Amplitude and phase-shift keying

Amplitude and phase-shift keying (APSK) is a digital modulation scheme that conveys data by modulating both the amplitude and the phase of a carrier wave.

This allows for a lower bit error rate for a given modulation order and signal-to-noise ratio, at the cost of increased complexity, compared to ASK or PSK alone.

The distinction between the two is in their production; QAM is produced from two orthogonal signals.

[2] The resilience of APSK to amplifier and channel non-linearities afforded by its low PAPR have made it especially attractive for satellite communications, including DVB-S2.

For the regular QAM constellations, a gap of 1.56 dB is observed.

A modified constellation diagram of 16-APSK. Typically 16-APSK will have 15 degree phase offset on the outer ring, which is not depicted here. Symbols can be easily distinguished from each other and, moreover, varying of the space between rings is a way to counteract transmission distortions. [ 4 ]