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CDM-600 Satellite Modem Revision 7
Forward Error Correction Options MN/CDM600.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. There is also a 225, 205 code available that it compatible with legacy EF Data
modems. 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 IESS-308 E1, and IESS-310 mode
225, 205 Standard IESS-308 T1
194, 178 Standard IESS-308 T2/E2
126, 112 Standard IESS-309 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-600, 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 RS 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 kbps) - not good for
voice, or IP applications
Adds to demod acquisition time.