Comtech EF Data CDM-600L, Installation And Operation Manual

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CDM-600L Satellite Modem Revision 0
Forward Error Correction Options MN/CDM600L.IOM
7–4
Closed Network Modes
A 220,200 code is used in transparent closed network 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 kbps. 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.
Open Network Modes
Code Rate Mode
219, 201 Standard IESS308 E1, and IESS 310 mode
225, 205 Standard IESS308 T1
194, 178 Standard IESS308 T2/E2
126, 112 Standard IESS309 modes
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-600L,
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.
Table 7-3. Concatenated R-S Coding Summary
FOR AGAINST
Exceptionally good BER performance
- several orders of magnitude
improvement in link BER under given
link conditions. Very pronounced threshold effect - does not fail
gracefully in poor Eb/No conditions. Additional coding
overhead actually degrades sync threshold, and reduces
link fade margin.
Very small additional bandwidth
expansion Significant processing delay (~25 kbits) - not good for
voice, or IP applications
Adds to demod acquisition time.