Amplitude and phase-shift keying

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Amplitude and phase-shift keying or asymmetric phase-shift keying (APSK), is a digital modulation scheme that conveys data by changing, or modulating, both the amplitude and the phase of a reference signal (the carrier wave). In other words, it combines both Amplitude-shift keying (ASK) and Phase-shift keying (PSK) to increase the symbol-set, it can be considered as a superclass of Quadrature amplitude modulation (QAM). The advantage over conventional QAM, for example 16-QAM, is lower number of possible amplitude levels.

Moreover, a careful design of the constellation geometry enables to approach the Gaussian capacity when the constellation size grows to infinity, for the regular QAM constellations, a gap of 1.56 dB is observed. The previous solution, where the constellation has a Gaussian shape, is called constellation shaping.