6.5 Combined amplitude and phase keying (QAM/APK)

Introduction

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.