Radio Burst Multiplexing

6.7.1 RADIO BURST MULTIPLEXING PRINCIPLES

Data is transmitted over the GSM Air interface in blocks. Each block is known as a ‘burst’.
One burst corresponds to one timeslot on an 8-timeslot TDMA frame.
GSM defines a number of burst types, dependant upon the burst’s function. However, they
all have a size of 156.25 bit periods with time duration of 0.577ms (see section on
voice/channel coding for explanation).
However, the Guard period of each burst exists to provide an inter-timeslot buffer and is
therefore not actually part of the information burst. Therefore the burst size is often referred
to by its information size. For example, a normal burst of 156.25 bit periods has an 8.25 guard
period and therefore has an information burst size of 148 bits.
Each burst bit is numbered 0-156 with the last ¼ bit being 156. The 0 bit is always transmitted
first.

Types of Data Burst

• The 156.25 bit periods of a timeslot can hold different types of data
burst:

Normal Burst

(Traffic and most control channels)
Frequency Correction Burst (FCCH)
Data and tail bits are all 0
Synchronisation Burst (SCH)
Data to synchronise MS with BTS
Dummy Burst
Transmitted on BCCH carrier when there are no other
bursts - allows power level measurements
Access Burst (RACH)
Long guard period to avoid collisions

GSM defines 5 bursts types include:

· Normal Bursts
· Synchronisation Bursts
· Frequency Correction Bursts
· Access Bursts
· Dummy Bursts

6.7.2 NORMAL BURST

A normal burst is the most common burst type and is generally used for carrying user data
(voice) over the Traffic Channel (TCH) and for control channels other than the FCCH, SCH
and RACH (see different burst types below). It has an information burst size of 148 bits
(156.25-8.25 guard period). The burst comprises the following elements:
· Data Blocks. The burst includes 2 x 57-bit voice/data blocks. These blocks (plus the
stealing bits) are encrypted (see section on voice/channel coding for explanation of
how blocks are inserted into bursts).
· Tail Bits. The burst includes 2 x 3-bit tail blocks. These blocks are always set to
0,0,0. Their purpose is to assist in the equalisation process by providing a start/stop
reference for the equaliser.
· Training Bits. Each burst contains a 26-bit training sequence. This sequence is used
by the equaliser to compensate for timing variations on the channel (see section on
Equalisation). GSM defines 8 distinct 26-bit training patterns. The sequence used is
determined at call setup.
· Stealing Bits. Each burst contains two stealing bits, one associated with each
voice/data block. A stealing bit is set to 1 if its associated data block has been ‘stolen’
for use by the FACCH (see section on GSM control channels for further explanation).
· Guard Period. The 8.25-bit guard period is unused space with allows for a short time
gap between consecutive timeslots to prevent inter-timeslot interference over the
channel. The guard period also allows time for the transmitter to ramp -up and rampdown.

6.7.3 FREQUENCY CORRECTION BURST

The Frequency Correction burst is used for frequency synchronisation of the MS. The data
bits are all set to 0, the equivalent of an unmodulated carrier with a specific frequency offset.
The repetitions of these bursts are collectively known as the Frequency Correction Channel
(FCCH). It has an information burst size of 148 bits (156.25-8.25 guard period). The tail bits
and guard band are the same as for a normal burst.

6.7.4 SYNCHRONISATION BURST

The synchronisation burst is used for time-synchronisation of the mobile with the BTS. It
differs from the Normal Burst primarily in the fact that it has an extended 64-bit training
sequence, with no stealing bits and reduced-size data blocks. The repetitions of these bursts
are collectively known as the Synchronisation Channel (SCH). It has an information burst
size of 148 bits (156.25-8.25 guard period). A single burst comprises the following elements:
· Data Blocks. The burst includes 2 x 39-bit encrypted data blocks. These blocks carry
information regarding the TDMA frame number and Base Station Identity Code
(BSIC). It is broadcast in conjunction with the Frequency Correction burst. The frame
Number (FN) is encoded in a 19-bit pattern which repeats every 2,715,648 frames (3.5
hours). The TDMA frame number is used by the mobile to determine the type of
logical channel being transmitted on the control channel, TS0. The BSIC is also used
by the mobile for checking the identity of the BTS when making power
measurements.
· Training Bits. The extended 64-bit training sequence is used to allow for timesynchronisation
between the mobile and the BTS.
The tail bits and guard band are the same as for a normal burst.

6.7.5 DUMMY BURST

The format of Dummy Burst is the same as that for the Normal Burst (the stealing bits have
relevance here). However, it contains no information. It is generally transmitted by the BTS
on any BCH carrier timeslot (including TCHs) when no other information is to be sent so that
neighbouring cells can still carry out power measurements.

6.7.6 ACCESS BURST

The Access Burst is used to gain access to the network and is therefore uplink only. It is also
used to request resources from the new cell on handover. It has a shortened information
burst size of 88 bits (156.25-68.25 guard period). The burst comprises the following elements:
· Data Blocks. The burst includes a 36-bit data block. This block contains information
from the MS requesting network resources
· Tail Bits. The burst includes a normal 3-bit tail block and an extended 8-bit tail block.
· Training Bits. Each burst contains an extended 41-bit training sequence to allow for
the equaliser to provide adequate timing adjustments. This sequence is used to
synchronise the MS to the BTS. The BTS detects the 41-bit access sequence and
computes the timing advance value which is then transmitted to the MS.
· Guard Period. The extended 68.25-bit guard period is to allow for the fact that when
an MS initially connects to the network, it has no information about timing advance.
This guard period allows for maximum initial timing advance and hence maximum
cell size (max 64-bit timing advance value, up to 37.5km round-trip distance from the
BTS1) until the MS is informed about its correct timing advance level (see above).