Comtech EF Data SLM-5650B, Installation And Operation Manual

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SLM-5650B Satellite Modem
Revision 2
Appendix D D–4 MN-SLM-5650B
D.5 Closed Network Mode
A 225,205 or 220,200 code is used in closed network mode. For a rate of 225,205 data is put into
blocks of 225 bytes, of which 205 bytes are data, and 20 bytes are FEC overhead. The code was
chosen because it is compatible with legacy Comtech EF Data modems.
For closed network Viterbi Reed Solomon, an interleaver depth of 4 or 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 low data rates, this equates to an
appreciable part of a second, when combined with the round trip delay makes 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.
Table D-3. Open Network Modes
Code Rate Mode
225, 205, 219, 201
208, 192, 194, 178
126, 112
IESS-308
219, 201 IESS-309 IBS, VSAT-IBS and Extended
219, 201 IESS-310
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 SLM-5650B Satellite Modem 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.
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 D-4. 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
Interoperable with legacy Intelsat networks Adds to demod acquisition time.