Digital modulation (also referred to as shift keying) is a modulation in which the modified parameter of the carrier signal can take only discrete values. This type of modulation can sometimes be referred to also as discrete modulation or manipulation, e.g.: discrete frequency modulation, amplitude manipulation (on-off keying), etc. Modulation is the process of varying a carrier signal, typically a sinusoidal signal, in order to use that signal to convey information. ... A carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) to represent the information to be transmitted. ... A discrete signal is a signal that has been sampled from a continuous signal. ... On-off keying (OOK) is a type of modulation that represents digital data as the presence or absence of a carrier wave. ...
A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator.
In digitalmodulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol).
Digitalmodulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system.
As with all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal.
As for many digitalmodulation schemes, the constellation diagram is a useful representation and is relied upon in this article.
Since in digital telecommunications the data is usually binary, the number of points in the grid is usually a power of 2 (2,4,8...).
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