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CDM-600L Satellite Modem
Forward Error Correction Options
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 at this time is 2.048 Mbps.
FOR
Higher coding gain (1-2 dB) at lower data
rates, compared to Viterbi.
7.4
Reed-Solomon Outer Codec
IMPORTANT
The concatenation of an outer Reed-Solomon (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 Reed-Solomon 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 Reed-Solomon 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
Reed-Solomon decoder do not exceed its capacity to correct those errors. In the case of the
CDM-600L, different Reed-Solomon code rates are used, according to the mode of operation:
Table 7-2. Sequential Decoding Summary
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 at Ku-band, are
extremely important, and reducing link EIRP can seriously degrade the
availability of such a link.
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.
7–3
Revision 2
MN/CDM600L.IOM
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