Comtech EF Data CDM-550T - Manual (Page 76)

CDM-550T Satellite Modem Revision 3
Forward Error Correction Options MN/CDM550T.IOM
Note: In BPSK mode, the CDM-550T 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 Sequential.
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.
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 appear 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 kbits 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 lower data rates there are sufficient number of
computational cycles per input symbol to permit the decoding process to perform optimally.
However, as the data rate increases, there are fewer cycles available, leading to a reduction in
coding gain. This is clearly illustrated in the performance curves which follow. For data rates
above ~1 Mbps, Viterbi should be considered the better alternative.
Table 7-2 Sequential Decoding Summary
FOR AGAINST
Higher coding gain (1 -2 dB) at lower
data rates, compared to Viterbi. Pronounced threshold effect - does not fail gracefully in
poor Eb/No conditions.
Higher processing delay than Viterbi
(~4 k bits) - not good for low-rate coded voice.
Coding gain varies with data rate - favors lower data
rates.
7–2