Ttypes of Burst in GSM

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In almost every wireless communication, we use various kind of channel types. Each of those different channel tend to have its own physical (or transport layer) structure. In GSM as well, we have many different types of physical and logical channels and each of these channel types are using its own physical structure at the level of a slot (Burst). Followings are the types of Burst being used in GSM/GPRS.

 Frequency Correction Burst 
  • This burst format is used by FCCH channel only.
  • The whole data space (142 bits) is used for unmodulated carrier (pure sinusoid) or carrier modulated with all zero bits.
  • The frequency is 1625/24 kHz (or approximately 67 kHz).
  • This pure carrier is the ‘identity’ of a beacon frequency (also called BCCH-frequency or base-frequency) and FCCH slot.

Synchronization Burst
  • This burst format is used by SCH channel only.
  • This channel makes a mobile station time-synchronized with the base station clock. That is why the synchronization training sequence is very large for this burst comparing to other burst types.
  • Only one training sequence is defined for this burst.

 Access Burst 
  • This burst format is used by RACH and AGCH channels.

When a mobile station sends an RACH message and receives an AGCH reply, neither MS nor the BTS does have the timing-advance information. For that reason, the actual message is relatively short and have a long guard band (GB) in order to make sure that there will be no overlap with the next burst. The length of the guard band in the access burst (68.25 bits x 3.69 = 251.16 ms) is equivalent to 37.5 km propagation delay. The GSM allows a cell radius up to of 35 km. That is, an RACH message from an MS at a distance of up to 35 km from the base station can reach to the base station antenna without overlapping the next burst. The FACCH channel uses this burst during handover operation (when the timing advance of new cell is not yet known). Only one training sequence is defined for this burst.

 Normal Burst
  • This burst format is used by all other channels (except FCCH, SCH, RACH and AGCH).
  • This normal burst is used by TCH, SDCCH, SACCH, FACCH, BCCH and PCH.
A few important features of the burst is stated below.
o Maximum 57 x 2 = 114 bits of voice/data per burst
o Flag bit is to indicate if the channel is carrying user traffic (Flag = 0) or control message bits (Flag = 1).
That is the flag is 0 for TCH and 1 for others.

 Dummy Burst 

This is like normal burst but has no meaning of its payload bits.

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