Mocomtech CDM-QX operation manual Closed Network Modes, Concatenated R-S Coding Summary

CDM-Qx Satellite Modem	Revision 5
Forward Error Correction Options	MN/CDMQx.IOM

6.3.1Closed 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. 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.

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 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.

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 6-2. Concatenated R-S Coding Summary

FOR	AGAINST
Exceptionally good BER performance	Very pronounced threshold effect - does not fail
- several orders of magnitude	gracefully in poor Eb/No conditions. Additional coding
improvement in link BER under given	overhead actually degrades sync threshold, and reduces
link conditions.	link fade margin.
Very small additional bandwidth	Significant processing delay (~25 kbits) - not good for
expansion	voice, or IP applications
	Adds to demod acquisition time.

6–4