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.H. Méric

Approaching The Gaussian Channel Capacity With APSK Constellations

''IEEE Communications Letters''. The previous solution, where the constellation has a Gaussian shape, is called constellation shaping.

References

Literature

DVB-Flexible Serially Concatenated Convolutional Turbo Codes with Near-Shannon bound performance for telemetry applications

CCSDS-131.2-O-1. * * De Gaudenzi, R., Guillén i Fàbregas, A. and Martinez, A., 2006

Turbo‐coded APSK modulations design for satellite broadband communications

International journal of satellite communications and networking, 24(4), pp.261-281. Category:Quantized radio modulation modes {{electronics-stub

Approaching The Gaussian Channel Capacity With APSK Constellations

''IEEE Communications Letters''. The previous solution, where the constellation has a Gaussian shape, is called constellation shaping.

References

Literature

DVB-Flexible Serially Concatenated Convolutional Turbo Codes with Near-Shannon bound performance for telemetry applications

CCSDS-131.2-O-1. * * De Gaudenzi, R., Guillén i Fàbregas, A. and Martinez, A., 2006

Turbo‐coded APSK modulations design for satellite broadband communications

International journal of satellite communications and networking, 24(4), pp.261-281. Category:Quantized radio modulation modes {{electronics-stub