What is Quadrature Amplitude Modulation (QAM)?

Quadrature amplitude modulation (QAM) is both a digital and an analog modulation scheme. Quadrature amplitude modulation conveys two digital bit streams or two analog message signals by modulating (changing) the amplitudes of two carrier waves, using either the amplitude shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme respectively.

These two waves, typically sinusoids, are out of phase with each other by 90 degrees and are thus called quadrature components or quadrature carriers. The modulated waves are summed resulting in a waveform that is a combination of both phase shift keying (PSK) and amplitude shift keying (ASK). In the analog case phase modulation (PM) and amplitude modulation (AM) are utilized. In the digital Quadrature amplitude modulation case, a fixed number of at least two phases and at least two amplitudes are used. Phase shift keying modulators are frequently designed using the quadrature amplitude modulation principle, but are not considered as QAM as the amplitude of the modulated carrier signal is constant. Quadrature amplitude modulation (QAM) is used widely as a modulation scheme for digital telecommunication systems.