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A.1.4.8.1
Specifications
Refer to Table A-14 for Viterbi specification.
A.1.4.8.2
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.
A–44
Table A-14. Viterbi Specification
Parameter
BER
See Chapter 1
Maximum Data Rate
2.048 Mbit/s (rate 1/2)
2.048 Mbit/s (rate 3/4)
2.048 Mbit/s (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
Phase compensation with an ambiguity resolver
De-puncturing
Computing branch metric values
Add-Select-Compare (ASC) computer processing
Memory storage
Descrambling
Differential decoding
SDM-300A Satellite Modem
Specification
Rev. 4
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