Type of modulation for digital signals (DVB-S). The digital, serial signal components I and Q directly control phase shift keying. The constellation diagram with its four discrete states is obtained by representing the signal components using the I and Q signals as coordinate axes. Due to the high nonlinear distortion in the satellite channel, this type of modulation is used for satellite transmission.

The four discrete states all have the same amplitude that is why nonlinear amplitude distortions have no effect.