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ECE DepartmentFlorida Institute of Technology

Wireless Data Communication Networks

Lecture 20: Introduction to GSM


Outline

GSM logical channelsRadio resource management


GSM Logical Channels

Logical channels – provide services to upper layers of the protocol stack

They belong to different layers (1-3) of the OSI model

GSM Logical Channels

TCH

TCH/F TCH/H

CCH

BCH CCCH DCCH

CBCH

ACCH SDCCH

FACCHSACCH

FCCH

SCH

BCCH

PCH

AGCH

RACH

TCH - Traffic Channel

TCH/F - Traffic Channel (Full Rate)

TCH/H - Traffic Channel (Half Rate)BCH - Broadcast Channels

FCCH - Frequency Correction Channel

SCH - Synchronization Channel

BCCH - Broadcast Control Channel

CCCH - Common Control Channels

PCH - Paging Channel

AGCH - Access Grant Channel

RACH - Random Access Channel

DCCH - Dedicated Control Channels

SDCCH - Stand-alone Dedicated Control ChannelACCH - Associated Control Channels

SACCH - Slow Associated Control Channel

FACCH - Fast Associated Control Channel

CCH - Control Channel

CBCH - Cell Broadcast Channel


Traffic Channel (TCH)

Traffic channel carries speech and user data in both directions

o Full rate ~ 33.85 Kb/seco Half rate ~ 16.93 Kb/seco Full rate uses 1 slot in every frameo Half rate uses 1 slot in every other frame

Data rates differ due to differences in Error Control Coding

Most contemporary GSM networks operate AMR vo-coders

Full Rate TCH can carry:• Voice (13 Kb/sec)• Data at rates:

- 9.6 Kb/sec- 4.8 Kb/sec- 2.4 Kb/sec

Half Rate TCH can carry:• Voice (6.5 Kb/sec)• Date at rates:

-4.8 Kb/sec- 2.4 Kb/secAdaptive Multi-Rate Coder (AMR)

• Voice 12.2, 10.2, 7.95 and 7.40 in FR• Voice 6.7, 5.9, 5.15, 4.75 in HR


Control Channels

GSM Defines 3 types of Control Channels:1. Broadcast Channels (BCH)

Broadcast information that helps mobile system acquisition, frame synchronization, etc. They advertise properties and services of the GSM network.

Forward link only2. Common Control Channels (CCCH)

Facilitate establishment of the link between MS and system

Both forward and reverse link3. Dedicated Control Channels (DCCH)

Provide for exchange the control information when the call is in progress

Both forward and reverse – in band signaling

CC H

BC H

C C C H

D C C H


Broadcast Channels (BCH) Three types of BCH:

1. Synchronization channel (SCH) Provides a known sequence that helps

mobile synchronization at the baseband

Communicates with S-burst Broadcasts Base Station Identity Code

(BSIC)

2. Frequency Correction channel (FCH) Helps mobile tune its RF oscillator Communicates with F-burst

3. Broadcast Control Channel (BCCH) Provides mobile with various information

about network, its services, access parameters, neighbor list, etc.

BC H

SC H

FC H

BC C H


Common Control Channel (CCCH)

Three types of CCCH:1. Random Access Channel (RACH)

Used by mobile to initialize communication

Mobiles use slotted ALOHA Reverse link only

2. Paging Channel (PCH) Used by the system to inform the mobile

about an incoming call Forward link only GSM Supports DRX

3. Access Grant Channel (AGC) Used to send the response to the mobiles

request for DCCH Forward link only

C C C H

R AC H

PC H

AG C


Dedicated Control Channels (DCCH)

Three types of DCCH:1. Stand Alone Dedicated Control Channel

(SDCCH) Used to exchange overhead information when

the call is not in progress

2. Slow Associated Control Channel (SACCH) Used to exchange time delay tolerant

overhead

information when the call is in progress

3. Fast Associated Control Channel (FACCH) Used to exchange time critical information

when the call is in progress

DCCH

SDCCH

SACCH

FACCH


Logical Channels - Summary

Channel UL Only

DLOnly

UL/DL Point to Point

Broadcast Dedicated Shared

BCCH X X X

FCCH X X X

SCH X X X

RACH X X X

PCH X X X

AGCH X X X

SDCCH X X X

SACCH X X X

FACCH X X X

TCH X X X

Channel UL Only

DLOnly

UL/DL Point to Point

Broadcast Dedicated Shared

BCCH X X X

FCCH X X X

SCH X X X

RACH X X X

PCH X X X

AGCH X X X

SDCCH X X X

SACCH X X X

FACCH X X X

TCH X X X

ChannelChannel UL OnlyUL Only

DLOnlyDLOnly

UL/DL UL/DL Point to Point

Point to Point

BroadcastBroadcast DedicatedDedicated SharedShared

BCCHBCCH XX XX XX

FCCHFCCH XX XX XX

SCHSCH XX XX XX

RACHRACH XX XX XX

PCHPCH XX XX XX

AGCHAGCH XX XX XX

SDCCHSDCCH XX XX XX

SACCHSACCH XX XX XX

FACCHFACCH XX XX XX

TCHTCH XX XX XX

UL - Uplink DL - Downlink


GSM Radio Resource Management (RRM)

Time Advancement (TA)Mobile Assisted Handoff (MAHO)Dynamic Power Control (DPC)Discontinuous Transmission (DTX)Frequency Hopping (FH)


Time Advancement (TA)

Mobiles randomly distributed in space

Timing advance prevents burst collision on the reverse link

Maximum advancement is 63 bits

BTS

SLOT 0 SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5

MS 2

MS 1

d2, Slot 2

d1, Slot 1

d1 > d 2MS 2

MS 1

T 1

T 2

Collision

T 1 - Delay of MS 1Signal

T 2 - Delay of MS 2Signal

SLOT 7SLOT 6

km35bits10693.3bit63

sm103

21max 68

D

Maximum cell radius


Mobile Assisted Handoff (MAHO)

GSM Implements MAHO

In the process of evaluating handoff candidates, GSM systems evaluate measurements

Evaluation done at BSC Three types of

measurements Signal strength Signal quality Timing advance

Measurement type

Link Cell DTX Measurement Source

RSL Downlink Serving Cell Full Set Mobile

RSL Downlink Serving Cell Subset Mobile

RSL Downlink Neighbors N/A Mobile

Quality Downlink Serving Cell Full Set Mobile

Quality Downlink Serving Cell Subset Mobile

RSL Uplink Serving Cell Full Set BTS

RSL Uplink Serving Cell Subset BTS

RSL Uplink Neighbors Full Set BTS

RSL Uplink Neighbors Subset BTS

Quality Uplink Serving Cell Full Set BTS

Quality Uplink Serving Cell Subset BTS

Timing Advance Uplink Serving Cell N/A BTS

Measurement type

Link Cell DTX Measurement Source

RSL Downlink Serving Cell Full Set Mobile

RSL Downlink Serving Cell Subset Mobile

RSL Downlink Neighbors N/A Mobile

Quality Downlink Serving Cell Full Set Mobile

Quality Downlink Serving Cell Subset Mobile

RSL Uplink Serving Cell Full Set BTS

RSL Uplink Serving Cell Subset BTS

RSL Uplink Neighbors Full Set BTS

RSL Uplink Neighbors Subset BTS

Quality Uplink Serving Cell Full Set BTS

Quality Uplink Serving Cell Subset BTS

Timing Advance Uplink Serving Cell N/A BTS

Measurement typeMeasurement type

LinkLink CellCell DTXDTX Measurement SourceMeasurement Source

RSLRSL DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile

RSLRSL DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile

RSLRSL DownlinkDownlink NeighborsNeighbors N/AN/A MobileMobile

QualityQuality DownlinkDownlink Serving CellServing Cell Full SetFull Set MobileMobile

QualityQuality DownlinkDownlink Serving CellServing Cell SubsetSubset MobileMobile

RSLRSL UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS

RSLRSL UplinkUplink Serving CellServing Cell SubsetSubset BTSBTS

RSLRSL UplinkUplink NeighborsNeighbors Full SetFull Set BTSBTS

RSL RSL Uplink Uplink NeighborsNeighbors SubsetSubset BTSBTS

QualityQuality UplinkUplink Serving CellServing Cell Full SetFull Set BTSBTS

QualityQuality UplinkUplink Serving CellServing Cell SubsetSubset BTSBTS

Timing AdvanceTiming Advance UplinkUplink Serving CellServing Cell N/AN/A BTSBTS


MAHO - Signal Strength Measurements Performed on uplink and

downlink Reported as a quantized value

RXLEV:RXLEV = RSL[dBm] + 110

Minimum RXLEV: -110, MAX RXLEV = -47

On the downlink, measurement performed for both serving cell and up to 32 neighbors

Up to 6 strongest neighbors are reported back to BTS through SACCH

Measurements of the neighbors are performed on the BCCH channels – not affected by the DTX

Measurements on the serving channel – affected by the DTX.

Perform over a subset of SACCH that guarantees transmission even in the case of active DTX

Before processing, the RXLEV measurements are filtered to prevent unnecessary handoffs

Example MAHO measurements


MAHO – Signal Quality Measurements

Performed on uplink and downlink

Only on the serving channel Reported as a quantized

value RXQUAL For a good quality call

RXQUAL < 3 Measurements are averaged

before the handoff processing If DTX is active, the

measurements are performed over the subset of SACCH that guarantees transmission

Mapping between RxQUAL and BER


Discontinuous Transmission (DTX) Idea: No voice – no need for transmission Benefits of DTX

o Uplink: System interference reduction Lower battery consumption

o Downlink System interference reduction Reduction of the inter-modulation products Lower power consumptions

Downsides of DTX usage:o MAHO measurements are less accurateo Voice quality is degraded due to slowness of VAD

Implemented both for BTS and MS Uses Voice Activity Detection (VAD) to detect periods of silence


Dynamic Power Control (DPC) Three reasons

o Elimination of near-far problemo Reduction of system interferenceo Improvement of MS battery life

DPC for MSo Depending on its power class, MS

can adjust its power between the max and min value in 2dB steps

o MS can perform 13 adjustments every SACCH period, i.e., 480ms

o Large adjustments > 24 dB will not be completed before the arrival of new command

o Commonly implemented as BSC feature. Many vendors are moving it at the BTS level

DPC for BTSo Vendor specifico Based on MAHO reports

GSM power classes for some popular bands


Frequency hopping (FH)

FH - multiple carriers used over the course of radio transmission

Two kinds of FH:o Slow Hopping – change of carrier

frequency happens at the rate slower than the symbol rate

o Fast Hoping – carrier frequency changes faster than the symbol rate

GSM implements slow FH Scheme Carrier frequency is changed once per

time slot Two reasons for FH

o Frequency diversityo Interference avoidance


Frequency Diversity of FH

Mobile environment is characterized with small scale fading

The depth of signal fade is a function frequency

If two signals are sufficiently separated in frequency domain they fade independently

Frequency diversity gain diminishes for fast moving mobiles


Interference avoidance of FH

FH averages interferenceAllows for tighter reuse of frequencies Increases the capacity of the system

User 1

User 2

User 3

User 4

User 5

f1

f4

f1

f1 f1

f1

f1

f2

f2

f2

f2

f3

f4

f1

f1

f2

f3

f3f4

f1

f4 f3

f1

f3

f4

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T

4TT 2T 3T 5T


Synthesized FH in GSM

Synthesized hoppingo The hopping frequencies assigned in an arbitrary wayo Needs relatively expensive broadband combiners

TX/RX

TX/RX

TX/RX

CarrierFreuqnacyf0,f 1,...,f m

BroadbandCombiner

1

2

n



Each radio is hopping in an independent way

Radios retune – “real time”


FH Algorithms

Cyclic hoppingo Frequencies are used in consecutive ordero If the radio is performing cyclic FH the order of frequencies in the

sequence goes from the lowest ARFCN to the highest ARFCN

Random hoppingo Implemented in a pseudo-random wayo Uses one of 63 available PN sequenceso The actual frequency obtained through module operation with the total

number of frequencies in the mobile allocation list

,,,,,,, 3214321 fffffff

,,,,,,, 3234421 fffffff


Frequency Planning and Reuse Strategies BCCH Channels – fixed assignment TCH Channels

o Fixed, or o Frequency hopping

Frequency plan is critical for GSM performance

Used plans: 7/21, 4/12 or 3/9 Frequency plan determines the

amount of interference in the system GSM requires at least 9dB of C/I Modern GSM systems implement “ad-

hock” frequency planning obtained from AFP tools

1

10

100

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25Required C/I [dB]

Size

of t

he C

lust

er (N

)

n = 3

n = 3.5

n = 4

n = 4.5

n = 5

Relationship between cluster size and C/I


Review questions

What is the difference between TCH/FR and TCH/HR?

What is the BCCH channel?What is TCH?Why is time advancing necessary

in GSM?What is MAHO?What is RxLev?What is RxQual?What are advantages of DTX?Can DTX be used on the BCCH?

How often does GSM mobile adjust its power?

What is the power adjustment step for the mobile?

What is frequency hopping?Can frequency hopping be used on

BCHH?What are the advantages of FH?What are typical reuse schemes

for BCCH in GSM?

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