CDM-Qx Satellite Modem
Forward Error Correction Options
they can correct errors that occur in bursts. This, unfortunately, is the nature of the
uncorrected errors from Viterbi decoders, which produce clusters of errors that are
multiples of half the constraint length. For this reason, the data must be interleaved
following R-S encoding, and is then de-interleaved prior to decoding. This ensures that a
single burst of errors leaving the Viterbi decoder is spread out over a number of
interleaving frames, so errors entering the R-S decoder do not exceed its capacity to
correct those errors.
In the case of the Modem, different R-S code rates are used, according to the mode of
operation:
6.3.1
Closed 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
Exceptionally good BER performance
- several orders of magnitude
improvement in link BER under given
link conditions.
Very small additional bandwidth
expansion
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
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.
Significant processing delay (~25 kbits) - not good for
voice, or IP applications
Adds to demod acquisition time.
6–3
Revision 6
MN/CDMQx.IOM
AGAINST
Need help?
Do you have a question about the CDM-Qx and is the answer not in the manual?
Questions and answers