GSM Fundamentals RF

GSM Fundamentals & RF

What you will learn here?• GSM Network Architecture• GSM channel Architecture• GSM Optimization Steps ( Performance, Drive testing &

Benchmarking)• Parameters Introduction• Case Study

Radio InterfaceAnother MSC

HLR/AUCSMC

PSTN ISDN

OMC

MS

Um

MS

A-bis

BSC

A

BTS

MSC/VLR

Power

Frequency

Time

FDMA

Frequency

Power Time

TDMA

FrequencyCDMA

PowerTime

Multiple Access Methods

Frequency Reuse Pattern

Three types of frequency reuse patterns

• 7 Cell reuse pattern

• 4 cell reuse pattern

• 3 cell reuse pattern

3 Site Reuse Patternc2

c1c3

a1a2

a3

b1

b2

b3

c1c2

c3Cell Re-use

Channels in GSM Air interface

Logical Channels

Control Channels Traffic Channels

Full rate => Used for speech at 13 Kbits/s

or sending data at 9.6 Kbits/s

Half rate => Used for speech at 6.5 Kbits/s or sending data at 4.8 Kbits/s

Enhanced Full rate => Used for speech at 13 Kbits/s

or sending data at 9.6 Kbits/s but with almost Land line quality

TCH = TRAFFIC CHANNEL

Broadcast control channel (BCH)

Control channelCommon control channel (CCCH)

Voice channel(TCH)

FCCH SCH BCCH(system information)

TCH/FAGCH RACH SDCCH FACCH

SACCH

TCH/HPCH

Common channel (CCH)

Dedicated channel (DCH)

Logical channel

Logical Channel Type• GSM900 and DCS1800 have the same logical channel category

FCCHSCHBCCH

PCH

AGCH

BCCH

CCCH

Common Channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated Channel

Downlink Logical Channel

RACH CCCH

Common channel

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

Dedicated channel

Uplink Logical Channel

Use of Logical Channels

FCCH

Allocate signaling channel

Power-on Search for frequency correction burst

Search for synchronous burst

Listen to the system information

Monitor paging message

Send access burst

Set up the call

Allocate voice channel

Conversation

Release the call

Idle mode

SCHBCCH

PCH

RACHAGCHSDCCHSDCCHTCHFACCH

Dedicated mode

Idle mode

Network Identity Parameter — CGI

• Network identity parameters mainly include cell global identity (CGI) and base station identity code (BSIC).

CGI=MCC+MNC+LAC+CI

• Once MS receives SYS INFO, it decodes the CGI information, and decides whether it can stay in the cell according to the MCC and MNC indicated by CGI. At the same time, it judges whether the current location area is changed, so as to decide whether to execute location update. During the location update process, MS will report the new LAI to the network, so that the network can know the LA in which MS is currently located.

Mobile Country Code （ MCC）

• Definition– MCC consists of 3 decimal numbers. It

indicates the home country of the mobile subscriber.

• Format– MCC is composed of 3 decimal numbers. The

coding range is decimal 000-999.

Mobile Network Code （ MNC）

• Definition– MNC is used to uniquely identify a specific

GSM PLMN network in a certain country (decided by MCC).

• Format– MNC is composed of two decimal numbers.

The coding range is decimal 00-99.

Location Area Code （ LAC）• Definition

– To locate the location of MS, the whole area covered by each GSM PLMN is divided into different location areas. LAC is used to identify different location areas.

• Format– LAI contains LAC, which is composed of two bytes. LAC

adopts hexadecimal coding. The available range is from 0001H to FFFEH. The code 0000H and FFFFH cannot be used (please refer to specification GSM0303, 0408, and 1111). One location area can contain one or more cells.

Cell Identity （ CI）• Definition

– To uniquely identify each cell in the GSM PLMN, the network operator needs to allocate one code for each cell, which is the cell identity (CI). Cell identity, together with LAI, is used for identity of each cell in the world. (specification 0303).

• Format– CI is composed of 16 bits, The available range is

0~65535.

Network Identity Parameter — BSIC

• BSIC=NCC+BCC• In GSM system, each BTS is allocated with a color code, which is

called BSIC. MS can identify two cells with the same BCCH by the help of BSIC. In network planning, effort should be made to make sure that BCCH of neighbor cells are different from the serving cell’s BCCH to reduce the interference. Practically it is still possible that a same BCCH is re-used in the surrounding cells. For cells using the same BCCH in a relevant near distance, their BSIC must be different so that MS can identify two neighbor cells with same BCCH.

Network Color Code （ NCC）• Definition

– NCC is a part of BSIC. MS uses it to distinguish adjacent BTS that belong to different GSM PLMN.

• Format– NCC is composed of 3 bits, with the range of 0

to 7.

BTS Color Code （ BCC）• Definition

– BCC is a part of BSIC. For its function, please refer to above sections.

• Format– BCC is composed of 3 bits. The available

range is from 0 to 7.

Mobile Maximum Range

• Range = (Timing advance * bit period * velocity) / 2

• Range = (63 * (3.693 * 10e-6) * (3 * 10e5))/2

• So range is 34.9 Kms.

Example: Impact of PC & DTX

1.00

1.50

2.00

2.50

3.00

3.50

1.00000% 2.00000% 3.00000% 4.00000% 5.00000% 6.00000% 7.00000% 8.00000%

Effective Frequency Load

DC

R (%

)

Non BCCH RF Hopping 2/2

Non BCCH RF Hopping 2/2 DLPC

Non BCCH RF Hopping 2/2 DLPC DTX

No Power Control no DTX

Power ControlPower Control & DTX

100% more speech traffic with the same DCR

Real Netw

ork

Hopping principlesHopping principles

Frequency

F3

F2

F1

RANDOM

TDMA frame

CYCLIC

cycle

Call is transmitted through several frequencies in order to

• average the interference (interference diversity)

• minimise the impact of fading (frequency diversity)

1 2 3 4 5 6 etc.

BB-FH vs. RF-FH

Flexibility• hopping with small configurations and small BW is possible

Maximum FH gain or easy allocation possible• hopping enabled over large number of frequencies

Number of antennas• in large configurations the number of antennas increases if AFE used

HW dependencies• old BTS generations do not support RF-FH• wideband antenna coupling equipment needed (AFEs)

Less HW restrictions• supported by all BTS generations• all antenna combining methods feasible (RTC or AFE)

Minimum number of antennas requiredEasy to Implement (switch on)

•existing planning tools•the same interferers and frequency plan than with IUO

Limitations with small configurations (<3 TRX) and small BW• hopping may not be possible on both layers (IFH)

DL PC• limited PC range in downlink (mobile AGC problems)

BB-FH

RF-FH

No simple answer, selection on case by case basis

BB Hopping ManagementBB Hopping Management

BCCH 1 2 3 764

0 1 2 3 764

0 1 2 3 764

0 1 2 3 764

TRX-1

TRX-2

TRX-3

TRX-4

RTSL-0 RTSL-2RTSL-1 RTSL-4RTSL-3 RTSL-5 RTSL-6 RTSL-7

f1

f2

f3

f4

BCCH timeslot, does not hop.

5

5

5

5

Timeslot 0 of TRXs 2-4 hop over MA(f2,f3,f4).This hopping group uses HSN-1

All timeslots 1-7 hop over MA(f1,f2,f3,f4).This hopping group uses HSN-2

Multipath Fading results in variations in signal strength which is known as Rayleigh Fading.

Rayleigh Fading phenomenon is dependent on path difference and hence frequency of reception.

A fast moving mobile may not experience severe effect of this fading since the path difference is continuously changing.

A slow moving mobile ( or a halted mobile ) may experience severe deterioration in quality.

But, if the frequency of reception is changed when this problem occurs, could solve it.

The fading phenomenon is fast and almost continuos, this means the frequency change should also be continuos.

This process of continuously changing frequency is known as Frequency Hopping.

Frequency Hopping

RF Hopping ManagementRF Hopping Management

BCCH 1 2 3 764

0 1 2 3 764

0 1 2 3 764

0 1 2 3 764

TRX-1

TRX-2

TRX-3

TRX-4

RTSL-0 RTSL-2RTSL-1 RTSL-4RTSL-3 RTSL-5RTSL-6 RTSL-7

f1

BCCH TRX does not hop.

5

5

5

5

MAIOs are differentfor different TRXswithin the samehopping group-> no collisions.

MA1 = {f2, f3, f4,..}

HSN-1

AMR INTRODUCTION

AMR Introduction• AMR (Adaptive Multi Rate) consists of a family of codecs with

different bit-rates operating in GSM FR and HR• The aim is to improve channel (FR/HR) quality by adapting the most

appropriate channel codec based on current radio condition• Codec mode adaptation (link adaptation) is based on received channel

quality in both MS and BTS (the codec adaptation possible each 40ms)• The basic AMR codec mode sets for MS and BTS are provided by BSC

via layer 3 signalling• MS shall support all speech codec modes, although only a set of up to 4

speech codec modes are used during a call• AMR Capacity benefits:

– Improved robustness in Full Rate allows tightening of re-use patterns - increased spectral efficiency

– Operation of Half Rate channels will free available capacity for data traffic - reduced blocking

• AMR Coverage benefits:– Improved performance at cell edge

AMR Speech Codec

Voice quality

Robustness

AMR codecs:8 for Full rate and 6 for Half Rate:

1.0

2.0

3.0

4.0

5.0

No Errors 16 dB C/I 13 dB C/I 10 dB C/I 7 dB C/I 4 dB C/I

MOS (Mean Opinion Score)

EFRAMR FR

AMR Full Rate performance compared to Full Rate EFR in Clean Speech

1.0

2.0

3.0

4.0

5.0

No Errors 19 dB C/I 16 dB C/I 13 dB C/I 10 dB C/I 7 dB C/I 4 dB C/I

FRAMR HR

MOS (Mean Opinion Score)

AMR Half Rate performance compared toFull Rate in Clean Speech

Full Rate Half rateEFR 12.2

10.27.957.95 (*)

IS 136 7.47.46.76.75.95.95.155.154.754.75Speech bit rate

(*) Requires 16 kbit/s TRAUNot supported by Nokia BSS

Adaptive Multi Rate• Channel cross bit-rate is 22.8 kbit/s in GSM FR/EFR: 13 kbit/s speech

coding and 9.8 kbit/channel coding (HR channel gross bit rate 11.4 kbit/s)• In the AMR case, codec mode can be changed and more error correction

bits can be used whenever channel requires

0

5

10

15

20

25

FR12.2

FR10.2

FR7.95

FR 7.4 FR 6.7 FR 5.9 FR5.15

FR4.75

HR7.95

HR 7.4 HR 6.7 HR 5.9 HR5.15

HR4.75

AMR codec mode

Cha

nnel

bit-

rate

(kbi

t/s)

Channel codingSpeech coding

AMR Codec Modes

Channel mode

Channel codec Mode

Source coding bit-rate, speech

Net bit-rate, in-band channel

Channel coding bit-rate, speech

Channel coding bit-rate, in-band

CH0-FS 12.20kbit/s (GSMEFR) 0.10 kbit/s 10.20 kbit/s 0.30 kbit/s CH1-FS 10.20 kbit/s 0.10 kbit/s 12.20 kbit/s 0.30 kbit/s CH2-FS 7.95 kbit/s 0.10 kbit/s 14.45 kbit/s 0.30 kbit/s

TCH/FR CH3-FS 7.40 kbit/s (IS-641) 0.10 kbit/s 15.00 kbit/s 0.30 kbit/s CH4-FS 6.70 kbit/s 0.10 kbit/s 15.70 kbit/s 0.30 kbit/s CH5-FS 5.90 kbit/s 0.10 kbit/s 16.50 kbit/s 0.30 kbit/s CH6-FS 5.15 kbit/s 0.10 kbit/s 17.25 kbit/s 0.30 kbit/s CH7-FS 4.75 kbit/s 0.10 kbit/s 17.65 kbit/s 0.30 kbit/s

CH8-HS 7.95 kbit/s (*) 0.10 kbit/s 3.25 kbit/s 0.10 kbit/s TCH/HR CH9-HS 7.40 kbit/s (IS-641) 0.10 kbit/s 3.80 kbit/s 0.10 kbit/s

CH10-HS 6.70 kbit/s 0.10 kbit/s 4.50 kbit/s 0.10 kbit/s CH11-HS 5.90 kbit/s 0.10 kbit/s 5.30 kbit/s 0.10 kbit/s CH12-HS 5.15 kbit/s 0.10 kbit/s 6.05 kbit/s 0.10 kbit/s CH13-HS 4.75 kbit/s 0.10 kbit/s 6.45 kbit/s 0.10 kbit/s

Table: Channel and speech codec modes for AMR

(*) Requires 16 kbit/s TRAU. Therefore it is not seen as a feasible codec mode and will not be supported by Nokia.

AMR Link adaptation

Codec mode adaptation• Codec mode changed according

to channel conditions (UL/DL C/I)

• Only up to four codecs can be used during a call

• Goal—the highest MOS (Mean Opinion Score)

• Mode indications inform the receiver about the currently applied codec mode

• Mode Command informs MS about the codec mode to be applied on the uplink

Channel mode adaptation

• FR <-> HR changed by handover (packing and unpacking)

• Based on BTS load (BSC level) and channel condition (RxQual)

AMR FR codec mode adaptation example

speech coding

chan. coding

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Time[s]

[dB

]C/I EFR operation AMR mode

12.2 kbit/s

7.95 kbit/s

6.70 kbit/s

5.90 kbit/s

AMR Mode

C/I

AMRFR HR12.210.27.957.4 7.46.7 6.75.9 5.95.15 5.154.75 4.75

Benefits of AMR 1/2• Speech quality enhancement: AMR maintains good speech quality in

the situation where the connection faces low C/I or low signal level• Capacity and coverage gain: Link level simulation results illustrated

improvement in terms of TCH FER (up to 6dB at 1% FER in C/I)• Signalling channel performance: due to retransmissions schemes used

by these channels the probability of signalling success maintain very high even for very degraded conditions

• Improved BCCH plan: tighter frequency reuse or better quality with same frequency reuse, potentially releasing frequencies to be used on the non-BCCH layer. This is applicable when EFR legacy terminals do not represent a significant portion of the traffic.

Benefits of AMR 2/2• Mixed EFR – AMR traffic networks: use more aggressive power control for

AMR mobiles in order to decrease the average interference level in the network. Due to better error correction capability against the channel errors lower C/I target can be set for AMR mobiles hence lower PC thresholds can be used. Therefore, the overall interference decreases in the network (smaller average transmission power) and thus the quality of the existing EFR connections increase

• HR utilisation increases the hardware capacity of the cell since two half-rate connections can be allocated to fill only one timeslot.

• When compare AMR HR to previous GSM HR codec, it is noticed that AMR HR obtains remarkable better speech quality

DISCUSSION & QUESTIONS

Frequency Hopping is done in both Uplink and Downlink . Frequency is changed in every TDMA Frame Mobile can Hop on maximum 64 frequencies The sequence of Hopping can be Cyclic or Non-Cyclic There are 63 Non-Cyclic Hopping sequences possible Different Hopping sequence can be used in the same cell.

BCH Timeslot can never HOP, but the remaining Timeslots can very well hop.

Frequency Hopping

Reduction in Average Interference With Frequency Hopping consistent interference will become bursty. So even though, both the co-channel cells will be using the same set of ARFCN's for

Hopping, interference will not be continuos. This is because, GSM cells are not Frame synchronized, and change in frequency is

related to Frame nos. If same HSN is used in two cells, then either the interference will be nil , or if a phase

correlation exists then it will be continuos. So the two cells should preferably use different HSN's . Sectorial cells ( controlled by the same BTS) can use same HSN, since the sectors don't

come up at the same time. Cells if they are synchronized, can use same HSN, if each cell has an offset of some

TDMA frames. Offset of TDMA frames is also required to avoid SACCH occurring at the same time in all

synchronized cells, as they kills away the objective of DTX.

Frequency Hopping

HandoversHard Handoff

Analog, TDMA and GSMSoft Handoff

CDMA

Break before Make Make before Break

Cleaner Handovers• The mobile measures up to 32 adjacent cells for

– Signal Strength (RxLevel)– Signal Quality (RxQual)– updated every 480 mS and sends to BTS

• Sophisticated Handover based on– RxLevel– Interference– RxQual– Timing Advance– Power Budget

BTS

BTS

BTS

BTS

BTS

BTS

BTS

BTS

BSC

VLR

HLR EI

R

OMC

SMSCB

C

AUC

VMSC

MSC

Abis

A

MS

BTS

BTS

BTS

BTS BTS

BTS

BTS

BTS

BSC

BSC

PSTN

VLR

TRAU HLREIR

OMC

SMSC

BC

AUC

VMSC

MSC

Abis

A

OML

GSM NETWORK ELEMENTS

GSM Network Components• Mobile Station consists of two parts-

– Mobile Equipment (ME)– Subscriber Identity Module (SIM)

• ME – Hardware e.g. Telephone, Fax Machine, Computer.

• SIM– Smart Card which plugs into the ME.

Channels On Air Interface• Physical Channel • Logical Channel

• Physical Channel– Physical channel is the medium over which the

information is carried.

• Logical Channel– Logical channels consists of the information carried

over the Physical Channel.

GSM Channels

Traffic Channel

TCH carries payload data - speech, fax, data

• Connection may be:

- Circuit Switched - voice or data or - Packet Switched – data

• TCH may be:

• Full Rate (TCH/F)

- one channel per user

- 13 kb/s voice, 9.6 kb/s data or

• Half Rate (TCH/H)

- one channel shared between two users

- 6.5 kb/s voice, 4.8 kb/s data

Traffic Channels

TCH/FFull rate 22.8kbits/s

TCH/HHalf rate 11.4 kbits/s

• Time is divided into discrete periods called “Timeslots”

Control Channel

DCCH(Dedicated Channels)Downlink & Uplink

CCCH(Common Control Chan)Downlink & Uplink

Synch.Channels

RACHRandom

Access ChannelCBCH

Cell Broadcast Channel

SDCCHStandalone dedicated

control channel

ACCHAssociated

Control Channels

SACCHSlow associated Control Channel

FACCHFast Associated

Control Channel

PCH/AGCH

Paging/Access grant

FCCHFrequency

Correction channel

Control Channels

BCH ( Broadcast channels )Downlink only

BCCHBroadcast

control channel

SCHSynchronization

channel

Broadcast Channels (BCH)

BCH channels are all downlink and are allocated to timeslot zero.

Channels are:

• FCCH: Frequency control channel sends the mobile a burst of all ‘0’ bits which allows it to fine tune to the downlink frequency

• SCH: Synchronization channel sends the absolute value of the frame number (FN), which is the internal clock of the BTS, together with the Base Station Identity Code (BSIC)

• BCCH: Broadcast Control Channel sends radio resource management and control messages, Location Area Code and so on.

Some messages go to all mobiles, others just to those that are in the idle state

Common Control Channels (CCCH)

CCCH contains all point to multi-point downlink channels (BTS to

several MSs) and the uplink Random Access Channel:

• CBCH: Cell Broadcast Channel is an optional channel for general information such as road traffic reports sent in the form of SMS

• PCH: Paging Channel sends paging signal to inform mobile of a call

• RACH: Random Access Channel is sent by the MS to request a channel from the BTS or accept a handover to another BTS.

A channel request is sent in response to a PCH message.

• AGCH: Access Grant Channel allocates a dedicated channel (SDCCH) to the mobile

• NCH: Notification Channel informs MS about incoming group or

broadcast calls

Dedicated Control Channels (DCCH)

SDCCH( Standalone Dedicated Control Channel )

Uplink and Downlink

Used for call setup, location update and SMS.

SACCH( Slow Associated Control Channel )

Used on Uplink and Downlink only in dedicated mode.

Uplink SACCH messages - Measurement reports.

Downlink SACCH messages - control info.

FACCH( Fast Associated Control Channel )

Uplink and Downlink.

Associated with TCH only.

Call Scenarios

• Mobile to Mobile– Intra-city – Inter-city

• Mobile to Land– Intra-city – Inter-city

• Land to Mobile– Intra-city – Inter-city

Mobile To Land Sequence

1

3

CHANNEL REQUEST

DCCH ASSIGN

SIGNALLING LINKESTABLISHED

REQUEST FOR SERVICE

SET CIPHER MODE

SET-UP

EQUIPMENT ID REQUEST

AUTHENTICATION

MS BSS MSC VLR HLR PSTNEIR

RACH

AGCH

SDCCH

SDCCH

Call Info7

4

6

5

2CR

CC

8 COMPLELTE CALLCALL PROCEEDING

9 ASSIGNMENT COMMAND

INITIAL & FINALADDRESS (IFAM)

ASSIGNMENT COMPLETE (ACM)

10

ANSWER(ANS)

11

CONNECT ACKNOWLEDGE

SDCCH

SDCCH

ASSIGNMENT COMPLELTE

MS HEARS RINGTONEFROM LAND PHONE

ALTERING

RING TONESTOPS

CONNECT

(channel)

(TCH)

FACCH

FACCH

FACCH

TCH

(circuit)

FAACH

BILLING STARTS

Hello!

MS BSS MSC

VLR HLR PSTN EIR

Call Contt.

Customer..Expectation

• Good coverage – where ever he goes• Good quality• No blocking• Value added services

– SMS– Voice mail– MMS– Call forward/call waiting– Data/internet at high data rates– prepaid

Basic Network Design Objectives

The basic objectives of a wireless system are:– COVERAGE: provide sufficient cell sites to deliver RF coverage

of the entire desired area.– BUILDING/VEHICLE PENETRATION: deliver sufficient signal

levels to adequately penetrate buildings and vehicles where appropriate.

– TRAFFIC: ensure that no cell captures more traffic than it can handle at the desired grade of service (i.e., blocking percentage)

– PERFORMANCE: design, construct, and adjust the network to deliver reliable service free from excessive origination and call delivery failures, dropped calls, quality impairments, and service outages.

– ECONOMICS: provide return on investment sufficient to support operating and capital expenses, expand the network to take advantage of growth opportunities, and retire costs of construction prior to depreciation of the network equipment.

What is Performance Optimization?

• The words “performance optimization” mean different things to different people, viewed from the perspective of their own jobs

• System Performance Optimization includes many different smaller processes at many points during a system’s life– recognizing and resolving system-design-related issues (can’t

build a crucial site, too much overlap/soft handoff, coverage holes, etc.)

– “cluster testing” and “cell integration” to ensure that new base station hardware works and that call processing is normal

– “fine-tuning” system parameters to wring out the best possible call performance

– identifying causes of specific problems and customer complaints, and fixing them

– carefully watching system traffic growth and the problems it causes - implementing short-term fixes to ease “hot spots”, and recognizing problems before they become critical

Optimization• Optimisation is an ongoing process of analysing network performanceagainst Quality of Service targets:

Performance•Measurements of network performance cover:

• Traffic in erlangs

• TCH and SDCCH Grade of Service (Congestion)

• Call success rate

• Handover failure

• Coverage area

• Coverage quality

• Subscriber base and growth

• Key Performance Indicators (KPI) are measurable dynamic

parameters that help to target areas of concern

KPI’s• Appropriate KPIs to use depend on:

• The nature of the network• Data sources available• Measurement tools available• Ability of engineering team• Cost of network infrastructure

• Sources of data include:• Surveyed data - from drive tests• Network statistics - from OMC • Field engineer reports

Radio Interface Optimization• Power Control

• Diversity

• Frequency Hopping

• Antenna Parameters ( Height, Azimuth, Tilts )

Antenna Tilts

Benchmarking•Surveyed data from test-mobile measurements can be used to

benchmark system performance against that of a competitor

• Problems that may be identified from surveyed data:

• Poor coverage

• Unexpected interference

• Missing handover definitions

• Installation problems at BTS

• Test-mobile measurements should include:

• continuous calls to test coverage

• repetitive short calls to test call-success

GSM Fundamentals RF

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