To date, we have considered only single property
modulators using either phase, amplitude or frequency symbols for conveying the data. One
might think that a modulation method combining two or more symbol types could give
improved performance in the inevitable trade-off between bandwidth efficiency and noise
performance and this is indeed the case.
The most commonly used combination is amplitude and phase signalling, sometimes classed as
M-ary APK and sometimes as Quadrature
Amplitude Modulation (QAM), depending on the constraints put on the amplitude/phase
relationship.
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