Angle modulation is a class of analog modulation. These techniques are based on altering the angle (or phase) of a carrier signal to transmit data. This as opposed to varying the amplitude of the carrier, such as in amplitude modulation transmission.
Angle Modulation is modulation in which the angle of a sine-wave carrier is varied by a modulating wave. Frequency modulation (FM) and phase modulation (PM) are the two main types of angle modulation. In frequency modulation, the modulating signal causes the carrier frequency to vary. These variations are controlled by both the frequency and the amplitude of the modulating wave. In phase modulation, the phase of the carrier is controlled by the modulating waveform.