QPSK

   Quadrature Phase Shift Keying is a digital modulation technique in which the carrier phase can have one of four possible values of 0, 90, 180, 270 degrees on the equivalent of a 90 degrree rotation. There are even more advanced concepts based upon 8-phase (45 degree rotation), 16 phase (22.5 degree rotation) and so on to 32 phase, etc.