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CDM-600L Satellite Modem
Forward Error Correction Options
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.
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.
CAUTION
As an example, if 64 kbps QPSK uncoded is used, the symbol rate will be half of this
rate, or 32 ksymbols/second. One quarter of this equals 8 kHz. Therefore, the absolute
maximum acquisition sweep range which should be considered is ± 8 kHz. If there is any
frequency uncertainty on the incoming carrier, this should be subtracted from the sweep
width. The problem becomes progressively better with increasing symbol rate.
Comtech EF Data cannot be held responsible for incorrect operation if the user does not
adhere to these guidelines when using uncoded operation.
7–10
Revision 0
MN/CDM600L.IOM
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