Quadrature Amplitude Modulation (QAM) is a complex digital modulation scheme to achieve high levels of spectral efficiency. It will be used in microwave links and other RF-transmission systems or in digital cable television and cable modem applications (e.g. DSL) to convey data over large distances. Digital QAM is a combination of both phase-shift keying and amplitude-shift keying. The most common forms in microwave transmission are QAM with 16 to 1024 symbols.

By using a higher-order constellation, it is possible to transmit more bits for constant occupied bandwidth. Otherwise the signal is more susceptible to noise and other corruption. The increased system gain must be balanced by lager sized antennas.