Mocomtech CDM-570 - Manual (Page 199)

CDM-570/570L Satellite Modem with Optional IP Module Revision 4
Forward Error Correction Options MN/CDM570L.IOM
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’.
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.
This problem has been almost entirely eliminated in the CDM-570/570L with the fast
acquisition algorithm which includes Fast Fourier Transform (FFT) techniques. However,
there is a very small probability that a data false lock could still occur in uncoded mode,
and in this circumstance Comtech EF Data cannot be held responsible for incorrect
operation.
7.7 Rates above 2.5 Msymbols/sec
IMPORTANT
Starting with Release 1.4.1 of the CDM-570/570L firmware, the maximum symbol
rate has been increased from 2.5 to 3.0 Msymbols/sec. This has been done without
modification to the hardware, and as a consequence, there may be a small
degradation in BER versus Eb/No performance for rates above 2.5 Msymbols/sec.
The degradation is as follows:
Rates from 2.5 to 2.65 Msps: degradation < 0.1 dB
Rates from 2.65 to 2.80 Msps: degradation < 0.2 dB
Rates from 2.80 to 3.00 Msps: degradation < 0.3 dB
Users should take this into account when considering the BER versus Eb/No graphs
that follow.
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