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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.
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GSM Mobile Computing IT644
GSM System Architecture Network Subsystem MSC ?? Radio Subsystem BTS, BSC Operation Support Subsystem
GSM System Hieararchy GSM Network . . . . MSCR. MSC Region Location Area LocationArea BSC BSC . . . . . . . . LocationArea MSCR.
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
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.)
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.
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
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!
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.
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)
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
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
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
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)
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)
Transmission Bursts • Normal Burst • Synchronization Burst • Frequency Correction Burst • Dummy Burst • Access Burst
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.
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
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
Normal Burst Tail bit 3 57 Data bits 26 bit Training seq 57 Data bits 3 8.25 Bit GP Stealing Flags Fig.
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
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.
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
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
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
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
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
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.
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.