Amplitude and Phaseshift keying or Asymmetric Phaseshift 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 Amplitudeshift keying (ASK) and Phaseshift keying (PSK) to increase the symbolset. It can be considered as a superclass of Quadrature amplitude modulation (QAM). The advantage over conventional QAM, for example 16QAM, is lower number of possible amplitude levels, resulting in fewer problems with nonlinear amplifiers. Applications The DVBS2 specification permits the use of 16APSK and 32APSK modes, allowing 16 and 32 different symbols respectively and are intended for mainly professional, semilinear applications. They can be also used for broadcasting but they require a higher level of available C/N and an adoption of advanced predistortion methods in the uplink station in order to minimize the effect of transponder nonlinearity.
References DVBS2 — ready for lift off, article in the EBU technical review Retrieved from "http://en.wikipedia.org/" 
