Comtech EF Data CDM-570A - Manual (Page 430)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix B Revision 2
B–2
B.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost universal
standard for satellite communications. The CDM-570A/570AL complies with the Intelsat IESS
308/309 standards for Viterbi decoding with a constraint length of seven. This is a de facto
standard, even in a closed network environment, which means almost-guaranteed
interoperability 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. It has a short constraint length, fixed at 7, for all code
rates. (The constraint length is defined as the number of output symbols from the encoder that
are affected by a single input bit.)
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%.
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 CDM-570A/570AL only permits a coding
rate of 1/2. Because the method of convolutional coding used with Viterbi, the encoder does
not preserve the original data intact, and is called non-systematic.
Table B-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.