GSM Bursts & Frames

Bursts and time slots in the Air Interface

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Bursts from Mobile StationsBursts from Mobile Stations

BTSBTS

2Mbit/s to BSC2Mbit/s to BSC

TDMA Time SlotTDMA Time Slot

TDMA FrameTDMA Frame

Burst: It is defined as the information

present in a particular time slot Each GSM RF carrier supports eight

timeslots and the data are transmitted in the form of bursts that are designed to fit within these slots.

GSM burst types

tailbits

3

fixed bits ("0")142

tailbits

3

guardperiod

8,25 bits

FREQUENCY CORRECTION BURST

tailbits

3

encrypted bits57

SB1

training sequence

26

SB1

encrypted bits57

tailbits

3

guardperiod

8,25 bits

NORMAL BURST

TDMA FRAME ~ 4.615 ms

576.9 µs

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 training sequence

64

encrypted bits39

tailbits

3

guardperiod8,25 bits

SYNCHRONISATION BURST

tailbits

3

mixed bits142

tailbits

3

guardperiod

8,25 bits

DUMMY BURST

Burst types

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

Normal burstUsed to send all other logical channel information.

Frequency correction burstUsed to transmit the FCCH channel.

Synchronisation burstUsed to transmit synchronisation information.

Access burstUsed to send RACH information.It has a long guard period to allow the BTS to calculate the MS

distance from 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.

Normal Burst

T3 Coded Data57

TrainingSequence 26

CodedData 57

GP8.25

S1 S1 T3

148 bits = 0.54612 msec

This burst is used to carry information on TCH and on the control channelsBCCH, PCH, AGCH, SACCH and FACCH.

Note: GP (Guard Period)

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, of 57

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 of 26 bits. It is

used to adapt parameters of receiver to the current path propagation characteristics. It synchronizes the receiver, thus masking out multipath propagation effects.

Strongest signal selected in case of multipath propagation The guard period (GP), with a length of 8.25 bits, is

used to avoid a possible overlap of two mobiles information.

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 their internal timebase s.

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

Synchronization Burst The synchronization burst is used to transmit

synchronization channel information (SCH). As its name suggests, this burst carries

details of the GSM frame structure and allows an MS to fully synchronize with the BTS.

The synchronization burst is the first burst that the MS has to demodulate and, for this reason, the training sequence is extended to 64 bits. It also allows larger multipath delay spreads to be resolved.

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

access burst comes in a rather unique format because of its special tasks.

A mobile station uses the access burst only for the initial access to a BTS.

MS does not know the current distance to the BTS it generally is uncertain if the access burst arrives with in 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 for the access burst was set to only 88 bits along with increased guard band bits of 68.25.

The purpose of this extra free space is to measure the distance between MS and BTS at the beginning of a connection.