SLM-3650 Satellite Modem
Revision 3
Options
MN/SLM3650.IOM
A–97
A.6
Uncoded Operation (No FEC)
There are occasions where a user may desire to operate a satellite link with
No FEC
of
any kind. For this reason, the SLM-3650 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 {O}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 {O}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.
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.
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