However, as one would expect, since the modulated
waveform generation involved a state machine, an optimal receiver, such as a Maximum Likelihood
Sequential Estimator, should also make use of this information [67].
In Fig. 2.85 is a comparison of various modulation schemes presented so far. The potential benefits
of applying cross-correlation techniques can be dramatic.
Similarly, the comparison can be made with the /4-FQPSK modulation scheme shown in Fig 2.86.
100 CHAPTER TWO
MODULATION THEORY 101
FIGURE 2.85 PSD comparison of FQPSK.
Transmit Power Spectral Density (PSD)
??“60
??“50
??“40
??“30
??“20
??“10
0
0 1 2 3 4
Normalized Frequency (f Ts)
Power Spectral Density (dB)
GMSK (BT = 0.5)
FQPSK
DJ-FQPSK
FQPSK-KF
Transmit Power Spectral Density (PSD)
??“60
??“50
??“40
??“30
??“20
??“10
0
0 1 2 3 4
Normalized Frequency ( f Ts)
Power Spectral Density (dB)
DJ-FQPSK
FQPSK-KF
??/4-FQPSK
GMSK (BT = 0.3)
FIGURE 2.86 PSD comparison of /4-FQPSK.
There exists an enhanced FQPSK version, where the I- and Q-channel waveforms are calculated
at the symbol-by-symbol rate rather than the conventional bit-by-bit rate. This now means the subtle
phase discontinuity that existed was removed or smoothed.
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