Comtech EF Data SDM-300A - Manual (Page 238)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–2
9.2 Viterbi
The combination of convolutional coding and Viterbi decoding has become an almost
universal standard for satellite communications. The SDM-300A 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 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). This is not true of the Sequential decoding method, as
explained in the section below. Note that in BPSK mode, the SDM-300A 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.