Communications engineering. A combination of amplitude- and phase modulation. It is used for the transmission of digital signals over analog channels, for example DSL, or for data transmission in high frequency nets.
There is an analog version as well as a digital one.
The analog type employs two sinusoidal carriers with a phase shift of 90° between them. Each carrier is amplitude modulated and the two modulated signal are then added to form the transmission signal.
For the digital version the (digital) input signal is split into bit groups whose length depends on the amount of phase angles used, for example
4QAM = 2 bit (4 states consisting of 2 different amplitudes * 2 phase angles),
8QAM = 3 bit (8 states consisting of 2 different amplitudes * 4 phase angles),
64QAM = 6 bit (64 states consisting of 2 different amplitudes * 32 phase angles). These bit groups modulate the sinusoidal carrier.
4QAM is identical to 4 Phase Shift Keying (4PSK, also called Quadrature Phase Shift Keying (QPSK)). This is not valid for other QAM- and PSK-modulations.
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