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CDM-600L Satellite Modem
Forward Error Correction Options
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-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.
Good BER performance - very useful coding gain.
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 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
Table 7-1. Viterbi Decoding Summary
FOR
7–2
MN/CDM600L.IOM
AGAINST
Higher coding gain possible with
other methods
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