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CDM-600 Satellite Modem Revision 7
Forward Error Correction Options MN/CDM600.IOM
7–12
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, a new signal processing algorithm avoids this
problem.
CAUTION
When using Firmware Versions prior to 1.3.1, 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. For Firmware version 1.3.1 or higher, the problem has been been
eliminated.
Example 1: A Firmware Version prior to 1.3.1 is being used, and the user selects
64 kbps QPSK, uncoded. 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.
Example 2: A Firmware Version of 1.3.1, or higher is used. There is no limitation
on acquisition sweep width.
Comtech EF Data cannot be held responsible for incorrect operation if the user does not
adhere to these guidelines when using uncoded operation.