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GSM

GSM. Mobile Computing IT644. GSM System Architecture. Network Subsystem MSC ?? Radio Subsystem BTS, BSC Operation Support Subsystem. GSM System Hieararchy. GSM Network. MSC R. MSC Region. Location Area. Location Area. BSC. BSC. Location Area. MSC R.

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GSM

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  1. GSM Mobile Computing IT644

  2. GSM System Architecture Network Subsystem MSC ?? Radio Subsystem BTS, BSC Operation Support Subsystem

  3. GSM System Hieararchy GSM Network . . . . MSCR. MSC Region Location Area LocationArea BSC BSC . . . . . . . . LocationArea MSCR.

  4. Operations and Maintenance Centre OMC G MSC MSC BTS BSC Home Location Register HLR BSC VLR Visitor Location Register BTS Equipment Identity Register EIR BTS MS Authentication centre AVC MS

  5. Comprises : Mobile Equipment SIM (Subscriber Identity Module ??) The Mobile Station (MS) • Other Identification : • International Mobile Station Equipment Identity (IMEI) • International Mobile Subscriber Identity (IMSI) • Mobile Subscriber ISDN Number (Mobile Telephone No.)

  6. Radio Network Base Subsystem Controller ? (BSC) -Handles essential control and protocol intelligence. - Handover is executed at the BSC. Base Transceiver System ? (BTS) - Is a high frequency Transmitter/Receiver. - Handles error protection; coding/decoding for the radio channel.

  7. 124 1 2 3 4 5 6 7 8 2 1 Frequency Channels in GSM • Uplink frequency band : 890 - 915 MHz • Downlink frequency band : 935 - 960 MHz • 124 channels (of 200 kHz each) in each band. ?? • Each channels has a TDMA structure with 8 timeslots. • ( => upto 8 users per freq. channel ) 960 Mhz TDMA structure of each channel 935 Mhz

  8. 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 Downlink Delay Uplink So the MS does not have to Transmit and Receive at the same time instance!

  9. Logical Channels • Traffic Channels (TCH) • Signaling Channels • Broadcast Channel (BCH) • Common Control Channel (CCH) • Dedicated/Associated Control Channel (DCCH/ACCH) Note: These logical channels are then mapped onto Physical channels. A GSM Physical channel comprises a particular timeslot on a given freq. Channel.

  10. Signalling channel contd. .... • BCH : • Broadcast Control Channel (BCCH) • Frequency Correction Channel (FCCH) • Synchronization Channel (SCH) • CCH : • Random Access Channel (RACH) • Paging Channel (PCH) • D/ACCH • Stand-alone Dedicated Control Channel (SDCCH) • Slow Associated Control Channel (SACCH)

  11. Traffic 1 2 3 4 5 6 7 8 Traffic channel structure for full rate coding TDMA slots 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 9 10 11 12 13 14 26 TDMA Frames on a given channel Signaling (S) S-contains information about the signal strength in neighboring cells

  12. 26 Traffic channel structure for half rate coding 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Burst for one user 1 2 3 4 5 6 7 8 26 Burst for another user

  13. Why 26 burst Traffic Channel? • Mobile needs to send information about signal strength from surrounding cells to the network. • Capacity required to send measurement info is 1/24 the capacity required to send voice! Signaling Channel – Control Channel Associated Control Channel Slow Associated Control Channel

  14. Adaptive Frame Synchronization • Timing Advance: • MS advances its burst transmission by a time corresponding • to round trip time. • The delay is quantiled as a 6 bit number. • => 64 steps (0-63); each step advances the Timing by one • bit duration ie 3.7 ms. • 64 steps allows compensation over a maximum propagation • time of 31.5 bit periods ie 113.3 ms ( => a maximum distance • of ~ 35 km)

  15. Timing Advance : How it works. (Sent by BS on down link) 1 2 3 4 5 6 7 8 ||||| ||||| ||| |||| 8 1 2 3 4 5 6 7 One way Propagation delay ||| (received by BS on up link) Twowaypropagation delay (received by MS on down link) ||||| 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 (Sent by MS on up link)

  16. Transmission Bursts • Normal Burst • Synchronization Burst • Frequency Correction Burst • Dummy Burst • Access Burst

  17. Control Channel • Functions: • To help the MS find the control channels. ?? • To provide information about • - voice and control channel repetition cycle. • - parameters in the cell. • - surrounding cells. • - paging. • To allow random access attempts by the MS.

  18. The Downlink Control Channel • 51 cycle burst (2x26 -1) • Third burst on the control channel provides content • information. • Basic structure of control channel : • FSxxx xxxxx FSxxx xxxxx FSxxx xxxx .... • F: Frequency Correction Channel • (occurs every 10th burst) • S: Synchronization Channel

  19. TDMA Bursts in GSM 3 39 data 64 bit Training seq 39 data 3 8.25 142 fixed bits 3 3 8.25 FB SB Dummy Burst 3 26 bit Training seq 3 8.25 8 41 bit Training seq 36 data 3 68.25 Access Burst

  20. Normal Burst Tail bit 3 57 Data bits 26 bit Training seq 57 Data bits 3 8.25 Bit GP Stealing Flags Fig.

  21. F F F S S S B B B B B B B B B C D R D C R D R C C R Frequency synchronization Time synchronization BCCH Request (Random access) Grant

  22. Full Rate Speech Coding • Coder for 20ms segments - 120 bits at the output. • 13 Kbps. • Unequal error protection: • Out of 260 bits • - 182 bits are protected. • - 78 bits are not protected.

  23. Error Coding Class 1a - 3 parity bits from 50 bits. Class 1b - 132 bits are not parity checked, but fed to convolutional encoder. Class 2 - 78 bits are not protected. 182 78

  24. A Block Encoder (53, 50) G(D) = 1 + D+ D3 SW D D D + + First 50 bits data Last 3 parity bits 1 - 50 clock cycles, SW closed 51 - 53 clock cycles, SW open

  25. Convolutional Encoder K- constraint length k=5 R- rate of code r=2/1 K – storage location G0 = d4 + d3 + 1 G1 = d4 + d3 + d + 1

  26. Class 1a 50 bits Class 1b 132 bits Class 2 78 bits 50 3 132 4 78 R=1/2 k=5; 378 78 456 bits Parity bits Tail bits 456 bits in 20 ms ~ 456/0.02 = 22.8 kbps

  27. Interleaving 1. Block Interleaving: Code words are written line by line to a matrix and read column by column.?? 8 (col) * 57 (row) matrix 2. Diagonal Interleaving 1 2 4 3 5 6 7 8

  28. Discontinuous Transmission • On an average speech actually lasts only 50% of the time. • So transmitter is kept off whenever there is no speech. • This reduces co-channel interference and saves battery power. • Voice Activity Detector (VAD) is used at the transmitter, and Comfort Noise Generation (CNG) is used at the receiver.

  29. VAD • Background noise is stationary over relatively long periods. • Measure the deviations from the spectral characteristics of • the background noise. • CNG • Comfort noise characteristics are matched to the transmitted • noise.

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