Uncoded Operation (No Fec) - Comtech EF Data SLM-7650 Installation And Operation Manual

SLM-7650 Satellite Modem
Theory of Operation
4.3.3.5

Uncoded Operation (No FEC)

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 modulator 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 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 QPSK/OQPSK. 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.
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.
The second problem inherent in PSK demodulators is that of 'data false locking'.
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.
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Revision 4
MN/SLM7650.IOM