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# 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. |