What is Digital Quadrature Amplitude Modulation (QAM)?

Quadrature amplitude modulation (QAM), like every modulation scheme, conveys data by changing an aspect of a carrier wave or the carrier signal, frequently a sinusoid, in response to a data signal. In the case of quadrature amplitude modulation the amplitude of two waves are 90 degrees out of phase from one another and are changed to represent the data signal. Amplitude modulating (AM) two carriers in quadrature can be equivalently viewed as both AM and phase modulation (PM) of a single carrier.

Analog phase modulation and digital phase shift keying (PSK) can be regarded as a special case of quadrature amplitude modulation. Whereby, the magnitude of the modulating signal is a constant with only the phase varying. This may also be extended to frequency shift keying (FSK) and frequency modulation (FM), as these can be regarded as a special case of phase modulation.