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CDM-600L Satellite Modem
Forward Error Correction Options
It can be seen that the 8-PSK Rate 3/4 Turbo performance closely approaches that of the
Rate 2/3 TCM/Reed-Solomon case – the BER performance is within approximately 0.4
dB. However, it should be noted that the Rate 3/4 Turbo mode is 20% more bandwidth
efficient than the TCM case. The additional advantages of Turbo (lower delay,
performance during fades etc) should also be considered.
Exceptionally good BER performance - significant improvement compared
with every other FEC method in use today
No pronounced threshold effect - fails gracefully
Exceptional bandwidth efficiency
Coding gain independent of data rate (in this implementation)
Low decoding delay
Easy field upgrade in CDM-600L
7.7
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-600L offers this uncoded mode for
three modulation types - BPSK, QPSK and Offset QPSK. 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/Offset QPSK. 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, an entirely
different scheme 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.
Table 7-7. Turbo Product Coding Summary
FOR
7–9
Revision 0
MN/CDM600L.IOM
AGAINST
Nothing!
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