Comtech EF Data CDM-570 - Manual (Page 174)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Forward Error Correction Options MN/CDM570L.IOM
8–8
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.
Table 8-6. Turbo Product Coding Summary
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.
• Coding gain independent of data rate
(in this implementation).
• Low decoding delay.
• Easy field upgrade in CDM-570/570L.
8.6 Uncoded Operation (No FEC)
There are occasions when you may wish to operate a satellite link with no forward error
correction of any kind. For this reason, the CDM-570/570L offers this uncoded mode for three
modulation types - BPSK, QPSK, and OQPSK. However, you 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. 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.
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,