Post on 27-Jan-2016
description
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
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.
.
..
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)
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8
1 2 3 4 5 6 7
8
1 2 3 4 5 6 7 8
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One wayPropagationdelay
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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.