AmplitudeShiftKeying

Amplitude shift keying (ASK) is a digital modulation technique in which the amplitude of a carrier wave is varied in accordance with a digital signal. The carrier frequency remains constant while its instantaneous amplitude encodes the data. Binary ASK uses two amplitude levels to represent bits, while M-ary ASK uses more than two levels. A common special case of binary ASK is on-off keying (OOK), where the amplitude is either zero or a fixed amplitude.

In transmission, the modulated signal can be described as a carrier with different amplitudes corresponding to the symbol sequence. For binary ASK, a '1' might correspond to a higher amplitude and a '0' to a lower or zero amplitude. In M-ary schemes, the data symbols map to multiple discrete amplitudes. The spectrum of an ASK signal consists of the carrier frequency and symmetric sidebands around it; the required channel bandwidth grows with the data rate and the shaping of the symbols.

Demodulation can be performed with envelope detection (coarse, noncoherent) or coherent detection using a synchronized reference carrier. ASK is relatively simple to implement, both at the transmitter and receiver, but it is sensitive to amplitude variations caused by fading, noise, or multipath, which can degrade performance. It is less spectrally efficient and more power-inefficient in noisy channels compared to some other digital modulation schemes.

Applications include low-cost wireless links, optical communication systems using on-off keying, and some RFID or NFC systems. ASK is often chosen for simplicity when the channel conditions are favorable and high-precision amplitude recovery is achievable.