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Wireless Data Communication Networks Lecture 20: Introduction to GSM. Outline. GSM logical channels Radio 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.

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Wireless data communication networks lecture 20 introduction to gsm

Wireless Data Communication Networks

Lecture 20: Introduction to GSM


  • GSM logical channels

  • Radio resource management

Gsm logical channels
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

Traffic channel tch
Traffic Channel (TCH)

  • Full Rate TCH can carry:

  • Voice (13 Kb/sec)

  • Data at rates:

    • 9.6 Kb/sec

    • 4.8 Kb/sec

    • 2.4 Kb/sec

  • Traffic channel carries speech and user data in both directions

    • Full rate ~ 33.85 Kb/sec

    • Half rate ~ 16.93 Kb/sec

    • Full rate uses 1 slot in every frame

    • 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

  • Half Rate TCH can carry:

  • Voice (6.5 Kb/sec)

  • Date at rates:

    • -4.8 Kb/sec

    • 2.4 Kb/sec

  • Adaptive 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
Control Channels

  • GSM Defines 3 types of Control Channels:

    • Broadcast Channels (BCH)

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

      • Forward link only

    • Common Control Channels (CCCH)

      • Facilitate establishment of the link between MS and system

      • Both forward and reverse link

    • Dedicated Control Channels (DCCH)

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

      • Both forward and reverse – in band signaling

Broadcast channels bch
Broadcast Channels (BCH)

  • Three types of BCH:

    • Synchronization channel (SCH)

      • Provides a known sequence that helps mobile synchronization

        at the baseband

      • Communicates with S-burst

      • Broadcasts Base Station Identity Code (BSIC)

    • 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.

Common control channel ccch
Common Control Channel (CCCH)

  • Three types of CCCH:

    • Random Access Channel (RACH)

      • Used by mobile to initialize communication

      • Mobiles use slotted ALOHA

      • Reverse link only

    • Paging Channel (PCH)

      • Used by the system to inform the mobile

        about an incoming call

      • Forward link only

      • GSM Supports DRX

    • Access Grant Channel (AGC)

      • Used to send the response to the mobiles

        request for DCCH

      • Forward link only

Dedicated control channels dcch





Dedicated Control Channels (DCCH)

  • Three types of DCCH:

    • Stand Alone Dedicated Control Channel (SDCCH)

      • Used to exchange overhead information when

        the call is not in progress

    • Slow Associated Control Channel (SACCH)

      • Used to exchange time delay tolerant overhead

        information when the call is in progress

    • Fast Associated Control Channel (FACCH)

      • Used to exchange time critical information

        when the call is in progress

Logical channels summary
Logical Channels - Summary

DL - Downlink

UL - Uplink

Gsm radio resource management rrm
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
Time Advancement (TA)

  • Mobiles randomly distributed in space

  • Timing advance prevents burst collision on the reverse link

  • Maximum advancement is 63 bits

Maximum cell radius

Mobile assisted handoff maho
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

Maho signal strength measurements
MAHO - Signal Strength Measurements

  • 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

  • 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

Example MAHO measurements

Maho signal quality 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
Discontinuous Transmission (DTX)

  • Idea: No voice – no need for transmission

  • Benefits of DTX

    • Uplink:

      • System interference reduction

      • Lower battery consumption

    • Downlink

      • System interference reduction

      • Reduction of the inter-modulation products

      • Lower power consumptions

  • Downsides of DTX usage:

    • MAHO measurements are less accurate

    • 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
Dynamic Power Control (DPC)

  • Three reasons

    • Elimination of near-far problem

    • Reduction of system interference

    • Improvement of MS battery life

  • DPC for MS

    • Depending on its power class, MS can adjust its power between the max and min value in 2dB steps

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

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

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

  • DPC for BTS

    • Vendor specific

    • Based on MAHO reports

GSM power classes for some popular bands

Frequency hopping fh
Frequency hopping (FH)

  • FH - multiple carriers used over the course of radio transmission

  • Two kinds of FH:

    • Slow Hopping – change of carrier frequency happens at the rate slower than the symbol rate

    • 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

    • Frequency diversity

    • Interference avoidance

Frequency diversity of fh
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
Interference avoidance of FH

  • FH averages interference

  • Allows for tighter reuse of frequencies

  • Increases the capacity of the system

Synthesized fh in gsm
Synthesized FH in GSM

  • Each radio is hopping in anindependent way

  • Radios retune – “real time”

  • Synthesized hopping

    • The hopping frequencies assigned in an arbitrary way

    • Needs relatively expensive broadband combiners

Fh algorithms
FH Algorithms

  • Cyclic hopping

    • Frequencies are used in consecutive order

    • If the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN

  • Random hopping

    • Implemented in a pseudo-random way

    • Uses one of 63 available PN sequences

    • The actual frequency obtained through module operation with the total number of frequencies in the mobile allocation list

Frequency planning and reuse strategies
Frequency Planning and Reuse Strategies

  • BCCH Channels – fixed assignment

  • TCH Channels

    • Fixed, or

    • 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

Relationship between cluster size and C/I

Review questions
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?