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CDM-600L Satellite Modem
Forward Error Correction Options
performance curves that follow. For data rates above ~1 Mbps, Viterbi should be
considered the better alternative. The practical upper limit at this time is 2.048 Mbps.
Higher coding gain (1-2 dB) at lower data
rates, compared to Viterbi.
7.4
Reed-Solomon Outer Codec
It cannot be emphasized strongly enough that the purpose of the concatenated Reed-
Solomon is to dramatically improve the BER performance of a link under given
noise conditions. It should NOT be considered as a method to reduce the link EIRP
requirement to produce a given BER. Factors such as rain-fade margin, particularly
IMPORTANT
at Ku-band, are extremely important, and reducing link EIRP can seriously degrade
the availability of such a link.
The concatenation of an outer Reed-Solomon 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 and Sequential, which are convolutional), and in order to be processed correctly
the data must be framed and de-framed. Additionally, Reed-Solomon codes are limited
in how well they can correct errors that occur in bursts. This, unfortunately, is the nature
of the uncorrected errors from both Viterbi and Sequential decoders, which produce
clusters of errors that are multiples of half the constraint length. (This is particularly
severe in the case of Sequential, where the constraint lengths are considerably longer than
Viterbi). 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-600L, different R-S code rates are used, according to the mode of
operation:
Table 7-2. Sequential Decoding Summary
FOR
7–3
AGAINST
Pronounced threshold effect - does not fail
gracefully in poor Eb/No conditions.
Higher processing delay than Viterbi
(~4 k bits) - not good for low-rate coded voice.
Upper data rate limit approximately 2Mbps
Coding gain varies with data rate - favors lower
data rates.
Revision 0
MN/CDM600L.IOM
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