Comtech EF Data CDM-550T, Руководство пользователя

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CDM-550T Satellite Modem Forward Error Correction Options
Rev. 1.3 7–7
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.
NOTE: Whenever uncoded operation is selected, the modem automatically uses the
differential encoder/decoder appropriate for the modulation type. It cannot be disabled.
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.