Comtech EF Data SNM-1001L - Manual (Page 230)

SNM-1001L Satellite Modem Revision 1
Decoder MN/SNM1001L.IOM
14–4
14.1.2.1 Viterbi Specification
Parameter Specification
BER See Chapter 1
Maximum Data Rate 2.048 Mbps (rate 1/2)
2.048 Mbps (rate 3/4)
2.048 Mbps (rate 7/8)
Synchronization Time 8000 bits (maximum)
Output Fault Indicators Activity detection of I and Q data sign bits
Raw BER Detection From 0 to 255 bits out of 1024 samples
Differential Decoding 2-phase or none
Constant Length 7
14.1.2.2 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 sign/magnitude 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
correct as many errors as possible. The code symbols produced by the encoder provide the data
for this task.
The seven functions used in processing the data stream are:
• Phase compensation with an ambiguity resolver
• De-puncturing
• Computing branch metric values
• Add-Select-Compare (ASC) computer processing
• Memory storage
• Descrambling
• Differential decoding
The data is first sent through an ambiguity resolver for compensating the potential 90
° phase
ambiguity inherent in a QPSK demodulator. The data is then de-punctured if the decoder is
operating in the 3/4 or 7/8 rates. The de-puncture pattern is the same as the puncture pattern
used in the encoder.
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.