Other variations were subsequently proposed that generated
output waveforms at the symbol rate, reducing possible phase discontinuities that may otherwise
arise.
As the system design parameter A decreased, the envelope variations decreased. Hence by varying
the parameter A, the cross-correlator can manipulate the constellation diagram to go from IJFOQPSK
to FQPSK-KF and thus change the spectral regrowth effects. Since the introduction of the
Feher-patented PSK modulation schemes, much research has been conducted to continue to strive for
more power and spectrally efficient modulation schemes. Also alternatives to those presented by
Dr. Feher have also emerged, such as those described in [52] and [53].
Numerous publications have demonstrated the FQPSK family of modulation schemes to have significant
advantages over QPSK, OQPSK, and MSK and /4-DQPSK, especially in the highly nonlinear
amplified channel application [54??“56].
In an effort to further improve spectral efficiency, a double-jump (DJ)-filtered cross-correlated
FQPSK modulation scheme, called DJ-FQPSK, was also proposed [45]. For this case, the crosscorrelator
output signals are filtered with the frequency response of the square root DJ filter. However,
prior to presenting the square root filters, let??™s present two forms of a DJ filter.
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