CDM-570A/570AL Satellite Modem with Optional Packet Processor
Revision 5
FEC (Forward Error Correction) Options B–19 MN-CDM570A
B.8 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.
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.
Comtech EF Data cannot be held responsible for incorrect operation if the user
does not adhere to these guidelines when using uncoded operation.
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-570A/570AL offers this uncoded mode for 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, 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.
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 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.
To Next Page
Loading...
