Below we emphasize the inherent correlation between the I- and Q-channels of a FSK modulated
waveform. There are two points to make, first being when one channel is at a minimum, the other is
at a maximum. The channels work in concert to produce the modulated waveform. The second point
is as one channel decreases (or increases), the other channel increases (or decreases). The combination
of the two channels produces a constant envelope. In this section, we will insert this crosscorrelation
property of the I- and Q-channels into the modulator in order to not only create a spectrally
efficient modulation scheme, but also have less spectral regrowth when a nonlinearity is introduced
to the modulated waveform (see Figs. 2.74 and 2.75). By controlling the amount of cross-correlation
inserted, the envelope variation can be significantly reduced [50].
Feher QPSK (FQPSK). The first approach is called FQPSK-1 [41]. Here the intent is to build on
OQPSK and try to reduce the envelope variations, in order to use the more power efficient, nonlinear
PA without the tremendous concern around the typical issues surrounding spectral
regrowth. Here intersymbol and jitter free (IJF) filters were used in the OQPSK modulation
scheme (see Fig.
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