Comtech EF Data SNM-1001L Installation And Operation Manual page 181

SNM-1001L Satellite Modem
Forward Error Correction Options
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 SNM-1001L 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 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.
Offset QPSK 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.
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.
CAUTION
Comtech EF Data strongly cautions users when using uncoded operation. If
the acquisition sweep width exceeds one quarter of the symbol rate, there is a
very high probability that the demodulator will false lock.
7–5
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MN/SNM1001L.IOM