Comtech EF Data CDM-625 - Manual (Page 274)

CDM-625 Advanced Satellite Modem Revision 15
Forward Error Correction Options MN-CDM625
7–2
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) you 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 unit 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 (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.