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 MSC Region . MSC R. Location Area
BSC
BSC . . . . . .
Location Area
MSC R.

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

5. 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.)

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. Frequency Channels in GSM
Uplink frequency band : MHz
Downlink frequency band : 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
124 1 2 1 2 7 3 4 5 6 8
TDMA structure of
each channel
935 Mhz

8. So the MS does not have to Transmit and 1 2 7 3 4 5 6 8
Downlink
Delay 1 2 7 3 4 5 6 8
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 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 1 2 7 3 4 5 6 8 9 10 11 12 13 14 26
TDMA Frames
on a given channel
Traffic
Signaling (S)
S-contains information about the signal strength in neighboring cells

12. Traffic channel structure for half rate coding1 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 26
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 ms ( => a maximum distance
of ~ 35 km)

15. Timing Advance : How it works.
(Sent by BS on
down link) 8
| | | | |
| | | | |
| | |
| | | | 8
One way
Propagation
delay
| | |
(received by BS on
up link)
Two way propagation
delay
(received by MS on
down link)
| | | | |
(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 3 142 fixed bits 3 8.25 FB SB 3 39 data 64 bit
Training seq
8.25
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 Fig. Tail bit 3 57 Data bits 26 bit Training seq 57
8.25
Bit GP
Stealing Flags
Fig.

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

22. Full Rate Speech Coding
Coder for 20ms segments 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 bits are not parity checked,
but fed to convolutional encoder.
Class bits are not protected. 78
182

24. A Block Encoder (53, 50) G(D) = 1 + D+ D3 SW D D D + +
First 50 bits
data
Last 3 parity bits
clock cycles, SW closed
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 Parity bits
Tail bits 50 3 4
132 78
R=1/2 k=5; 78
456 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 CNG 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.