Comtech EF Data CDM-600/600L - Manual (Page 122)

CDM-600/600L Open Network Satellite Modem Revision 3
Forward Error Correction Options MN/CDM600L.IOM
6–2
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 E
b
/N
o
). This is not true of the Sequential decoding
method, as explained in the section below. Note that in BPSK mode, the CDM-600/600L 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 6-1. 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 produces small error
bursts – good for coded voice.
No pronounced threshold effect – fails gracefully.
Coding gain independent of data rate.
6.3 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 E
b
/N
o
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 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 that follow. For