User Manual

ArchitectureNortel Networks Confidential 3–39
S8000/S8002/S8006 BTS Reference Manual
The DCU8 unit has five DSPs:
one EGAL DSP, which equalizes the reception signal
two DECOD DSPs, which handle reception signal decoding, and level 1
sequencing
two TRANS DSPs, which handle transmission signal processing, encoding, and
the interface with the remote transcoder
There is one DECOD DSP and one TRANS DSP in each chain.
SPU
The SPU carries out processing associated with the transmission layer, and to this
end, it executes a certain number of functions, such as: (see Figure 3–12 and
Figure 3–13):
demodulation of GMSK signal at reception
ciphering/deciphering of sent and received data
encoding/decoding and interleaving/de–interleaving of data from the various
channels
encoding/decoding of voice and data (from 13 kbit/s to 16 kbit/s and vice–versa)
transfer of discontinuous transmission (DTX) signal
control of transmitters and receivers
processing of radio measurements
Demodulation function
Demodulation consists of extracting, from the GMSK signal received, the binary
data transmitted, that is 144 bits for a normal burst and 36 bits for an access burst.
This is done for the eight time slots of the radio channel.
The demodulation principle selected takes into account the inter–symbol
interference resulting from smoothing of the transmission phase transitions
(limitation of the transmitted spectrum), multiple path phenomena, and distorsion
introduced by the channel filter upon reception.
Implementation of this type of demodulator requires modification of the
transmission channel as concerns pulse response, frequency deviation, and
reception times. Determining these parameters is part of the job of the demodulation
function.