01 Frames Structures and Burst

7/29/2019 01 Frames Structures and Burst

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GSM Bursts & Frames


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Bursts and time slots in the Air

Interface ... ...

Bursts from Mobile Stations

BTS

2Mbit/s to BSC

TDMA Time Slot

TDMA Frame


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Burst:

It is defined as the information presentin a particular time slot

Each GSM RF carrier supports eighttimeslots and the data are transmitted

in the form of bursts that are designedto fit within these slots.


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GSM burst types

tailbits

3

fixed bits ("0")142

tailbits

3

guardperiod

8,25 bits

FREQUENCY CORRECTION BURST

tailbits

3

encrypted bits57

SB1

trainingsequence

26

SB1

encrypted bits57

tailbits

3

guardperiod

8,25 bits

NORMAL BURST

TDMA FRAME ~ 4.615 ms

576.9 s


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GSM burst types (2)

ext. tailbits

8

synchronisationsequence

41

encrypted bits36

tailbits

3

extended guard period68,25 bits

ACCESS BURST

tailbits

3

encrypted bits39

extended trainingsequence

64

encrypted bits39

tailbits

3

guardperiod8,25bits

SYNCHRONISATION BURST

tailbits

3

mixed bits142

tailbits

3

guardperiod

8,25 bits

DUMMY BURST


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Burst types

The information contained in one time slot is a burst. 5 types of bursts are available

Normal burst

Used to send all other logical channel information. Frequency correction burst

Used to transmit the FCCH channel. Synchronisation burst

Used to transmit synchronisation information. Access burst

Used to send RACH information.

It has a long guard period to allow the BTS to calculate the MS distancefrom the BTS and to provide timing advance information to the MS.

Dummy burstUsed to fill up unused timeslots, which transmits the BCCH channel.

No real information.


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Normal Burst

T3 Coded Data

57

Training

Sequence 26

Coded

Data 57

GP

8.25

S1 S1 T3

148 bits = 0.54612 msec

This burst is used to carry information on TCH and on the control channels

BCCH, PCH, AGCH, SACCH and FACCH.

Note: GP (Guard Period)


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Normal burst The tail bits(T) are a group of 3 bits set to zero and placed at

the beginning and the end of a burst. The coded data bits corresponds to two groups, of57 bits

each, containing signaling or user data. The stealing flags(S) indicate, to the receiver, whether block

contains data or is "stolen" The training sequence has a length of26 bits. It is used to

adapt parameters of receiver to the current path propagationcharacteristics. It synchronizes the receiver, thus masking outmultipath propagation effects. Strongest signal selected in case of multipath propagation

The guard period(GP), with a length of8.25 bits, is used toavoid a possible overlap of two mobiles information.


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Frequency Correction Burst The most simple format of all the bursts is

used for the frequency correction burst,which is transmitted only in the frequency

correction channel (FCCH). The frequency correction burst is also used

by MSs as a frequency reference for theirinternal timebase s.

All 148 bits (142 bits + 6 tail bits) are codedwith 0 and frequency a pure sinewave istransmitted which is the frequency with whichMS has to tune with.


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Synchronization Burst The synchronization burst is used to transmit

synchronization channel information (SCH).

As its name suggests, this burst carriesdetails of the GSM frame structure and allowsan MS to fully synchronize with the BTS.

The synchronization burst is the first burst

that the MS has to demodulate and, for thisreason, the training sequence is extended to64 bits. It also allows larger multipath delayspreads to be resolved.


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Access Burst In contrast to the bursts described so far, the access

burst comes in a rather unique format because of itsspecial tasks.

A mobile station uses the access burst only for theinitial access to a BTS. MS does not know the current distance to the BTS it

generally is uncertain if the access burst arrives within specified time frame and there exists chances of

overlapping with other bursts. To ensure that an access burst arrives at the BTS

during the proper time period the number of bits forthe access burst was set to only 88 bits along withincreased guard band bits of 68.25.


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The purpose of this extra free space isto measure the distance between MS

and BTS at the beginning of aconnection.

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01 Frames Structures and Burst