GSM - RADIO INTERFACE

1. GSM - RADIO INTERFACE

2. IN THIS PRESENTATION • Radio Interface • Frequency Bands & Specifications • Multiple Access Method FDMA & TDMA • FDMA /TDMA Frame Representation • Logical Channels Traffic & Control • Operational Concepts • Other Salient Features of RF I/F- DTX, Time Alignment • Diversity, Fr. Hopping, • Power Control. GSM - RADIO INTERFACE

3. Inter-Exchange Junction Subscriber Line (2W) BSC BTS MS Telephone Exchange Mobile Switching Centre (MSC) Communication - Mobile

4. Most Important Interface GSMRADIO INTERFACE • Full Compatibility between mobile stations of various Manufacturers & Networks of different vendors to help roaming • To increase spectral efficiency -- Large number of simultaneous calls in a given bandwidth -- Frequency Reuse -- Interference -- Use of Interference Reduction Techniques

5. Frequency Bands GSM 900 Mhz DCS 1800 MHz UPLINK DOWNLINK B T S GSMUplink & Downlink

6. GSM Specifications RF Spectrum : GSM 900 Mobile to BS (UP-LINK) - 890 to 915 MHz BS to Mobile (DOWN -LINK) - 935 to 960 MHz Bandwidth - 25 MHz GSM 1800 ( DCS ) : Mobile to Cell (UP-LINK) - 1710 to 1785 MHz Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz Bandwidth - 75 MHz

7. Carrier Separation - 200 kHz Duplex Distance - 45 MHz No. of RF Carriers - 124 Access Method - TDMA/FDMA Modulation Method - GMSK Transmission Rate - 270.833 Kbps Speech Coding - Full rate 13 Kbps Half rate 6.5 Kbps GSM Specifications

8. GSM uses both FDMA & TDMA 1 2 3 4 5 6 124 Freq Mhz. 890.2 890.4 890.6 890.8 914.8 891.0 GSM - MULTIPLE ACCESS • FDMA Access along Frequency axis • Each RF carrier 200khz apart • Total 124 RF Channels available. • One or more carrier assigned to each base station ……...

9. Absolute Radio Freq Carrier Number (ARFCN) 1and 124 not used • until it is co-ordinated with Non -GSM operators in adjacent freq. bands. • Thus for practical purposes only 122 RF Carriers are available. • Frequency for any ARFCN ( n) can be calculated from : F up-link (n) = 890.2 +0.2* ( n-1 ) MHz F down-link (n) = 935.2 +0.2* ( n-1 ) MHz Here 124. GSM - MULTIPLE ACCESS

10. 890 915 935 960 25 MHz 25 MHz 1 2 1 2 0 0 Mobile to Base Base to Mobile (MHz) 890.4 890.6 935.4 935.6 890.2 935.2 200 kHz 200 kHz 45MHz Channel layout and frequency bands of operation GSMFDMA

11. 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 GSMTDMA Amplitude 45 MHz Frequency F1 (Cell Rx) F2 F1’ (Cell transmit) F2’ Typical TDMA/ FDMA frame structure

12. GSMDigital Voice Transmission Speech Coding - In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. - This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. - This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones.

13. GSMDigital Voice Transmission Channel Coding - It uses 260 bits from speech coding as input and outputs 456 encoded bits. Interleaving - These 456 bits for every 20 ms of speech are interleaved forming eight blocks of 57 bits each. - In one burst one block of 57 bits from one sample and another block from another sample are sent together.

14. - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. - One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). - GP enables MS/BTS to “ramp up” and “ ramp down”. GSMDigital Voice Transmission Burst Formatting To counteract the problems encountered in radio path:

15. D D D D D D D D D D D D D D D D 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 3 57 1 1 57 3 8.25 26 Interleaving & Burst Formating 1st Sample of 20 ms speech 2nd Sample of 20 ms speech 456 bits Sample 2 456 bits Sample 1 1 2 3 4 5 6 7 8 Stream of Time Slots Normal Burst

16. Analog Analog Speech Coding Speech Decoding Channel Coding Channel Decoding Interleaving De-interleaving Burst formatting Burst formatting Ciphering Deciphering Modulation Demodulation 200kHz BW GSMSpeech to Radio waves 200kHz BW

17. TIME BP2 BP1 BP8 BURST BP7 BP6 BP5 BP4 BP3 BP2 BP1 FREQ MHz 890.2 890.6 891.0 915.8 890.0 890.4 890.8 891.2 FDMA/TDMA Scheme F R A M E

18. 3 T 142 Fixed Bits 8.25 GP 3 T CC SMS SS CM • FCCH Burst 3 T 57 Encrypted 8.25 GP 3 T CC 57 Encrypted SMS SS 1S 26 Training 1 S CM • Normal Burst

19. 8 8 8 7 7 7 6 6 6 5 5 5 4 4 4 3 3 3 2 2 2 1 1 1 6 6 6 6 4 1 2 3 5 7 8 6 4 1 2 3 5 7 8 8 4 7 1 2 3 5 7 8 6 4 1 2 3 5 7 8 5 4 4 1 2 3 5 7 8 3 2 1 PHYSICAL CHANNELS 8 7 6 5 4 3 2 1 FRAME OF 8 TIME SLOTS FRAME REPETITION 8 7 6 5 4 3 2 1 PHYSICAL CHANNELS 1

20. GSM-- TDMA STRUCTURE • TDMA 8 Time Slots / RF Channel • Time slot duration 0.577m sec or 15 / 26 m sec • Frame 8 Burst Periods ( Time Slots) • = 8 15/26 = 4.615 m sec • Multi Frame Traffic 26  4.615 = 120 msec • Control 51  4.615 = 235.365 m sec • Super Frame 51  Traffic Multi frames • 26  Control Multi frames • Hyper Frame 2048 Super Frames = 3 28 52.76 • hr min sec

21. Hyperframe = 2048 Superframes 3 Hours 28 Minutes 53 Seconds and 760 milliseconds 0 2047 Superframe = 26× 51 multiframes 6.12 Seconds 0 50 0 25 26 Multiframe 120 mS 51 Multiframe Approx 235 mS 0 1 2 24 25 0 1 48 49 50 TDMA frame 4.615 mS 0 1 2 3 4 5 6 7 GSM Radio Interface - CYCLES

22. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 BP 0 BP 1 BP 2 BP 3 BP 4 BP 5 BP 6 BP 7 3 57 1 26 1 57 3 8.25 Organisation of Speech & Data Frames 0-11 : TCH Frames 12 : SACCH Frames 13-24 : TCH Frames 25 : Unused 26 – frame multiframe Duration: 120 ms TDMA frame Duration: 60/13 ms =4.615 ms Tail bits Tail bits Guard bits Training sequence Stealing bit Stealing bit Data bits Data bits Normal burst Duration 15/26 ms

23. Logical Channels Traffic Channels Control Channels Common Control Channels Dedicated Control Channels Broadcast Channels Logical Channels Full-Rate Half-Rate FCCH SCH BCCH RACH PCH AGCH SDCCH SACCH FACCH

24. GSM LOGICAL CHANNELS • USER INFORMATION( TRAFFIC) • SIGNALLING INFORMATION (CONTROL)

25. GSM • CONTOL CHHANELS OVER LOGICAL CHANNELS • Intended to carry signalling and synchronisation • THREE TYPES OF CONTROL CHANNELS • Broadcast control channel BCCH • Common control channel CCCH • Dedicated control channel DCCH

26. OPERATIONAL CONCEPTS • Subscribers are not allocated dedicated channels • TCH Allocated to users only when needed • Hence IDLE MODE & DEDICATED MODE • DEDICATED MODE -- When a full Bi -directional P to P CHL has been allocated during an established call • IDLE MODE MODE -- When MS is powered on (active) without being in dedicated mode

27. Broadcast control channel BCCH • P- MP For Freq Correction FCCH • For Syncronisation SCH • BCCH • Common control channel CCCH • For ACCESS Management PCH • RACH • AGCH • Dedicated control channel DCCH • P - P For Registration SDCCH • ,authentication SACCH • & Handover FACCH GSM • THREE TYPES OF CONTROL CHANNELS

28. OPERATIONALCONCEPTS IDLE MODE ACCESS PROCDURE DEDICATED MODE • IDLE MODE ----FCCH ---- SCH ----BCCH • MS O/G Call ----RACH ----AGCH ----SDCCH ----TCH • MS I/C Call ----PCH ----RACH ----AGCH ----SDCCH ----TCH

29. GSM – RF Interface Other Salient Features Of GSM RF I/F: - Control of Transmitted Power. • Discontinuous Transmission. - Timing Advance. - Diversity. - Frequency Hopping.

30. Speech activity only 40% of time. Needs Voice activity detection. Determination of voice threshold vis-à-vis noise. Annoying clicks/inefficient DTX. Generation of Comfort Noise at receiver to avoid the feeling of the set being dead. Discontinuous Transmission (DTX)

31. GSM – RF Interface Timing Alignment : - Large distance between BTS and MS causes the problem. - Each MS on call is allocated a timeslot on TDMA frame. - The problem occurs when the information transmitted by MS does not reach BTS on allocated timeslot. TDMA Frame 0 1 2 3 4 5 6 7 B –on TS2 TS3 TS2 A –on TS3 BTS

32. GSM – RF Interface Timing Advance : ( To counteract problem of Time Alignment ) - MS instructed to do its transmission certain bit-times earlier or later – to reach its timeslot at BTS in right time. - In GSM systems maximum 63 bit-times can be used. Start Sending - This limits the GSM cell size to 35 Km radius. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Time

33. GSM – RF Interface Antenna Diversity : Space Diversity - - Mounting two receiver antenna physically separated a distance. • Probability of both of them being affected by a deep fading dip • at same time is low. - At 900 MHz with antenna spacing of 5-6 m we get 3 db gain. Polarization Diversity - - Dual polarized antenna – vertical and horizontal arrays. Tx Tx Rx Rx (A) Rx ( B) No Diversity Antenna Diversity

34. FREQUENCY HOPPING • Change of frequency after every frame in a pre-determined manner • SFH improves performance in multi-path fading • Provides interference diversity • Decreases required C/I • Mandatory for MS when requested by BS • FCCH ,SCH ,BCCH are not hopped • Algorithm : Cyclic or pseudorandom

35. GSM - RADIO INTERFACE SUMMARY • Radio Interface • Frequency Bands & Specifications • Multiple Access Method FDMA & TDMA • FDMA /TDMA Frame Representation • Logical Channels Traffic & Control • Operational Concepts • Other Salient Features of RF I/F- DTX, Time Alignment • Diversity, Fr. Hopping, • Power Control.

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