Mocomtech CDM-QX operation manual Reed-Solomon Outer Codec

CDM-Qx Satellite Modem	Revision 5
Forward Error Correction Options	MN/CDMQx.IOM

6.3Reed-Solomon Outer Codec

IMPORTANT

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.

The concatenation of an outer Reed-Solomon Codec with Viterbi decoder first became popular when Intelsat introduced it 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

0.4dB 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, 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 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 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 Modem, different R-S code rates are used, according to the mode of operation:

6–3