The Logical
Channel is used in time multiplex in a physical channel, which is
categorized according to the types of messages transmitted in the physical
channel. Different logical channels are used in transmission of different types
of information between BS and MS, such as the signaling or traffic data. In GSM
system, five different types of burst sequences are specified for different
logical channels, which have different time-amplitude diagrams as shown in the
above diagrams.
The training sequence helps to discriminate radio
channels with same frequency so as to help to demodulate the signals. However,
there is no training sequence for FB and DB; for SB and AB, the training
sequence is constant, i.e. the synchronous bit; for NB, there are 8 different
training sequences specified in the specifications. These 8 different training
sequences of NB are numbered from 0 to 7, which are called training sequence
numbers. By allocating training sequences with distinct differences to channels
of the same frequency used in cells that are close to and may interfere with
each other, the co-frequency interference can be avoided efficiently during
modulation.
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