Rev. 2 | |
Forward Error Correction | CD/CIM550.IOM |
COMTECH EFDATA CANNOT BE HELD RESPONSIBLE FOR INCORRECT OPERATION IF THE USER DOES NOT ADHERE TO
IMPORTANT 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
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
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, another scheme entirely 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
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’.
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