CDD-562AL/564A/L Multi-Channel Demodulator
Revision 0
Forward Error Correction
MN-CDD562AL564AL
A–6
A.4
Uncoded Operation (No FEC)
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.
Comtech EF Data cannot be held responsible for incorrect operation if the user does not
adhere to these guidelines when using uncoded operation.
Example: If selecting 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 that 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.
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 CDD-56xA/L offers this uncoded mode for the modulation types BPSK and
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, known as ‘phase ambiguity’, 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, 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 four possible lock states leading to four 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.
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
Summary of Contents for CDD-562AL
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