Comtech EF Data SLM-5650A - Manual (Page 396)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–2
D.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The SLM-5650A complies with the Intelsat IESS 308 and
309 standards for Viterbi decoding, and has a short constraint length – defined as the number of
output symbols from the encoder that are affected by a single input bit – fixed at seven for all
code rates.
This constraint length of seven is a de facto standard, which facilitates the SLM-5650A’s inter-
operability with other manufacturer’s equipment. It provides very useful levels of coding gain,
and its short decoding delay and error-burst characteristics make it particularly suitable for low
data rate coded voice applications.
A major advantage of the Viterbi decoding method is that the performance is independent of data
rate, and does not display a pronounced threshold effect (i.e., does not fail rapidly below a certain
value of Eb/No). Note that in BPSK mode, the SLM-5650A permits code rates of 1/2, 3/4, and
7/8.
By choosing various coding rates (Rate 1/2, 3/4 or 7/8) the user can trade off coding gain for
bandwidth expansion:
Rate 1/2 coding gives the best improvement in error rate, but doubles the transmitted data rate,
and hence doubles the occupied bandwidth of the signal.
Rate 7/8 coding, at the other extreme, provides the most modest improvement in performance, but
only expands the transmitted bandwidth by 14 %.
Because the method of convolutional coding used with Viterbi, the encoder does not preserve the
original data intact, and is called non-systematic.
Table D-1. Viterbi Decoding Summary
FOR AGAINST
Good BER performance - very useful coding gain. Higher coding gain possible with other methods
Almost universally used, with de facto standards for
constraint length and coding polynomials
Shortest decoding delay (~100 bits) of any FEC
scheme - good for coded voice, VOIP, etc
Short constraint length produce small error bursts;
good for coded voice.
No pronounced threshold effect - fails gracefully.
Coding gain independent of data rate.