Comtech EF Data CDM-570 - Manual (Page 169)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Forward Error Correction Options MN/CDM570L.IOM
8–3
errors. In the case of the CDM-570/570L, two different RS code rates are used, according to the
mode of operation.
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 four 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 kbits. 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 RS coding is the very pronounced threshold effect. For any given
modem design, there will be a threshold value of E
b
/N
o
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-570/570L, and Rate 1/2 operation, this threshold
is around 4 dB E
b
/N
o
. 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 E
b
/N
o
.
Care should be taken not to operate the demodulator near its sync
threshold. Small fluctuations in E
b
/N
o
may cause total loss of the link,
with the subsequent need for the demodulator to re-acquire the signal.
Table 8-2. 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
kbits) - not good for voice, or IP
applications.
• Adds to demod acquisition time.