Comtech EF Data SNM-1001L Installation And Operation Manual page 230

SNM-1001L Satellite Modem
Decoder
14.1.2.1 Viterbi Specification
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:
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.
Parameter
BER
Maximum Data Rate
Synchronization Time
Output Fault Indicators
Raw BER Detection
Differential Decoding
Constant Length
•
Phase compensation with an ambiguity resolver
•
De-puncturing
•
Computing branch metric values
•
Add-Select-Compare (ASC) computer processing
•
Memory storage
•
Descrambling
•
Differential decoding
Specification
See Chapter 1
2.048 Mbps (rate 1/2)
2.048 Mbps (rate 3/4)
2.048 Mbps (rate 7/8)
8000 bits (maximum)
Activity detection of I and Q data sign bits
From 0 to 255 bits out of 1024 samples
2-phase or none
7
14–4
Revision 1
MN/SNM1001L.IOM