Comtech EF Data CDM-550T - Manual (Page 80)

CDM-550T Satellite Modem Revision 3
Forward Error Correction Options MN/CDM550T.IOM
It can be clearly seen that the concatenated Reed-Solomon cases increase the delay very
significantly (due mainly to interleaving/de-interleaving), while the TPC cases yield delays which
are less than or equal to Sequential decoding.
FOR AGAINST
Exceptionally good BER performance - significant
improvement compared with every other FEC method in
use today Nothing!
No pronounced threshold effect - fails gracefully
Exceptional bandwidth efficiency for Rate 3/4 QPSK -
40% less than Viterbi and concatenated Reed-Solomon
Coding gain independent of data rate (in this
implementation)
Low decoding delay
Easy field upgrade in CDM-550T
7.6 Uncoded Operation (No FEC)
There are occasions where a user may wish to operate a satellite link with no forward error
correction of any kind. For this reason, the CDM-550T offers this uncoded mode for all three
modulation types - BPSK, QPSK and OQPSK. However, the user should be aware of some of the
implications of using this approach.
PSK demodulators have two inherent undesirable features. The first of these is known as ‘phase
ambiguity’, and is due to the fact the demodulator does not have any absolute phase reference,
and in the process of carrier recovery, the demodulator can lock up in any of K phase states,
where K = 2 for BPSK, K = 4 for QPSK/OQPSK. Without the ability to resolve these ambiguous
states there would be a 1-in-2 chance that the data at the output of the demodulator would be
wrong, in the case of BPSK. For QPSK, the probability would be 3 in 4.
The problem is solved in the case of BPSK by differentially encoding the data prior to
transmission, and then performing the inverse decoding process. This is a very simple process,
but has the disadvantage that it doubles the receive BER. For every bit error the demodulator
produces, the differential decoder produces two.
The problem for QPSK is more complex, as there are 4 possible lock states, leading to 4
ambiguities. When FEC is employed, the lock state of the FEC decoder can be used to resolve
two of the four ambiguities, and the remaining two can be resolved using serial differential
encoding/decoding. However, when no FEC is being used, another scheme entirely must be used.
Therefore, in QPSK, a parallel differential encoding/decoding technique is used, but has the
disadvantage that it again doubles the receive BER.
OQPSK is a different situation again, where the ambiguities result not only from not having an
absolute phase reference, but also not knowing which of the two parallel paths in the demod, I or
Q, contains the half-symbol delay. Another type of differential encoding is used, but yet again the
error rate is doubled, compared to ideal.
7–6