Wireless Data Communication Networks Lecture 20: Introduction to GSM

1. Wireless Data Communication Networks Lecture 20: Introduction to GSM

2. Outline • GSM logical channels • Radio resource management

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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

9. Logical Channels - Summary DL - Downlink UL - Uplink

10. GSM Radio Resource Management (RRM) • Time Advancement (TA) • Mobile Assisted Handoff (MAHO) • Dynamic Power Control (DPC) • Discontinuous Transmission (DTX) • Frequency Hopping (FH)

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Interference avoidance of FH • FH averages interference • Allows for tighter reuse of frequencies • Increases the capacity of the system

20. 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

21. 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

22. 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

23. 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?