GSM 03 Frame Structure
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Transcript of GSM 03 Frame Structure
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7/30/2019 GSM 03 Frame Structure
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Aster RNPD Pre ared b Sumit Kumar
FRAME STRUCTURE
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NORMAL BURST
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
3 57 bits 26 bits 57 bits 3
Frame 1 (4.615 ms.) Frame 2
0.577 ms.
0.546 ms.
Guard
PeriodTail
BitsData
Flag
Bit
Training
Sequence
FlagBit Data Tail
Bit
Guard
Period
Normal Burst carries traffic channel and control channels BCCH, PCH,
AGCH, ADCH, SACCH and FACCH.
1 1
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DESCRIPTION
Data Two Blocks of 57 Bits each. Carries speech, data or control
information.
Tail Bits Used to indicate the start and end of each burst. Three bits always
000.
Guard Period 8.25 bits long. The receiver can only receive and decode if the
burst is received in the timeslot designated for it. Since the MS are moving.
Exact synchronization of burst is not possible practically. Hence 8.25 bits
corresponding to about 30us is available as guard period for a small margin oferror.
Flag Bits This bit is used to indicate if the 57 bits data block is used as
FACCH.
Training Sequence This is a set sequence of bits known by both the
transmitter and the receiver. When a burst of information is received theequalizer searches for the training sequence code. The receiver measures
and then mimics the distortion which the signal has been subjected to. The
receiver then compares the received data with the distorted possible
transmitted sequence and chooses the most likely one.
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EQUALISER
The equaliser will mainly address the problems of intersymbol interference
the problem is that the air interface affects the signal in some way thatcauses bit error on the receiving side. If it was known in which way the
signals were affected, the system could take measures to correct the errors.
In a normal burst, used for traffic there is a 26 bit training sequence in the
middle of the burst. The bit pattern in this training sequence is known tothe system. By analysing it the system will know what the air interface
looks like and how it is affecting the signal sent.
By analysing the training sequence a channel model can be built. This
channel model is like a filter affecting the transmitted bits in the same
manner that the air interface is affecting them. By running different bit
patterns through the channel model and comparing the resulting signal
with what was actually received, the system can reach a conclusion as
to what was actually sent.
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SYNCHRONIZATION BURST
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
3 39 bits 64 bits 39 bits 3
Frame 1 (4.615 ms.) Frame 2
0.577 ms.
0.546 ms.
GuardPeriod
TailBits
EncryptedBits
SynchronizationSequence
EncryptedBits
TailBit
GuardPeriod
Carries SCH channel
Enables MS to synchronize its timings with the BTS.
Contains BSIC and TDMA Frame number.
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FREQUENCY CORRECTION BURST
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
3 142 bits 3
Frame 1 (4.615 ms.) Frame 2
0.577 ms.
0.546 ms.
GuardPeriod
TailBits
Fixed Data TailBit
GuardPeriod
Carries FCCH channel
Made up of 142 consecutive zeros.
Enables MS to correct its local oscillator locking it to that of the BTS.
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ACCESS BURST
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
8 36 bits
Frame 1 (4.615 ms.) Frame 2
0.577 ms.
TailBits
EncryptedBits
GuardPeriod
Carries RACH
Has a bigger guard period since it is used during initial access and the MS.
does not know how far it is actually from the BTS.
41 bits 68.25 bits3
TailBits
SynchronizationSequence
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DUMMY BURST
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
3 57 bits 26 bits 57 bits 3
Frame 1 (4.615 ms.) Frame 2
0.577 ms.
0.546 ms.
Guard
PeriodTail
Bits Data
Flag
BitTraining
Sequence
FlagBitData
Tail
Bit
Guard
Period
Transmitted on the timeslot of the BCCH carrier in the downlink.
1 1
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HYPERFRAME AND SUPERFRAME STRUCTURE
0 1 2 2045 2046 2047
3 h 28 min 53 s 760 ms 1 Hyper frame = 2048 super frames = 2,715,648 TDMA frames
0 1 2 3 47 48 49 50
0 1 24 25
0 1 2 23 24 25 0 1 2 48 49 50
0 1 2 3 4 5 6 7
6.12s 1 Super frame = 1362 TDMA frames = 51(26fr) or 26(51fr) multiframes
120 ms 235.38 ms
4.615 ms
Traffic 26 frame multiframe Control 51 frame multiframe
TDMA Frame
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NEED FOR TIMESLOT OFFSET
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
BSS Downlink
MS Uplink
If Uplink and Downlink are aligned exactly, then MS will have to transmitAnd receive at the same time. To overcome this problem a offset of 3
Timeslot is provided between downlink and uplink.
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NEED FOR TIMESLOT OFFSET
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5
3 Timeslot
offset
BSS Downlink
MS Uplink
As seen the MS does not have to transmit and receive at the same time
This simplifies the MS design which can now use only one synthesizer.
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26 FRAME MULTIFRAME STRUCTURE
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
4.615 ms.
120 ms.
MS on dedicated mode on a TCH uses a 26 - frame multiframe structure.
Frame 0 - 11 and 13 - 24 used to carry traffic.
Frame 12 used as SACCH to carry control information from and to MSTo BTS.
Frame 25 is idle and is used by mobile to decode the BSIC of neighbour
cells.
T T T T T T T T T T T T S T T T T T T T T T T T T T
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25