Post on 11-Mar-2020
Mobile and Wireless Networking 2013 / 2014
192620010 Mobile & Wireless Networking
Lecture 5: Cellular Systems (UMTS / LTE) (1/2)
[Schiller, Section 4.4]
Geert Heijenk
Mobile and Wireless Networking 2013 / 2014
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Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2) q Evolution of cellular systems q GSM
l GSM Network Architecture l GSM radio interface l GPRS l EDGE
q 3G UMTS l UMTS Network Architecture l Wideband CDMA
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Source: Agilent Technologies, 2012
Evolution of cellular systems
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GSM Architecture
fixed network
BSC
BSC
MSC MSC
GMSC
OMC, EIR, AUC
VLR
HLR NSS with OSS
RSS
VLR
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1 2 3 4 5 6 7 8
higher GSM frame structures
935-960 MHz 124 channels (200 kHz) downlink
890-915 MHz 124 channels (200 kHz) uplink
time
GSM TDMA frame
GSM time-slot (normal burst)
4.615 ms
546.5 µs 577 µs
tail user data Training S guard space S user data tail
guard space
3 bits 57 bits 26 bits 57 bits 1 1 3
GSM Radio Interface: TDMA/FDMA
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GPRS (General Packet Radio Service) q packet switching q using free slots only if data packets ready to send q (~reservation Aloha) q Few changes to base station
(software) q New core network architecture
(router-based)
Class Receiving slots Sending
slots Maximum number of slots
1 1 1 2 2 2 1 3 3 2 2 3 5 2 2 4 8 4 1 5
10 4 2 5 12 4 4 5
Coding scheme 1 slot 2 slots 3 slots 4 slots 5 slots 6 slots 7 slots 8 slots CS-1 9.05 18.2 27.15 36.2 45.25 54.3 63.35 72.4 CS-2 13.4 26.8 40.2 53.6 67 80.4 93.8 107.2 CS-3 15.6 31.2 46.8 62.4 78 93.6 109.2 124.8 CS-4 21.4 42.8 64.2 85.6 107 128.4 149.8 171.2
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GPRS architecture and interfaces
MS BSS GGSN SGSN
MSC
Um
EIR
HLR/ GR
VLR
PDN
Gb Gn Gi
SGSN
Gn
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EDGE
EDGE (Enhanced Data rates for GSM Evolution): q New modulation technique: 8PSK instead of GMSK (bitrate x3) q Can be combined with GPRS q Adaptive Modulation and Coding q Incremental Redundancy
(Hybrid ARQ) q New BS hardware
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Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2) q Evolution of cellular systems q GSM
l GSM Network Architecture l GSM radio interface l GPRS l EDGE
q 3G UMTS l UMTS Network Architecture l Wideband CDMA
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UMTS architecture (original release (R99))
UTRAN UE CN
Iu Uu
UTRAN (UMTS Terrestrial Radio Access Network) q Cell level mobility q Radio Network Subsystem (RNS) q Encapsulation of all radio specific tasks
UE (User Equipment) CN (Core Network)
q Inter system handover q Location management if there is no dedicated connection between
UE and UTRAN
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UTRAN architecture
UTRAN comprises several RNSs
Node B can support FDD or TDD or both
RNC is responsible for handover decisions requiring signalingto the UE
Cell offers FDD or TDD
RNC: Radio Network Controller RNS: Radio Network Subsystem
Node B
Node B
RNC
Iub
Node B
UE1
RNS
CN
Node B
Node B
RNC
Iub
Node B
RNS
Iur
Node B
UE2
UE3
Iu
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Core network: architecture
BTS
Node B
BSC
Abis
BTS
BSS
MSC
Node B
Node B
RNC
Iub
Node B RNS
Node B SGSN GGSN
GMSC
HLR
VLR
IuPS
IuCS
Iu
CN
EIR
Gn Gi
PSTN
AuC
GR
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UMTS Protocol Architecture - User Plane
UMTS UMTS Transport Network Legend:
UE IuPS Iub Gn Gi GGSN
IP
UDP
IP
GTP-U
PHY
MAC
RLC
PDCP
Node B Uu
UDP UDP
IP
SGSN
GTP-U
IP
IP
TCP
RLC
RNC
IP
UDP
MAC
PDCP GTP-U
PHY L2
App
FP FP
L1
L2
L1
L2
L1
L2
L1
L2
L1
L2
L1
IP
L2
L1
TCP
App
L2
L1
Host
Internet Other
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UMTS Protocol Architecture – Control Plane
UMTS UMTS Transport Network Legend:
UE IuPS Iub
Signalling Bearer
SCCP
UMM/SM
RANAP
PHY
MAC
RLC
RRC
Node B Uu SGSN
UMM/SM
RLC
RNC
Signalling Bearer
SCCP
MAC
RRC RANAP
PHY L2
NBAP NBAP
L1
L2
L1
L2
L1
L2
L1
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Outline of Lecture 5
Cellular Systems (UMTS / LTE) (1/2) q Evolution of cellular systems q GSM
l GSM Network Architecture l GSM radio interface l GPRS l EDGE
q 3G UMTS l UMTS Network Architecture l Wideband CDMA
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Wideband CDMA
Direct Sequence CDMA, also known as Wideband CDMA
Chip rate 3.84 Mc/s Carrier spacing 5 MHz
Channel coding
Transport channels
Multiplexing
Mapping to physical channels
Spreading Spreading
Physical channels
Physical-layer procedures
and measurements
Channel coding
≈5 MHz
Modulation Modulation
3.84 Mc/s
Transport-channel processing
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How do we spread the data?
The operation of spreading in a CDMA system is divided into two separate parts q Spreading code = Scrambling code + Channelization code
Scrambling
q Separates different mobiles (in uplink) and different cells/sectors (in downlink)
Channelization q Separates different physical channels that are transmitted on the
same scrambling code q The purpose of channelization is most evident in the downlink
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Spreading and scrambling of user data
Constant chipping rate of 3.84 Mchip/s Different user data rates supported via different spreading factors
q higher data rate: less chips per bit and vice versa User separation via unique, quasi orthogonal scrambling codes
q users are not separated via orthogonal spreading codes q much simpler management of codes: each station can use the same
orthogonal spreading codes q precise synchronisation not necessary as the scrambling codes stay quasi-
orthogonal data1 data2 data3
scrambling code1
spr. code3
spr. code2
spr. code1
data4 data5
scrambling code2
spr. code4
spr. code1
sender1 sender2
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Orthogonal Variable Spreading Factor (OVSF) coding
1
1,1
1,-1
1,1,1,1
1,1,-1,-1
X
X,X
X,-X 1,-1,1,-1
1,-1,-1,1 1,-1,-1,1,1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
1,-1,1,-1,1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,1,-1,-1,1,1,-1,-1
1,1,-1,-1,-1,-1,1,1
1,1,1,1,1,1,1,1
1,1,1,1,-1,-1,-1,-1
SF=1 SF=2 SF=4 SF=8
SF=n SF=2n
...
...
...
...
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UMTS FDD frame structure
W-CDMA • 1920-1980 MHz uplink • 2110-2170 MHz downlink • chipping rate: 3.840 Mchip/s • soft handover • QPSK • complex power control (1500 power control cycles/s) • spreading: UL: 4-256; DL:4-512
0" 1" 2" 12" 13" 14"..."
Radio frame"
Pilot" FBI" TPC"
Time slot"
666.7 µs"
10 ms"
Data"
Data1"
uplink DPDCH"
uplink DPCCH"
downlink DPCH"TPC"TFCI" Pilot"
666.7 µs"
666.7 µs"
DPCCH"DPDCH"
2560 chips, 10 bits"
2560 chips, 10*2k-1 bits (k = 1...7)"
TFCI"
2560 chips, 10*2k bits (k = 0...7)"
Data2"
DPDCH" DPCCH"FBI: Feedback Information TPC: Transmit Power Control TFCI: Transport Format Combination Indicator DPCCH: Dedicated Physical Control Channel DPDCH: Dedicated Physical Data Channel DPCH: Dedicated Physical Channel Slot structure NOT for user separation
but synchronisation for periodic functions!
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Channel bit rate [kbps] User bit rate (bef. coding) [kbps] k
Spreading factor Uplink Downlink Uplink Downlink
0 512 N/A 15 kbps N/A 6 kbps
1 256 15 kbps 30 kbps 15 kbps 24 kbps
2 128 30 kbps 60 kbps 30 kbps 51 kbps
3 64 60 kbps 120 kbps 60 kbps 90 kbps
4 32 120 kbps 240 kbps 120 kbps 210 kbps
5 16 240 kbps 480 kbps 240 kbps 432 kbps
6 8 480 kbps 960 kbps 480 kbps 912 kbps
7 4 960 kbps 1920 kbps 960 kbps 1872 kbps
Bit rates and Spreading Factors
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Fading
Path loss – fading due to distance q 1/distanceα (α between 3 and 4)
Long term (slow) fading – caused by shadowing q Log-normal
Short term (fast) fading – caused by multipath propagation q Rayleigh fading amplitude
Signal level (dB) Path loss
Long term fading
Distance (log) Short term fading
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Purpose of Power Control
Goal q mobile station transmitted power is controlled such that all users in
the cell experience the same SIR (Signal to Interference Ratio) at the base station receiver
Open Loop (initial power setting) q compensate for pathloss and slow fading q uses downlink pilot channel
Closed Loop (fast power control) q compensates also for fast fading q needs dedicated downlink control channel for power control
commands
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Dynamic Range of Power Control
PI PC
Worst case: PC(dB) – PI(dB) = – 80 dB! Interferers are rejected by the processing gain:
Power control with a large dynamic range is essential!
G = = = 100 → 20 dB Rchip Rbit
106 104
⇒ = – 80 + 20 = – 60 dB! C I
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Why Soft Handover?
Soft handover essential for power control Soft handover reception
q combines signals from different base stations
BS 1 BS 2
RNC
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Time Dispersion – Rake receiver – Channel Estimation
τ1 τ2
h1
h0 h2
Channel
Diversity Combination Selective Equal gain Maximum Ratio
Channel Estimation Delay Delay Delay and complex amplitudes
a2 a1 a0
0 0 1
1/3 1/3 1/3
h2* h1* h0*
g g g
C(n) τ2 τ1
C(n) C(n)
a2 a1 a0
r(n)
Diversity Combination
a0
a1
a2
To Decoder
τ1 τ2
τ2
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Mobile Soft Handover Implementation with Rake Receiver
BS 1 BS 2
g g
C2(n)
τ2 τ1
C1(n)
a1 a2
Diversity Combination To Decoder
C1(n) C2(n)
h1 h2 τ1 τ2
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Softer Handover
Softer handover reception q combines signals from one base station
BS
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One cell reuse is typical for CDMA
In CDMA, all cells use the same carrier frequency (frequency reuse = 1) q makes soft handover possible q requires efficient power control q makes system load control more complex
FDMA/TDMA (reuse > 1) CDMA (reuse = 1)
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Capacity
WCDMA capacity limited by q Amount of interference that can be tolerated q Amount of interference generated by each user q Amount of downlink orthogonal codes
Any reduction in generated interference directly improves capacity q Voice activity q Bursty transmission (packet-like services) q Narrow-beam antennas
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Resource Planning versus Power Planning
GSM (TDMA) q Frequency planning q Slot assignment
CDMA q Increased output power ⇒ increased interference ⇒ lower capacity q Power planning!
Reducing interference (by any means) ⇒ direct increase of capacity
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Cellbreathing
GSM q Users have their own dedicated time(/frequency) slot q Number of users in cell does not directly influence cell size
UMTS q Cellsize is closely related to cell capacity q Capacity is determined by signal to noise ratio q Interference adds to the noise:
l other cells l other users in the same cell
q If there is a lot of noise, users at the cell border cannot increase their signal any further à cannot communicate
q So: cell size decreases as number of active users increases: Cell breathing
q Number of active users should be limited q This complicates cell planning
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Cell breathing: example