onto physical resource blocks. The information field for the resource block

assignment carried on the PDCCH contains a RIV (Resource Indication Value) from which a

starting VRB and a length in terms of contiguously allocated virtual resource blocks, can be

derived.


0 1 2 3 40 1 2 3 4

5 6 7 8

10 11 12

14 13

9

11

RIV

3.1.6 Modulation and Coding Scheme

One of the key parameters in the DCI messages is the MCS Index Parameter. Table 3-1

illustrates the mapping of the MCS index to the modulation and TBS (Transport Block Set)

Index.

Table 3-1 Modulation and TBS index table for PDSCH

MCS

Index

MCSI

Modulation

Order

mQ

TBS

Index

TBSI

MCS

Index

MCSI

Modulation

Order

mQ

TBS

Index

TBSI

0 2 0 16 4 15

1 2 1 17 6 15

2 2 2 18 6 16

3 2 3 19 6 17


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4 2 4 20 6 18

5 2 5 21 6 19

6 2 6 22 6 20

7 2 7 23 6 21

8 2 8 24 6 22

9 2 9 25 6 23

10 4 9 26 6 24

11 4 10 27 6 25

12 4 11 28 6 26

13 4 12 29 2 Reserved

14 4 13 30 4

15 4 14 31 6

The modulation order parameter indicates whether the scheduled transmission is QPSK (2

bits), 16QAM (4bits) or 64QAM (6bits). The UE is able to use this information, in

conjunction with the physical number of Resource Blocks, i.e. symbols, to receive all the bits.

Figure 3-9 illustrates an example of a scheduled message. As previously mentioned the

resource allocation, modulation order and precoding information enables the UE to determine

the number and location of the physical bits. The TBS (Transport Block Set) parameter in the

previous table enables the UE to identify the size of the transport block(s) using a mixture of a

table and equation. Since the coding is all predefined, the UE is able to replicate the number

of coded bits (pre puncturing) and therefore, using the RV (Redundancy Version) parameter,

identify which bits the eNB would have punctured/rate matched. Using this it can now

attempt to decoded the transport block and verify the CRC.

Figure 3-9 Using the TBS Size

5M

Hz

(25

Reso

urc

e B

lock

s)

Scheduled

5RB (MIMO SM)

16QAM

TBS(s) Size

RV

Local and Distributed

VRB (Virtual Resource

Block) OptionsTBS

size

Physical Bits

RV

1/3 Rate coding

Punctured

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3-9

3.1.7 Uplink Scheduling

The uplink scheduling process is similar. The eNB provides the relevant parameters in the

DCI Format 0 message. In order to simplify uplink signal processing and the DFT (Discrete

Fourier Transform) design the PUSCH can only be allocated in factors of 2, 3, and 5, i.e. 7

RBs are not allowed.

LTE TTI Bundling

LTE also supports subframe bundling where a bundle of PUSCH transmissions consists of

four consecutive uplink subframes in both FDD and TDD. The subframe bundling operation

is configured by the parameter “ttiBundling” provided by higher layers.

3.2 Scheduler Interaction

A good scheduler is one that is harmonized with all the flexibilities of the LTE air interface. In

so doing, it can quickly adapt to various issues and changes in the radio environment.

As previously mentioned, the scheduler needs to be QoS aware for different users and their

services. This is achieved by the scheduler interacting with different functions and the process

which manages those functions. In addition, it must have a mixture of pre-configured and

dynamic variables it can use and possibly change.

3.2.1 Radio Bearers

The scheduling of user information can be broadly broken down into three areas:

SRB (Signaling Radio Bearer) - each UE on the network will establish a SRB, i.e. RRC

connection, when it moves to the LTE Active state. There are three types of SRB, namely

SRB 0, SRB 1 and SRB 2. Each have different scheduling requirements.

Default EPS Bearer - The process of attaching to the network causes a default EPS

bearer to be established. The QoS attributes for this are part of the user ’s subscription.

This is passed to the eNB as part of the Initial Context Setup procedure.

Dedicated EPS Bearers - In addition to the default bearer, one or more dedicated bearers

can be established (each with their own QoS attributes). The process of E-RAB setup

from the MME typically activates these.

3.2.2 Scheduler Interaction with Layer 2 and Layer 1

It is mainly the responsibility of RRM and RRC to configure the bearers between the eNB and

the UE. As a result, configuration parameters can be sent for various layer 2 management

functions looking after scheduling, link adaptation, RLC, HARQ etc. Figure 3-10 illustrates

some of the interaction that may take place within the eNB.


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Training Manual



Issue 01 (2010-05-01)

Figure 3-10 Scheduler Interaction

Scheduler

Layer 3 RRM

Manager

HARQ

Layer 2

Layer 1

Link

Adaptation

Dynamic

Allocation

UE

MAC

Layer 1 Reports + UCI + SRS

Layer 2 RRM Manager

Buffer Status

Layer 3

Layer 2

In order to correctly schedule resources, various layer 1 and layer 2 indications and

configurations are required. The link adaptation function manages the current MCS

(Modulation and Coding Scheme) based on feedback from both layer 1 in the eNB and the

UE. In addition, SRS (Sounding Reference Signals) provide intelligence about the channel.

Other reports from the eNB Layer 1, as well as UCI (Uplink Control Information) from the

UE, can be collated to provide an up-to-date representation of the channel.

The scheduler also needs to interact closely with HARQ, since layer 1 NACKs and

subsequent retransmissions impact resources. Additional functionality which monitors the

relationship between retransmissions, the choice of MSC and power control is also vital,

enabling the system to adapt to the channel conditions.

3.3 Dynamic and Semi-persistent Scheduling

LTE supports Dynamic and Semi-persistent scheduling, the latter being used to reduce the

amount of control channel overhead/signaling. This enables the eNB scheduler to efficiently

schedule resources for application/bearers which have a continual allocation requirement, e.g.

VoIP. The semi-persistent allocation persists until the eNB scheduler changes it.

When Semi-Persistent Scheduling is enabled by RRC, the following information is provided:

Semi-Persistent Scheduling C-RNTI.

semiPersistSchedIntervalUL - This is the uplink Semi-Persistent Scheduling interval.

implicitReleaseAfter - This is the number of empty transmissions before implicit release

if Semi-Persistent Scheduling is enabled for the uplink.

semiPersistSchedIntervalDL - This is the downlink Semi-Persistent Scheduling interval .

numberOfConfSPS-Processes - This is the number of configured HARQ processes for

Semi-Persistent Scheduling (downlink).

Whether twoIntervalsConfig is enabled or disabled for uplink (only for TDD).

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3-11

3.3.1 Dynamic Scheduling

Figure 3-11illustrates the concept of dynamic scheduling, whereby an individual scheduling

message allocates a resource. Note that in the uplink TTI bundling could also be used.

Figure 3-11 Dynamic Scheduling

ACK/

NACK

Downlink

Uplink

0 1 2 3 4 5

PDSCH

DPDCCHMobile

Receives

Mobile

Sends

PUCCH

Dynamic

3.3.2 Downlink Semi-persistent Scheduling

After a Semi-Persistent downlink assignment is configured, the UE considers that the

assignment recurs in each subframe for which:

(10 * SFN + subframe) = [(10 * SFNstart time + subframestart time) + N *

semiPersistSchedIntervalDL] modulo 10240, for all N>0.

Where SFNstart time and subframestart time are the SFN (System Frame Number) and subframe,

respectively, at the time the configured downlink assignment were (re-)initialised. Figure 3-12

illlustrates the basic concept of uplink Semi-Persistent Scheduling.


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Issue 01 (2010-05-01)

Figure 3-12 Semi Persistent Scheduling

ACK/

NACK

Downlink

Uplink

0 1 2 3 4 5

PDSCH

SPDCCHMobile

Receives

PUCCH

Semi-

Persistent

ACK/

NACK

Mobile

Sends

Mobile

Sends

3.3.3 Uplink Semi-persistent Scheduling

When a Semi-Persistent Scheduling uplink grant is configured, the UE considers that the

grant recurs in each subframe for which:

(10 * SFN + subframe) = [(10 * SFNstart time + subframestart time) + N *

semiPersistSchedIntervalUL + Subframe_Offset * (N modulo 2)] modulo 10240, for all N>0.

Where SFNstart time and subframestart time are the SFN (System Frame Number) and subframe at

the time the configured uplink grant were (re-)initialised.

In additon, the Subframe_Offset is set to 0 unless twoIntervalsConfig is enabled. In this case

the Subframe_Offset is set according to a table in the 3GPP 36.321 specification.

Retransmissions for Semi-Persistent Scheduling can continue after clearing the configured uplink grant.

LTE Air Interface

Training Manual 4 Intra LTE Mobility



4-1

4 Intra LTE Mobility

Objectives


4.1 Describe intra-LTE mobility in ECM-CONNECTED and ECM-IDLE mode.

4.2 Explain the concept of event triggered periodical reporting.

4.3 Describe the mobility measurements.


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4.1 Intra-LTE Mobility

Intra-LTE mobility can be split into Idle State mobility and Active State mobility.

Figure 4-1 Intra-LTE Mobility

Idle State

Mobility

Active State

Mobility

A UE in the Idle State has previously registered on the network and is performing two main

procedures, namely cell reselection and listening to paging messages.

4.1.1 Idle State - Cell Reselection

The E-UTRA cell reselection process is similar (not identical) to the one used in UMTS. In

addition, various parameters are used to define if intra and inter frequency measurements

should be taken. Figure 4-2 illustrates the concept of intra-frequency and inter-frequency.

Figure 4-2 Intra-Frequency and Inter-frequency

eNBUE

Frequency

1 eNB

eNB

Frequency

2

Frequency

1

Intra-Frequency

Inter-Frequency

Intra-Frequency Measurements

Criteria to perform intra-frequency measurements are as follows.

If Squal > Sintrasearch - the UE may choose not to perform intra-frequency measurements.

If Squal <= Sintrasearch - the UE performs intra-frequency measurements.

If Sintrasearch is not sent for the serving cell then the UE performs intra-frequency

measurements.

Figure 4-3 illustrates the basic concept of Sintresearch.

LTE Air Interface




4-3

Figure 4-3 Sintrasearch Parameter

Time

Squal

Sintrasearch

Inter-frequency Measurements

E-UTRA includes a cell priority mechanism; as such, an inter-frequency cell may have a

higher or lower priority. The decision to monitor these cells is based on their priority, as well

as the presence of and the relationship to the Snonintrasearch parameter.

High and Medium Mobility State

In addition to the normal mobility state, a High-mobility and a Medium-mobility state are also

configurable in the E-UTRA. The presence of valid hysteresis values, counters and timers sent

in the system information broadcast messages activates this feature.

The parameters (TCRmax, NCR_H, NCR_M and TCRmaxHyst) are sent in the System Information

broadcast of the serving cell. In so doing the criteria for mobility state can be checked:

Medium-mobility state criteria - This is met if the number of cell reselections during

time period TCRmax exceeds NCR_M and does not exceed NCR_H.

High-mobility state criteria - This is met if the number of cell reselections during time

period TCRmax exceeds NCR_H.

If High-mobility state is detected the UE:

Adds the “sf-High” parameter (from Speed dependent ScalingFactor for Qhyst) to Qhyst.

Multiplies the TreselectionEUTRA by the “sf-High” (from Speed dependent ScalingFactor

for TreselectionEUTRA).

If Medium-mobility state is detected the UE:

Adds the “sf-Medium” parameter (from Speed dependent ScalingFactor for Qhyst) to

Qhyst.

Multiplies the TreselectionEUTRA by the “sf-Medium” (from Speed dependent

ScalingFactor for TreselectionEUTRA).

The Qhyst parameter is used as part of the ranking / reselection equations. The

TreselectionEUTRA parameter (with possible scaling) is used to identify the time duration a cell

must meet the criteria before reselection can take place.


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Training Manual



Issue 01 (2010-05-01)

Figure 4-4 Impact to Treselection

eNBUE

eNB

Cell is better for duration

of Treselection (may have

scaling applied )

Ranking of Cells

The cell reselection evaluation process is known as R (Cell Ranking). The calculations for the

Rs (serving cell) and Rn (neighbouring cells) are illustrated in Figure 4-5.

Figure 4-5 Ranking Equation

Rs = Qmeas,s + QHyst

Rn = Qmeas,n - Qoffset

Where:

Qmeas - This is the RSRP (Reference Signal Received Power) measurement quantity

used in cell reselections.

QHyst - This the hysteresis to apply to the servingcell. It may have had some scaling

applied due to mobility.

Qoffset - For intra-frequency cells this is equal to the Qoffsets,n parameter (if sent). For

inter-frequency cells this equals Qoffsets,n + Qoffsetfrequency.

4.1.2 Active State Mobility

When the UE is in the LTE Active State, i.e. RRC Connected, the eNB performs network

controlled UE assisted handovers. This process may be divided into three distinct phases.

These are:

Measurement and Reporting - In this phase the UE takes measurements of neighbor cells

and reports these measurements to the serving eNB.

Handover Preparation Phase - Once the serving eNB has identified that various criteria

for handover have been met it can select the target eNB. This could trigger signaling

directy between eNBs (across the X2 interface) or if that is not available the MME

(Mobility Management Entity) will get involved.

Perform Handover - The UE will be informed when to handover. It will also be provided

with sufficient information to access the target cell. The Random Access process will be

utilized since the UE and target eNB are not synchronized.

Figure 4-6 illustrates the main phases of an intra LTE handover.

LTE Air Interface




4-5

Figure 4-6 Intra-LTE Mobility

Serving Neighbor

Measurements and

Reporting

Margin

RSRP RSRP

UEeNB eNB

Handover

Preparation

Perform

Handover

4.1.3 Handover Procedure

There are various messages included in a LTE handover, Figure 4-7 illustrates the main

message which trigger the handover on the air interface, as well as the additional signaling

required in the e-UTRAN.

Figure 4-7 LTE Handover Procedure

Measurement

Report(s) Handover Request

Handover

Request AckRRC Connection

Reconfiguration

RequestSN Status

Transfer

Handover Confirmed

Path Switch

Request

Source

Path Switch

Request Ack

UE

eNB eNB MME S-GW

Target

Modify

Bearer

Initially the measurement reports need to be configured. This could be a mixture of fixed

configuration triggers, as well as some triggers which are dynamically provisioned.


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Training Manual



Issue 01 (2010-05-01)

In the “standard handover”, the mobile is configured to send MR (Measurement Reports)

based on the measurement configuration information in RRC signaling. These measurement

reports (discussed later in the section) form the basis for most handovers.

The actual handover process is initiated by the source eNB when it sends the Handover

Request message to the target eNB (assuming the X2 interface is present). This message

provides the target eNB with the necessary information, e.g. the target cell ID, security keys,

UE and RRC Context information, including the E-RAB (EPS Radio Access Bearer)

information and associated QoS.

The target eNB may then perform Admission Control based on the QoS requirements.

Assuming that the handover can be supported the target eNB allocates a C-RNTI and

optionally a RACH preamble. These are sent back in the Handover Request Acknowledge

message to the source eNB.

The UE receives a RRC Connection Reconfiguration message (with handover information)

from the source eNB. Using the information included it is then able to access the new cell and

complete the procedure.

Like UMTS, various timers are used in the event that the UE cannot access the target eNB.

4.2 Reporting Options

One of the main mobility reporting parameters is “MeasConfig”. This is an optional IE

(Information Element) in the RRC Connection Reconfiguration message. Figure 4-8 identifies

the main parameters included as part of “MeasConfig”.

Figure 4-8 Measurement Configuration Parameters

eNBMeasConfig

measObjectToRemoveList

measObjectId

measObject

reportConfigToRemoveList

reportConfigId

reportConfig

measIdToRemoveList

measGapConfig

s-Measure

timeToTrigger-SF

UE

RRC Connection Reconfiguration message

4.2.1 Measurement Configuration Parameter

In summary “MeasConfig” includes the following parameters:

measObjectToRemoveList - This is a list of measurement objects to remove.

measObjectId - This is used to identify a measurement object configuration.

measObject - Specifies measurement object configurations for E-UTRA, UTRA,

GERAN, or CDMA2000 measurements.

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4-7

reportConfigToRemoveList - This is a list of measurement reporting configurations to

remove.

reportConfigId - This is used to identify a measurement reporting configuration.

reportConfig - This specifies measurement reporting configurations for E-UTRA, UTRA,

GERAN, or CDMA2000 measurements.

measIdToRemoveList - This is a list of measurement identities to remove.

measGapConfig - This is used to setup and release measurement gaps.

s-Measure - This identifies the Serving cell quality threshold and controls whether or not

the UE is required to perform measurements of intrafrequency, inter-frequency and

inter-RAT neighboring cells. Value “0” indicates to disable s-Measure.

timeToTrigger-SF - This is the timeToTrigger which is multiplied with the scaling factor

applicable for the UE’s speed state.

4.2.2 Report Configuration Parameter

The IE ReportConfigEUTRA specifies criteria for the triggering of an E-UTRA measurement

reporting event. The E-UTRA measurement reporting events are labeled AN with N equal to 1,

2 and so on.

Event A1: Serving becomes better than the absolute threshold.

Event A2: Serving becomes worse than the absolute threshold.

Event A3: Neighbor becomes the amount of offset better than the serving.

Event A4: Neighbor becomes better than the absolute threshold.

Event A5: Serving becomes worse than the absolute threshold1 AND neighbor becomes

better than the another absolute threshold2.

Figure 4-9 illustrates the “reportConfig” parameter as part of the Measurement Configuration

parameter .

Figure 4-9 Report Configuration Parameters

eNBMeasConfig

reportConfigEUTRA

triggerType (event or Periodic)

triggerQuantity (RSRP, RSRQ)

reportQuantity

maxReportCells

reportInterval

reportAmount

ThresholdEUTRA

UE

RRC Connection Reconfiguration message

Figure 4-10 illustrates the difference between a periodic and event based reporting

mechanism.


LTE Air Interface

Training Manual



Issue 01 (2010-05-01)

Figure 4-10 Periodic and Event Reporting

UE

eNB

Threshold

Periodic

UE

Event Based

Event triggered based on

threshold, hysterisis and TTT

(Time To Trigger)

4.3 Mobility Measurements

4.3.1 Measurement Gaps

Time may need to be allocated to measure neighbor cells. This depends on whether they

utilize the same frequency (Intra-Frequency) or a different frequency (Inter-Frequency).

Typically when cells are on different frequencies they require “gap assisted” (the serving eNB

allocates time to take measurements) mode. Figure 4-11 illustrates scenarios when no

assistance is required.

Figure 4-11 Non Gap Assisted

Fc

Band

wid

th

Band

wid

th

Same frequency, same

bandwidth, non gap

assisted

Same frequency,

different bandwidth,

non gap assisted

Same frequency,

different bandwidth,

non gap assisted

Fc

Band

wid

th

Band

wid

th

Fc

Band

wid

th

Band

wid

th

In contrast, Figure 4-12 illustrates when gap assisted mode is required.

LTE Air Interface




4-9

Figure 4-12 Gap Assisted

Different frequency,

overlapping

bandwidth, gap

assisted


overlapping

bandwidth, gap

assisted


no overlapping

bandwidth, gap

assisted

FcB

and

wid

th

Fc

Band

wid

th

Band

wid

th

Fc

Band

wid

th

Band

wid

th

Band

wid

th

4.3.2 Gap Configuration

The measurement gap configuration parameter is sent in the RRC Connection

Reconfiguration message as part of the Measurement Configuration. This indicates the gap

pattern(s) in accordance with the received gapOffset parameter. Each gap starts at an SFN

(System Frame Number) and subframe, meeting the equations in Figure 4-13.

Figure 4-13 Gap Configuration

SFN mod T = FLOOR(gapOffset /10)

subframe = gapOffset mod 10

where: T= TGRP/10

Two Gap Patterns, with associated TGRP (Transmission Gap Repetition Period), are defined.

These indicate either 40ms or 80ms.

4.3.3 UE Measurements

There are various intra and inter system UE measurements.

E-UTRA Carrier RSSI

The E-UTRA Carrier RSSI (Received Signal Strength Indicator) comprises the total received

wideband power observed by the UE from all sources, including co-channel serving and

non-serving cells, adjacent channel interference, thermal noise etc.


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RSRP (Reference Signal Received power)

The RSRP (Reference Signal Received Power) is determined for a considered cell as the

linear average over the power contributions (Watts) of the resource elements that carry cell

specific Reference Signals within the considered measurement frequency bandwidth.

If receiver diversity is in use by the UE, the reported value is equivalent to the linear average

of the power values of all diversity branches.

RSRQ (Reference Signal Received Quality)

RSRQ (Reference Signal Received Quality) is defined as the ratio:

RSSI)carrier UTRA -(E

RSRP×N

where N is the number of RBs of the E-UTRA carrier RSSI measurement bandwidth. The

measurements in the numerator and denominator are made over the same set of resource

blocks.

LTE Air Interface

Training Manual 5 Glossary



5-1

5 Glossary

Numerics

16 QAM (Quadrature Amplitude

Modulation

2G (Second Generation)

3G (Third Generation)

3GPP (Third Generation

Partnership Project)

4G (Fourth Generation)

A

ACK (Acknowledgement)

AM (Acknowledged Mode)

AMPS (Advanced Mobile

Telephone System)

AMS (Adaptive MIMO

Switching)

APN (access Point Name)

ARQ (Automatic Repeat Request)

AS (Access Stratum)

AWS (Advanced Wireless

Services)

B

BCCH (Broadcast Control

Channel)

BCH (Broadcast Channel)

C

CATT (China Academy of

Telecommunications Technology)

CC (Chase Combining)

CCCH (Common Control

Channel)

CCE (Control Channel Element)

CDD (Cyclic Delay Diversity)

CDMA (Code Division Multiple

Access)

CFI (Control Format Indicator)

CP (Cyclic Prefix)

CPC (Continuous Packet

Connectivity)

CQI (Channel Quality Indicator)

CRC (Cyclic Redundancy Check)

C-RNTI (Cell - Radio Network

Temporary Identifier)

CS (Circuit Switched)

CS (Cyclic Shift)

CSG (Closed Subscriber Group)

D

DAI (Downlink Assignment

Index)

D-AMPS (Digital - Advanced

Mobile Phone System)

DC (Direct Current)

DCCH (Dedicated Control

Channel)

DC-HSDPA (Dual Cell - HSDPA)

DCI (Downlink Control

Information)

DCS (Digital Cellular Service)

DFT (Discrete Fourier Transform)

DL (Downlink)

DL-SCH (Downlink - Shared

Channel)

DL-SCH (Downlink Shared

Channel)

DRB (Dedicated Radio Bearer)

DRS (Demodulation Reference

Signal)

DRX (Discontinuous Reception)

DSSS (Direct Sequence Spread

Spectrum)

DTCH (Dedicated Traffic

Channel)

DTX (Discontinuous

Transmission)

DwPTS (Downlink Pilot Time

Slot)

5 Glossary

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E

EARFCN (E-UTRA Absolute

Radio Frequency Channel

Number)

EDGE (Enhanced Data Rates for

Global Evolution)

E-GSM (Extended GSM)

EMM (EPS Mobility

Management)

eNB (Evolved Node B)

EPC (Evolved Packet Core)

EPLMN (Equivalent HPLMN)

EPS (Evolved Packet System)

E-RAB (EPS Radio Access

Bearer)

ESM (EPS Session Management)

ETACS (Extended Total Access

Communication System)

ETSI (European

Telecommunications Standards

Institute)

ETWS (Earthquake and Tsunami

Warning System)

E-UTRA (Evolved - Universal

Terrestrial Radio Access)

E-UTRAN (Evolved - Universal

Terrestrial Radio Access

Network)

EV-DO (Evolution-Data

Optimized)

F

FDD (Frequency Division

Duplex)

FDM (Frequency Division

Multiplexing)

FDMA (Frequency Division

Multiple Access)

FEC (Forward Error Correction)

FFT (Fast Fourier Transform)

FHSS (Frequency Hopping

Spread Spectrum)

FM (Frequency Modulation)

FSTD (Frequency Shift Time

Diversity)

G

GF(2) (Galois Field (2))

GP (Guard Period)

GPRS (General Packet Radio

System)

GSM (Global System for Mobile

communications)

GSMA (GSM Association)

GUTI (Globally Unique

Temporary Identifier)

H

HARQ (Hybrid ARQ)

HARQ (Hybrid Automatic Repeat

Request)

HeNB (Home eNB)

HI (HARQ Indicator)

HPLMN (Home PLMN)

HSDPA (High Speed Downlink

Packet Access)

HSPA (High Speed Packet

Access)

HS-SCCH (High Speed - Shared

Control Channel)

HSUPA (High Speed Uplink

Packet Data)

I

IDFT (Inverse Discrete Fourier

Transform)

IEEE (Institute of Electrical and

Electronics Engineers)

IFFT (Inverse Fast Fourier

Transform)

IMEI (International Mobile

Equipment Identity)

IMS (IP Multimedia Subsystem)

IMSI (International Mobile

Subscriber Identity)

IMT Advanced (International

Mobile Telecommunications

Advanced)

IMT2000 (International Mobile

Telecommunications - 2000)

IP (Internet Protocol)

IR (Incremental Redundancy)

IS-136 (Interim Standard 136)

ISI (Inter Symbol Interference)

ITU (International

Telecommunication Union)

L

LCID (Logical Channel

Identifier)

LCR (Low Chip Rate)

LTE (Long Term Evolution)

M

MAC (Medium Access Control)

MBSFN (MBMS over Single

Frequency Network)

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5-3

MCS (Modulation and Coding

Scheme)

MGW (Media Gateways)

MIB (Master Information Block)

MIMO (Multiple Input Multiple

Output)

MME (Mobility Management

Entity)

MSC (Mobile Switching Centre)

Msg3 (Higher Layer Message)

MU-MIMO (Multi User - MIMO)

N

NACK (Negative

Acknowledgement)

NAS (Non Access Stratum)

NDI (New Data Indicator)

O

OFDM (Orthogonal Frequency

Division Multiplexing)

OFDMA (Orthogonal Frequency

Division Multiple Access)

P

PAPR (Peak to Average Power

Ratio)

PBCH (Physical Broadcast

Channel)

PCCH (Paging Control Channel)

PCFICH (Physical Control

Format Indicator Channel)

PCH (Paging Channel)

PCS (Personal Communications

Service)

PDCCH (Physical Downlink

Control Channel)

PDCP (Packet Data Convergence

Protocol)

PDN-GW (Packet Data Network -

Gateway)

PDSCH (Physical Downlink

Shared Channel),

PF (Paging Frame)

P-GSM (Primary GSM)

PH (Power Headroom),

PHICH (Physical Hybrid ARQ

Indicator Channel)

PHR (Power Headroom Report),

2-85

PHY (Physical Layer)

PL (Pathloss)

PLMN (Public Land Mobile

Network)

PMI (Precoding Matrix Indicator)

PO (Paging Occasion)

PRACH (Physical Random

Access Channel)

PRB (Physical Resource Block)

PS (Packet Switched)

P-S (Parallel to Serial)

PSS (Primary Synchronization

Signal)

PTM (Point-To-Multipoint)

PTP (Point-To-Point)

PUCCH (Physical Uplink Control

Channel)

PUSCH (Physical Uplink Shared

Channel)

Q

QoS (Quality of Service)

QPP (Quadratic Permutation

Polynomial)

QPSK (Quadrature Phase Shift

Keying)

R

R (Cell Ranking)

RA (Random Access)

RACH (Random Access Channel)

RAN (Radio Access Network)

RAPID (Random Access

Preamble Identifier)

RA-RNTI (Random Access -

RNTI)

RB (Radio Bearer)

RB (Resource Block)

RBG (Resource Block Groups)

RE (Resource Element)

REG (Resource Element Group)

R-GSM (Railways GSM)

RI (Rank Indication)

RIV (Resource Indication Value)

RLC (Radio Link Control)

RNC (Radio Network Controller)

RRC (Radio Resource Control)

RS (Reference Signals)

RSRP (Reference Signal Received

Power)

RSRQ (Reference Signal

Received Quality)

RSSI (Received Signal Strength

Indicator)

RV (Redundancy Version)

S

S (Cell Selection)

5 Glossary

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SAW (Stop And Wait)

SC-FDMA (Single Carrier -

Frequency Division Multiple

Access)

SFBC (Space Frequency Block

Coding)

SFN (System Frame Number),

S-GW (Serving Gateway)

SI (System Information)

SIB (System Information Block)

SIB 1 (System Information Block

Type1)

SI-RNTI (System Information -

Radio Network Temporary

Identifier)

SM (Spatial Multiplexing)

SMS (Short Message Service)

S-P (Serial to Parallel)

SR (Scheduling Request)

SRB (Signaling Radio Bearer)

SRS (Sounding Reference Signal)

SSS (Secondary Synchronization

Signal)

STC (Space Time Coding)

SU-MIMO (Single User - MIMO)

T

TA (Timing Alignment)

TAC (Tracking Area Code)

TACS (Total Access

Communications System)

TAI (Tracking Area Identity)

TB (Transport Block)

TBS (Transport Block Set)

TBS (Transport Blok Size)

TD (Transmit Diversity)

TD-CDMA (Time Division

CDMA)

TDD (Time Division Duplex)

TDMA (Time Division Multiple

Access)

TD-SCDMA (Time Division

Synchronous CDMA)

TF (Transport Format)

TFT (Traffic Flow Template)

TM (Transparent Mode)

TPC (Transmit Power Control)

TPMI (Transmitted Precoding

Matrix Indicator)

TTI (Time Transmission Interval)

TX (Transmit)

U

UCI (Uplink Control Information)

UE (User Equipment)

UL (Uplink)

UL-SCH (Uplink Shared

Channel)

UM (Unacknowledged Mode)

UMB (Ultra Mobile Broadband)

UpPTS (Uplink Pilot Time Slot)

USIM (Universal Subscriber

Identity Module)

V

VRB (Virtual Resource Block)

W

WCDMA (Wideband CDMA)

WiMAX (Worldwide

Interoperability for Microwave

Access)

Z

ZC (Zadoff-Chu)

LTE Air Interface




5-5

LTE air interface

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