Premier Mounts CDM-600 specifications Reed-Solomon Outer Codec, Sequential Decoding Summary

CDM-600 Satellite Modem	Revision 7
Forward Error Correction Options	MN/CDM600.IOM

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.

Table 7-2. Sequential Decoding Summary

FOR

AGAINST

Higher coding gain (1-2 dB) at lower data	Pronounced threshold effect - does not fail
rates, compared to Viterbi.	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.

7.4Reed-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

IMPORTANT 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.

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 and Sequential, which are 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 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-600, different R-S code rates are used, according to the mode of operation:

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