Mocomtech SLM-5650 - Manual (Page 118)

SLM-5650 Satellite Modem Revision 1
Options MN/SLM5650.IOM
A–2
A.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The SLM-5650 complies with the Intelsat
IESS 308 and 309 standards for Viterbi decoding with a constraint length of seven. This is a
de facto standard, which means 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. 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 SLM-5650 permits code rates of 1/2, 3/4, and 7/8.
Because the method of convolutional coding used with Viterbi, the encoder does not preserve
the original data intact, and is called non-systematic.
Table A-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.