Comtech EF Data SLM-5650A - Manual (Page 398)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–4
D.4 Closed Network Modes
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 four or eight 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-2. 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-5650A, 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.
Do not operate the demodulator near its sync threshold. Small fluctuations in
Eb/No can cause total loss of the link, and the demodulator must then re-
acquire the signal.
Table D-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
Interoperable with legacy Intelsat networks Adds to demod acquisition time.