Comtech EF Data CDM-600/600L - Manual (Page 131)

CDM-600/600L Open Network Satellite Modem Revision 3
Forward Error Correction Options MN/CDM600L.IOM
6–11
6.8 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-600/600L offers this uncoded mode for 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. 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,
contains the half-symbol delay. Another type of differential encoding is used, but yet again the error
rate is doubled, compared to ideal.
NOTE: Whenever uncoded operation is selected, the modem offers the choice between enabling
and disabling the differential encoder/decoder appropriate for the modulation type.
The second problem inherent in PSK demodulators is that of ‘data false-locking’.
In order to accomplish the task of carrier recovery, the demodulator must use a non-linear process.
A second-order non-linearity is used for BPSK, and a fourth-order non-linearity is used for QPSK.
When data at a certain symbol rate is used to modulate the carrier, the demodulator can lock at
incorrect frequencies, spaced at intervals of one-quarter of the symbol rate away from the carrier.
Fortunately, when FEC decoding is used, the decoder synchronization state can be used to verify
the correct lock point has been achieved, and to reject the false locks.
However, if uncoded operation is used, there is no way to recognize a data false lock. The
demodulator will indicate that it is correctly locked, but the data out will not be correct. In Firmware
Version 1.3.1 or higher for both the CDM-600L and CDM-600, a new signal processing algorithm
avoids this problem.