GSM System Architecture
Network Subsystem MSC ??
Radio Subsystem BTS, BSC
Operation Support Subsystem
GSM System Hieararchy
Location Area
MSC R......
GSM Network
MSC Region
Location Area
BSC
BSCLocation Area
.
.
.
.
.
..
MSC R.
Operations and Maintenance Centre
BSC MSC
G MSC
BSC
BTS
BTS
MS BTS
MS
OMC
HLR
VLR
EIR
AVC
Home Location Register
Visitor Location Register
Equipment Identity Register
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 kHz each) in each band. ?? • Each channels has a TDMA structure with 8 timeslots. ( => upto 8 users per freq. channel )
•
124
•
•
•
1
2
1 2 73 4 5 6 8
TDMA structure of each channel
935 Mhz
960 Mhz
1 2 73 4 5 6 8
1 2 73 4 5 6 8
Delay
Downlink
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.
Note: These logical channels are then mapped onto Physical channels.
A GSM Physical channel comprises a particular timeslot on a given freq. Channel.
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)
Signalling channel contd. ....
Traffic channel structure for full rate coding
1 2 73 4 5 6 8 1 2 73 4 5 6 8 1
1 2 73 4 5 6 8 9 10 • 11 12 13 14 • 26
Traffic
Signaling (S)
TDMA slots
TDMA Frameson a given channel
S-contains information about the signal strength in neighboring cells
Traffic channel structure for half rate coding
1 2 73 4 5 6 8 1 2 73 4 5 6 8 1
1 2 73 4 5 6 8 • • • • • • 26
1 2 73 4 5 6 8 9 10 11 12 13 14 • • 26
• •
Burst for one user
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 s.• 64 steps allows compensation over a maximum propagation time of 31.5 bit periods ie 113.3 s ( => a maximum distance of ~ 35 km)
Timing Advance : How it works.
1 2 3 4 5 6 7
(Sent by BS on down link)
1 2 3 4 5 6 7
8
(received by BS on up link)
(received by MS on down link)
(Sent by MS on up link)
|||||
8
1 2 3 4 5 6 7
8
1 2 3 4 5 6 7 8
||||
One wayPropagationdelay
||||||
|
|
|
|
||| |
||
Two way propagation delay
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
142 fixed bits 33
339
data39
data3
64 bitTraining seq
8.25
3 326 bit
Training seq8.25
8.25
841 bit
Training seq36 data 3 68.25
FB
SB
DummyBurst
Access
Burst
F S B B B
F S B B B
F S B B B
Frequency synchronization
Time synchronizatio
nBCCH
R R R RRequest
(Random access)
C C C C Grant
D D D
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.
18278
A Block Encoder (53, 50)
D D D+ +
G(D) = 1 + D+ D3
1 - 50 clock cycles, SW closed51 - 53 clock cycles, SW open
SW
data
Last 3 parity bits
First 50 bits
Convolutional Encoder
K- constraint length k=5R- rate of code r=2/1
K – storage location
G0 = d4 + d3 + 1G1 = d4 + d3 + d + 1
R=1/2 k=5; 378 78
456 bits
50 3 4132 78
Class 1a50 bits
Class 1b132 bits
Class 278 bits
456 bits in 20 ms ~ 456/0.02 = 22.8 kbps
Tail bits
Paritybits
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 3 4
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.