Comtech EF Data SLM-5650A - Manual (Page 401)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–7
D.7 Sequential (FAST Option)
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 (i.e.,
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 lower data rates, there
are a 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. For data rates above ~1 Mbps, Viterbi should be considered
the better alternative. The practical upper limit at this time is 2.048 Mbps.
Table D-6. 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.
Upper data rate limit approximately 2Mbps
Coding gain varies with data rate - favors lower
data rates.