User Manual

ArchitectureNortel Networks Confidential 3–41
S8000/S8002/S8006 BTS Reference Manual
The receiver has the space diversity function. Both received channels are combined
in an equalizer which carries out joint equalization.
For each of these channels, the pulse response as well as the C/I+N ratio are
estimated. These ratios are used to weight the predictions and samples of each
channel.
The symbols from the equalizer are then decrypted, de–interleaved and decoded to
restore the control messages and traffic sent by the mobile.
Ciphering/deciphering function
The fluxes of binary symbols sent and received on each time slot on the TCH or
SDCCH are encrypted one bit at a time, in compliance with the
ciphering/deciphering algorithm.
The ciphering or deciphering operation protects confidentiality of voice and
signaling. It consists of adding binary bits, one by one, between sent and received
data and a binary train (the ciphering sequence), generated from a ciphering key and
the TDMA frame number of the time slot.
Encoding/decoding and interleaving/de–interleaving functions
All traffic and control logic channels are encoded to protect useful information
against transmission errors. Each channel has its own encoding scheme, usually
including the following steps for each block:
protection of data bits with parity bits or a block code
encoding of the ”data bits + check bits” unit with a convolutional code; this
operation gives encoded bits
rearrangement and interleaving of the encoded bits
burst formating
For data, the encoding procedure depends on the rate: the interleaving level is
higher for data than for voice.
Some channels do not use the encoding schemes described above, in particular the
RACH, FCCH and SCH channels, for which the notion of interleaving on several
times slots does not exist.
Mobile transmission timing advance function
The BTS must measure the delay on the received signal when the mobile station
makes itself known.
This measurement, known as timing advance, is forwarded in the dedicated channel
assignment message (immediate assignment) to the MS, which uses this parameter
to anticipate its transmission timing.