Comtech EF Data CDM-550T, Руководство пользователя

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CDM-550T Satellite Modem Forward Error Correction Options
Rev. 1.3 7–3
Table 7-2 Sequential Decoding Summary
FOR AGAINST
Higher coding gain (1 -2 dB) at lower
data rates, compared to Viterbi. 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.
Coding gain varies with data rate - favors lower
data rates.
7.4 Reed-Solomon Outer Codec (Option)
VERY IMPORTANT NOTE:
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 RS 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-