1. GSM - RADIO INTERFACE

2. IN THIS PRESENTATION GSM - RADIO INTERFACE Radio Interface Frequency Bands & Specifications Multiple Access Method FDMA & TDMA Modulation Method 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.

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

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

5. GSM Uplink & Downlink Frequency Bands GSM 900 Mhz DCS 1800 MHz BTSBTS UPLINK DOWNLINK

6. GSM Specifications 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 RF Spectrum :

7. GSM Specifications Carrier Separation -200 kHz 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 Duplex Distance -45 MHz

8. GSM - MULTIPLE ACCESS GSM uses both FDMA & TDMA Freq Mhz. 890.2 1 890.4 2 890.6 3 890.8 4 891.0 56 914.8 124 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) 1 and 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. F up-link (n) = 890.2 +0.2* ( n-1 ) MHz F down-link (n) = 935.2 +0.2* ( n-1 ) MHz Frequency for any ARFCN ( n) can be calculated from : Here 1 ≤ n ≤ 124.

10. GSM FDMA 25 MHz Mobile to Base 0 12 890.2 890.4890.6 ( MHz) Base to Mobile 0 12 935.2 935.4 935.6 200 kHz 45MHz Channel layout and frequency bands of operation 890935960915 200 kHz

11. GSM TDMA 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 45 MHz Frequency F2’ F1’ (Cell transmit) F2F1 (Cell Rx) Amplitude Typical TDMA/ FDMA frame structure

12. GSM Digital Voice Transmission - 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. Speech Coding

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

14. GSM Digital Voice Transmission - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - To counteract the problems encountered in radio path. Burst Formatting - 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”.

15. GSM Gaussian Minimum Shift Keying (GMSK) The modulation method in GSM is GMSK which facilitates the use of narrow bandwidth and coherent detection capability. Rectangular pulses are passed through a Gaussian filter prior to their passing through a modulator. The modulation scheme almost satisfies the adjacent channel power spectrum density requirements of -60dB specified by CCIR.

16. GSM Speech to Radio waves Speech Coding Channel CodingInterleavingBurst formattingCiphering Burst formattingDeciphering Analog Modulation 200kHz BW Speech Decoding Analog 200kHz BW DemodulationDe-interleavingChannel Decoding

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

18. CM CCSMSSS Normal Burst 3T3T 57 Encrypted 1S1S 26 Training 1S1S 57 Encrypted 3T3T 8.25 GP CM CCSMSSS 3T3T 142 Fixed Bits 3T3T 8.25 GP FCCH Burst

19. 87654321FRAME OF 8 TIME SLOTS FRAME REPETITION PHYSICAL CHANNELS 876543218765432187654321 87654321 1 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 6 6 6 6 6 7 7 7 7 7 8 8 8 8 8 5 5 5 5 5 1 2 4 5 6 7 8 3

20. GSM Radio Interface - CYCLES 20470 Hyperframe = 2048 Superframes 3 Hours 28 Minutes 53 Seconds and 760 milliseconds 500 250 Superframe = 26× 51 multiframes 6.12 Seconds 51 Multiframe Approx 235 mS 26 Multiframe 120 mS 2524210 50494810 76543210 TDMA frame 4.615 mS

21. Organisation of Speech & Data 242523222120191817161514131211109876543210 BP 7BP 6BP 5BP 4BP 3BP 2BP 1BP 0 8.253571261573 Frames 0-11 : TCHFrames 12 : SACCHFrames 13-24 : TCHFrames 25 : Unused 26 – frame multiframe Duration: 120 ms TDMA frame Duration: 60/13 ms =4.615 ms Tailbits Data bits Stealingbit Training sequence Stealingbit Data bits TailbitsGuardbits Normal burst Duration 15/26 ms

22. 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

23. GSM LOGICAL CHANNELS USER INFORMATION( TRAFFIC) SIGNALLING INFORMATION (CONTROL)

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

25. 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

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

27. OPERATIONAL CONCEPTS IDLE MODE -- When MS is powered on (active) without being in dedicated mode MS stays continuously in touch with BS Listens to transmissions from BS to intercept Paging Messages ( for incoming calls) Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS Listens to BS to avail short message broadcast service

28. OPERATIONAL CONCEPTS ACCESS PROCEDURE -- Access to system ( switch over from IDLE to DEDICATED Mode) MS indicates to BS that it needs a connection BS accepts the request & indicates which traffic CHL it may use For above purpose specific transmission is done over “ Common Channels”

29. OPERATIONAL CONCEPTS MOBILE O/G Call MS sends access over RACH System allocates SDCCH through AGCH Lastly TCH is assigned through SDCCH when a conversation can start Set up information exchanged over SDCCH ( Authentication, Measurement Reports, Power Control)

30. OPERATIONAL CONCEPTS MOBILE I/C Call Paging to MS through PCH since MS is monitoring PAGCH MS responds by sending a page response over RACH Set up information exchanged over SDCCH ( Authentication, Call set-up messages, Power Control) As a result system allocates SDCCH to MS over AGCH Lastly TCH is allocated to mobile over SDCCH. Mobile starts conversation.

31. OPERATIONAL CONCEPTS IDLE MODE ACCESS PROCDURE DEDICATED MODE MS O/G Call MS I/C Call ----FCCH ---- SCH ----BCCH ----RACH ----AGCH ----SDCCH ----TCH ----PCH ----RACH ----AGCH ----SDCCH ----TCH

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

33. Discontinuous Transmission (DTX) 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.

34. Timing Alignment : GSM – RF Interface - 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. 01234567 BTS TDMA Frame A –on TS3 B –on TS2 TS3 TS2

35. 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. - This limits the GSM cell size to 35 Km radius. 0123456701234567 Time Start Sending GSM – RF Interface

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

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

38. SUMMARY GSM - RADIO INTERFACE Radio Interface Frequency Bands & Specifications Multiple Access Method FDMA & TDMA Modulation Method 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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