Comtech EF Data CDM-550T - Manual (Page 77)

CDM-550T Satellite Modem Revision 3
Forward Error Correction Options MN/CDM550T.IOM
7.4 Reed-Solomon Outer Codec (Option)
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 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
RS 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 which occur in
bursts. This, unfortunately, is the nature of the uncorrected errors from both Viterbi and
Sequential decoders, which produce clusters of errors which 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 RS
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 RS decoder do not exceed its capacity to correct those errors.
In the case of the CDM-550T, two R-S code rates are used. A 220,200 code is used in transparent
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. When Viterbi decoding is used as the primary FEC, an
interleaver depth of 4 is used. When Sequential decoding is used, an interleaver depth of 8 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.
A characteristic of concatenated R-S coding is the very pronounced threshold effect. For any
given modem design, there will be a threshold value of Eb/No below which the demodulator
cannot stay synchronized. This may be due to the carrier-recovery circuits, or the synchronization
threshold of the primary FEC device, or both. In the CDM-550T, and Rate 1/2 operation, this
threshold is around 4 dB Eb/No. Below this value, operation is not possible, but above this value,
the error performance of the concatenated R-S system produces exceptionally low error rates for a
very small increase in Eb/No.
CAUTION
Care should be taken not to operate the demodulator near its sync threshold.
Small fluctuations in Eb/No may cause total loss of the link, with the subsequent
need for the demodulator to re-acquire the signal.
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