1. ECE Department Florida Institute of Technology Wireless Data Communication Networks Lecture 20: Introduction to GSM

2. ECE Department Florida Institute of Technology Outline  GSM logical channels  Radio resource management Page 2

3. ECE Department Florida Institute of Technology 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 Page 3

4. ECE Department Florida Institute of Technology Traffic Channel (TCH) Page 4  Traffic channel carries speech and user data in both directions oFull rate ~ 33.85 Kb/sec oHalf rate ~ 16.93 Kb/sec oFull rate uses 1 slot in every frame oHalf 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/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

5. ECE Department Florida Institute of Technology Control Channels Page 5  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 only 2.Common Control Channels (CCCH)  Facilitate establishment of the link between MS and system  Both forward and reverse link 3.Dedicated Control Channels (DCCH)  Provide for exchange the control information when the call is in progress  Both forward and reverse – in band signaling

6. ECE Department Florida Institute of Technology 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. Page 6

7. ECE Department Florida Institute of Technology 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 Page 7

8. ECE Department Florida Institute of Technology 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 Page 8 DCCH SDCCH SACCH FACCH

9. ECE Department Florida Institute of Technology Logical Channels - Summary Page 9 UL - Uplink DL - Downlink

10. ECE Department Florida Institute of Technology GSM Radio Resource Management (RRM)  Time Advancement (TA)  Mobile Assisted Handoff (MAHO)  Dynamic Power Control (DPC)  Discontinuous Transmission (DTX)  Frequency Hopping (FH) Page 10

11. ECE Department Florida Institute of Technology Time Advancement (TA)  Mobiles randomly distributed in space  Timing advance prevents burst collision on the reverse link  Maximum advancement is 63 bits Page 11 Maximum cell radius

12. ECE Department Florida Institute of Technology 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 Page 12

13. ECE Department Florida Institute of Technology 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 Page 13  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

14. ECE Department Florida Institute of Technology MAHO – Signal Quality Measurements Page 14  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

15. ECE Department Florida Institute of Technology Discontinuous Transmission (DTX)  Idea: No voice – no need for transmission  Benefits of DTX oUplink:  System interference reduction  Lower battery consumption oDownlink  System interference reduction  Reduction of the inter-modulation products  Lower power consumptions  Downsides of DTX usage: oMAHO measurements are less accurate oVoice quality is degraded due to slowness of VAD  Implemented both for BTS and MS  Uses Voice Activity Detection (VAD) to detect periods of silence Page 15

16. ECE Department Florida Institute of Technology Dynamic Power Control (DPC)  Three reasons oElimination of near-far problem oReduction of system interference oImprovement of MS battery life  DPC for MS oDepending on its power class, MS can adjust its power between the max and min value in 2dB steps oMS can perform 13 adjustments every SACCH period, i.e., 480ms oLarge adjustments > 24 dB will not be completed before the arrival of new command oCommonly implemented as BSC feature. Many vendors are moving it at the BTS level  DPC for BTS oVendor specific oBased on MAHO reports Page 16 GSM power classes for some popular bands

17. ECE Department Florida Institute of Technology Frequency hopping (FH) Page 17  FH - multiple carriers used over the course of radio transmission  Two kinds of FH: oSlow Hopping – change of carrier frequency happens at the rate slower than the symbol rate oFast 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 oFrequency diversity oInterference avoidance

18. ECE Department Florida Institute of Technology 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 Page 18

19. ECE Department Florida Institute of Technology Interference avoidance of FH  FH averages interference  Allows for tighter reuse of frequencies  Increases the capacity of the system Page 19

20. ECE Department Florida Institute of Technology Synthesized FH in GSM  Synthesized hopping oThe hopping frequencies assigned in an arbitrary way oNeeds relatively expensive broadband combiners Page 20  Each radio is hopping in an independent way  Radios retune – “real time”

21. ECE Department Florida Institute of Technology FH Algorithms  Cyclic hopping oFrequencies are used in consecutive order oIf the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN  Random hopping oImplemented in a pseudo-random way oUses one of 63 available PN sequences oThe actual frequency obtained through module operation with the total number of frequencies in the mobile allocation list Page 21

22. ECE Department Florida Institute of Technology Frequency Planning and Reuse Strategies  BCCH Channels – fixed assignment  TCH Channels oFixed, or oFrequency 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 Page 22 Relationship between cluster size and C/I

23. ECE Department Florida Institute of Technology 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? Page 23  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?