Bursts in GSM

  Bursts

Bursts are instantaneous transmissions of data over an RF channel, with voice or data information.  There are five types of burst in GSM which are defined as follows:

·        Normal – carries voice and control information.

·        Frequency Correction Burst – used for frequency synchronization.

·        Synchronization Burst – used for timing synchronization between base station and mobile station.

·        Dummy Burst – mixture of bits sent when there is not any other communications taking place on the BCCH.

·        Access Burst – used on access and handover where timing advance information is unknown.

Each timeslot is divided into 156.25 bit periods, where every bit period is referenced to a bit number.  The first bit period is numbered 0 and the final .25 bit period is numbered 156.  Bit 0 is transmitted first and continues through to bit 156.  The description of bit numbering is important because the following sections describe the transmission timing and data associated with bit period during a burst.  Guard periods occur between consecutive bursts appearing in successive timeslots.

  Normal burst (NB)

The normal burst is used for carrying control and traffic information. Tail bits are located before and after the encrypted data, and occupy four consecutive bit periods.  Twenty-six bits form the training sequence, allowing the receiver’s equalizer to adapt its filters to the local RF environment.  Before and after the training sequence bits, is a stealing flag bit.  Stealing flags are used only for traffic channels and are used during FACCH messaging. Bits 148 through 156 provide a guard period to avoid collisions with the next subsequent burst.

0 TB

1 TB

2 TB

3 ED

… ED

… ED

60 F

61 TS

62 TS

… TS

… TS

86 TS

87 F

88 ED

89 ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure : Normal Burst

Where:

TB       = Tail Bits

ED       = Encrypted Data

F          = Flag

TS        = Training Sequence

GP       = Guard Period

    Frequency correction burst (FB)

The frequency correction burst is used for frequency alignment of the mobile.  This is accomplished by adjusting the mobile’s internal oscillator, or frequency source, to the same frequency of the serving base station.  Tail bits are located before and after the encrypted data, and occupy four consecutive bit periods.  142 bits form the sequence of frequency correction information, which essentially appears as an unmodulated carrier with a frequency offset of 67.7 kHz.  The frequency correction channel is made up of consecutive frequency correction bursts. Bits 148 through 156 provide a guard period to avoid collisions with the next subsequent burst.

0 TB

1 TB

2 TB

3 ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure : Frequency Correction Burst

Where:

TB       = Tail Bits

ED       = Encrypted Data

GP       = Guard Period

   Synchronization burst (SB)

The synchronization burst is used to adjust the time reference of the mobile station. This is accomplished by adjusting the mobile’s clock to the relative time of the serving base station.  This will allow the mobile unit’s transmission to be received by the base station in the correct time slot.  Contained within the burst is a long synchronization sequence, as well as TDMA frame number and Base Station Identity Code (BSIC) information. Tail bits are located before and after the encrypted data, and occupy four consecutive bit periods. Bits 148 through 156 provide a guard period to avoid collisions with the next subsequent burst.

0 TB

1 TB

2 TB

3 ED

4 ED

… ED

… ED

41 ED

42 SS

43 SS

… SS

… SS

105 SS

106 ED

107 ED

… ED

… ED

144 ED

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure : Synchronization Burst

Where:

TB       = Tail Bits

ED       = Encrypted Data

SS        = Synchronization Sequence

GP       = Guard Period

     Dummy Burst

The dummy burst is used on the BCCH when there are no other channels sending information.  This will allow the mobile scanning the BCCH to receive valid power measurements.  Contained within the burst is a mixture of bits carrying no relevant data and twenty-six training sequence bits.  Tail bits are located before and after the encrypted data, and occupy four consecutive bit periods. Bits 148 through 156 provide a guard period to avoid collisions with the next subsequent burst.

0 TB

1 TB

2 TB

3 MB

4 MB

… MB

… MB

60 MB

61 TS

62 TS

… TS

… TS

86 TS

87 MB

88 MB

… MB

… MB

144 MB

145 TB

146 TB

147 TB

148 GP

149 GP

…

156 GP

Figure: Dummy Burst

Where:

TB       = Tail Bits

MB      = Mixed Bits

TS        = Training Sequence

GP       = Guard Period

     Access burst (AB)

The access burst is used for random access and handover access by the mobile.  The burst is characterized by a long guard period, required when the mobile does not have timing advance information during access or handover.  Access bursts are used by the RACH and TCH after a handover.  Contained within the burst is a series of forty-one synchronization bits, followed by twenty-six encrypted data bits.  Tail bits are located before and after the encrypted data, the first series of tail bits occupy eight bit positions and the final four occur after the final encrypted bits. Bits 88 through 156 provide a guard period to avoid collisions with the next subsequent burst.

0 TB

1 TB

… TB

… TB

7 TB

8 SS

9 SS

… SS

… SS

48 SS

49 EB

50 EB

… EB

… EB

84 EB

85 TB

86 TB

87 TB

88 GP

89 GP

90 GP

… GP

… GP

… GP

156 GP

Figure : Access Burst

Where:

TB       = Tail Bits

SS        = Synchronization Sequence

EB       = Encrypted Bits

GP       = Guard Period

      Guard period

The guard period is provided to allow mobile station’s transmissions to be attenuated for some time between bursts.  This allows the mobile time to ramp-up and ramp-down during the guard periods.  The base station is not required to have the same ramping capabilities between adjacent bursts, but is required to have the ramping capabilities for non-used time-slots.