GSM 03 Frame Structure

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



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


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.


7/12Aster RNPD Pre ared b Sumit Kumar

ACCESS BURST

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

8 36 bits


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



DUMMY BURST

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7



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



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



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.



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.



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

Documents

GSM 03 Frame Structure