Amplitude and phase-shift keying

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) because all QAM schemes are APSK schemes but some APSK modulation schemes have either all real or all imaginary symbols and therefore have either only an in-phase component or just a quadrature component that can represent the whole symbol alphabet. The advantage over conventional QAM, for example 16-QAM, is lower number of possible amplitude levels and lower PARP.
Moreover, a careful design of the constellation geometry can approach the Gaussian capacity as 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.