Comtech EF Data SDM-300A - Manual (Page 241)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–5
9.4.1 Closed Network Modes
A 225, 205 code is used in EFD closed network modes. (225, 205 means that data is put
into blocks of 225 bytes, of which 205 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.
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 deinterleaver frame synchronization method can add significantly to the
time taken for the demodulator to declare acquisition.
9.4.2 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 SDM-300A,
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 RS 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.