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CDM-625 Advanced Satellite Modem
Forward Error Correction Options
FOR
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
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 1990's. 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), and 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.
7.3.1 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.
Table 7-1. Viterbi Decoding Summary
7–2
MN-0000036 (Ref MN/CDM625.IOM)
AGAINST
Higher coding gain possible with other methods
Revision 1
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