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.