GSM RADIO INTERFACE GSM RADIO INTERFACE IN THIS

GSM - RADIO INTERFACE

GSM - RADIO INTERFACE 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.

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

GSM RADIO INTERFACE • Most Important 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

GSM Uplink & Downlink • Frequency Bands GSM 900 Mhz DCS 1800 MHz K N LI UP W O D K N I L N B T S

GSM Specifications RF Spectrum : GSM 900 Mobile to BS BS to Mobile (UP-LINK) - 890 to 915 MHz (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

GSM Specifications Carrier Separation - 200 k. Hz 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 - MULTIPLE ACCESS • GSM uses both FDMA & TDMA • FDMA Access along Frequency axis • Each RF carrier 200 khz apart • Total 124 RF Channels available. One or more carrier assigned to each base station 1 2 3 4 5 6 124 ……. . . Freq 890. 2 890. 4 890. 6 890. 8 891. 0 914. 8 Mhz.

GSM - MULTIPLE ACCESS • 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. • 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 FDMA 890 915 25 MHz 1 0 2 890. 4 200 k. Hz 45 MHz 890. 6 960 25 MHz 1 0 Mobile to Base 890. 2 935 2 Base to Mobile (MHz) 935. 2 935. 4 935. 6 200 k. Hz Channel layout and frequency bands of operation

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

GSM Digital Voice Transmission Speech Coding - In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. - This calculates as 50 X 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.

GSM Digital 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.

GSM Digital Voice Transmission Burst Formatting To counteract the problems encountered in radio path: - 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”.

Interleaving & Burst Formating 1 st Sample of 20 ms speech 2 nd Sample of 20 ms speech 456 bits Sample 1 456 bits Sample 2 D D D 1 2 3 D D 4 5 D 6 7 8 1 2 3 4 5 D D 1 4 5 2 3 D D 6 7 8 Stream of Time Slots 3 57 1 26 1 Normal Burst 57 3 8. 25

GSM Speech to Radio waves Analog Speech Coding Speech Decoding Channel Coding Channel Decoding Interleaving De-interleaving Burst formatting Ciphering Deciphering Modulation Demodulation 200 k. Hz BW

FDMA/TDMA Scheme TIME BP 2 BP 1 BP 8 BURST BP 7 BP 6 F BP 5 R BP 4 A BP 3 M BP 2 E BP 1 FREQ 890. 0 890. 2 890. 4 890. 6 890. 8 891. 0 891. 2 915. 8 MHz

3 CC SMS T CM SS 142 3 8. 25 Fixed Bits T GP FCCH Burst 3 CC SMS 57 SS T CM Encrypted 1 S 26 1 57 3 8. 25 Training S Encrypted T GP Normal Burst

PHYSICAL CHANNELS 8 7 6 5 4 3 2 1 FRAME OF 8 TIME SLOTS 8 7 6 5 4 3 2 1 FRAME REPETITION 8 7 6 5 4 3 2 1 8 1 2 3 4 5 6 7 8 7 6 1 2 3 4 5 6 7 8 5 1 2 3 4 5 6 74 8 1 2 3 4 5 6 7 8 3 2 1 2 3 4 5 6 7 8 1 PHYSICAL CHANNELS 1

GSM-- TDMA STRUCTURE • TDMA • Time slot duration • Frame • Multi Frame 8 Time Slots / RF Channel 0. 577 m sec or 15 / 26 m sec 8 Burst Periods ( Time Slots) = 8 15/26 = 4. 615 m sec Traffic 26 4. 615 = 120 msec Control • Super Frame 51 4. 615 = 235. 365 m sec 51 Traffic Multi frames 26 Control Multi frames • Hyper Frame 2048 Super Frames = 3 28 52. 76 hr min sec

GSM Radio Interface - CYCLES Hyperframe = 2048 Superframes 3 Hours 28 Minutes 53 Seconds and 760 milliseconds 0 2047 Superframe = 26× 51 multiframes 6. 12 Seconds 0 50 0 0 1 25 26 Multiframe 51 Multiframe 120 m. S Approx 235 m. S 2 24 25 0 1 48 TDMA frame 4. 615 m. S 0 1 2 3 4 5 6 7 49 50

Organisation of Speech & Data Frames 0 -11 : TCH 0 1 2 3 4 Frames 12 : SACCH 5 6 7 8 9 10 11 Frames 13 -24 : TCH 12 13 14 15 16 17 18 Frames 25 : Unused 19 20 21 22 23 24 25 26 – frame multiframe Duration: 120 ms BP 0 3 BP 1 57 Tail Data bits BP 2 1 BP 3 BP 4 26 BP 5 1 BP 6 BP 7 57 Stealing Training Stealing Data bits sequence bit TDMA frame Duration: 60/13 ms =4. 615 ms 3 8. 25 Tail Guard bits Normal burst Duration 15/26 ms

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

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

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

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

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

OPERATIONAL CONCEPTS 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

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

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.

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 TS 2 TS 3 TS 2 A –on TS 3 BTS

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. - This limits the GSM cell size to 35 Km radius. 0 1 2 3 4 5 6 7 Time Start Sending 0 1 2 3 4 5 6 7

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 Rx No Diversity Rx (A) Tx Rx ( B) Antenna Diversity

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

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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