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CDM-600 Satellite Modem Revision 7
Forward Error Correction Options MN/CDM600.IOM
7–2
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 CDM-600 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 7-1. 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.
7.3 Sequential
Although the method of convolutional coding and Sequential decoding appears to be very
similar to the Viterbi method, there are some fundamental differences. To begin with, the
convolutional encoder is said to be systematic - it does not alter the input data, and the FEC
overhead bits are simply appended to the data. Furthermore, the constraint length, k, is much
longer (Rate 1/2, k=36. Rate 3/4, k= 63. Rate 7/8, k=87). This means that when the decoding
process fails (that is, when its capacity to correct errors is exceeded) it produces a burst of
errors which is in multiples of half the constraint length. An error distribution is produced
which is markedly different to that of a Viterbi decoder. This gives rise to a pronounced
threshold effect. A Sequential decoder does not fail gracefully - a reduction in Eb/No of just a
few tenths of a dB can make the difference between acceptable BER and a complete loss of
synchronization. The decoding algorithm itself (called the Fano algorithm) uses significantly
more path memory (4 kbps in this case) than the equivalent Viterbi decoder, giving rise to
increased latency. Furthermore, a fixed computational clock is used to process input symbols,
and to search backwards and forwards in time to determine the correct decoding path. At