Comtech EF Data SDM-9000 - Manual (Page 145)

SDM-9000 Satellite Modem Theory of Operation
Rev.4 5–17
5.4.3 Theory of Operation
The Viterbi decoder processes 3-bit quantized R0 and R1 parallel code bits or symbols
from the demodulator. The quantization is 3-bit soft decision in offset binary format.
This data is a representation of the data transmitted, corrupted by additive white
Gaussian noise. The task of the decoder is to determine which symbols have been
corrupted by the transmission channel and to correct as many errors as possible. The
code symbols produced by the encoder provide the data for this task.
The Viterbi decoder performs four functions for providing FEC. The functions used in
processing the data stream are:
•
Computing Branch Metric Values
•
ASC Computer Processing
•
Memory Storage
•
Synchronization Detect
A set of branch metric values is then computed for each of the received symbol pairs.
This is related to the probability that the received symbol pair was actually transmitted as
one of the four possible symbol pairs.
The branch metrics are then processed by the Add-Select-Compare (ASC) computer. The
ASC computer makes decisions about the most probable transmitted symbol stream.
These decisions are made when the ASC processes the current branch metrics with the
state metrics computed for the previous 64 decoder inputs.
The results of the ASC computer are stored in the path memory (80 states in depth). The
path with the maximum metric is designated as the survivor path and its data is used for
output. The difference between the minimum and maximum path metrics is used as the
means of determining synchronization of the decoder. A synchronization signal is used
for lock-detect and sent to the M&C.
The raw BER count is generated from the minimum and maximum metrics and sent to
the M&C for further processing. Refer to Table 1-2 and Figure 1-4 for typical Viterbi
decoder BER performance specifications.