GSM Mobile Computing IT 644 GSM System Architecture

GSM Mobile Computing IT 644

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

GSM System Hieararchy GSM Network MSC Region Location Area BSC BS C Location Area . . . Area Location MSC R. . . MSC R.

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

The Mobile Station (MS) Comprises : – Mobile Equipment – SIM (Subscriber Identity Module ? ? ) 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.

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

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

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

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

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. 1 One way Propagation delay | | | | | 2 3 4 5 | | | 1 | | | 1 2 3 4 1 6 7 2 (Sent by BS on down link) 8 3 4 5 Two way propagation delay 5 2 6 8 7 (received by BS on up link) 8 (received by MS on 7 3 6 down link) 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 (2 x 26 -1) • Third burst on the control channel provides content information. • Basic structure of control channel : FSxxx xxxxx FSxxx xxxx. . F: Frequency Correction Channel (occurs every 10 th burst) S: Synchronization Channel

TDMA Bursts in GSM FB 3 SB 3 142 fixed bits 39 data Burst 39 data 26 bit Training seq Dummy 3 Burst Acce ss 64 bit Training seq 8 41 bit 36 data 3 Training seq 3 8. 25 68. 25

Normal Burst Tail bit 3 57 Data bits 26 bit Training seq Stealing Flags Fig. 57 Data bits 8. 25 3 Bit GP

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

Full Rate Speech Coding • Coder for 20 ms 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 1 a - 3 parity bits from 50 bits. Class 1 b - 132 bits are not parity checked, but fed to convolutional encoder. Class 2 - 78 bits are not protected. 78 182

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

Convolutional Encoder K- constraint length R- rate of code K – storage location G 0 = d 4 + d 3 + 1 G 1 = d 4 + d 3 + d + 1 k=5 r=2/1

Parity bits Class 1 a 50 bits 50 R=1/2 k=5; Class 1 b 132 bits 3 132 378 Class 2 78 bits 4 78 78 456 bits in 20 ms ~ 456/0. 02 = 22. 8 kbps Tail bits

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 5 4 3 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.