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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 BS
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
Norm al bu rs tUsed to send all other logical channel information.
Freq uen cy correc ti on burs tUsed to transmit the FCCH channel.
Syn chr oni sation burstUsed to transmit synchronisation information.
Acces s burstUsed 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.
Dum my b urs tUsed 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 bi ts (T) are a group of 3 bits set to zero and placed at
the beginning an d the end of a burst. The coded data bi ts corresponds to two groups, of57 bits
each, containing signaling or user data. The stea l ing fl ag s(S) indicate, to the receiver, whether block
contains data or is "stolen" The train ing 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 peri od (GP), with a length of8.2 5 bits , is used toavoid a possible overlap of two mobiles information.
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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 BTSduring 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.