Comtech EF Data CDM-625 - Manual (Page 275)

CDM-625 Advanced Satellite Modem Revision 15
Forward Error Correction Options MN-CDM625
7–3
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 data rates above ~1 Mbps, Viterbi should be considered the
better alternative; the practical upper limit in this implementation is 2.048 Mbps.
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 grace-fully in
poor Eb/No conditions.
• Higher processing delay than Viterbi (~4 k bits) – not good
for low-rate coded voice.
• Upper data rate limit 2.048Mbps
• Coding gain varies with data rate – favors lower data rates.
7.4 Reed-Solomon Outer Codec
The concatenation of an outer Reed-Solomon (R-S) Codec with Viterbi decoder first became popular
when it was introduced by Intelsat in the early 1990s. It permits significant improvements in error
performance without significant bandwidth expansion. The coding overhead added by the R-S outer
Codec is typically around 10%, which translates to a 0.4 dB power penalty for a given link.
Reed-Solomon codes are block codes – as opposed to Viterbi, which is convolutional; in order to
be processed correctly, the data must be framed and de-framed. Additionally, R-S codes are
limited in how well they can correct errors that occur in bursts. This, unfortunately, is the nature
of the uncorrected errors from Viterbi decoders, which produce clusters of errors that are
multiples of half the constraint length. For this reason, the data must be interleaved following R-
S encoding, and is then de-interleaved prior to decoding. This ensures that a single burst of
errors leaving the Viterbi or Sequential decoder is spread out over a number of interleaving
frames, so errors entering the R-S decoder do not exceed its capacity to correct those errors. In
the case of the CDM-625, different R-S code rates are used according to the mode of operation:
Closed Network Modes and Open Network Modes.
7.4.1 Closed Network Modes
A 220,200 code is used in transparent closed network modes, and a 200,180 code is used in
framed (EDMAC) modes. (220,200 means that data is put into blocks of 220 bytes, of which 200
bytes are data, and 20 bytes are FEC overhead.) These two codes were chosen because they fit
well into Comtech EF Data’s clock generation scheme, and they have almost identical coding
gain. There is also a 225, 205 code available that it compatible with legacy EF Data modems.
When Viterbi decoding is used as the primary FEC, an interleaver depth of 4 is used. The
increase in coding gain is at the expense of delay. The interleaving/de-interleaving delay and the
delay through the decoder itself can be as high as 25 kbits. At very low data rates, this equates
to several seconds, making it highly unsuitable for voice applications. Additionally, the de-
interleaver frame synchronization method can add significantly to the time taken for the
demodulator to declare acquisition.