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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

outline
Outline
  • 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

DCCH

SDCCH

SACCH

FACCH

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?
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