Quadrature Phase Shift Keying
A Modulation Scheme where the Phase of the modulated signal is shifted depending on the incoming bitstream by an angle of -135°, -45°, +45° or +135°. Thus 4 states (a two-digit binary symbol) can get expressed in every transition.
In order to minimize the used Bandwidth allocated on the transponder the outgoing signal is usually filtered using digital FIR Filters with smooth roll-off so that the high-order noise of the sudden state changes gets removed.
let's place the famous four-modulation-point-phasediagram-picture here...