Editing Quadrature amplitude modulation (section)

{{short description|Family of digital modulation methods}}
{{Redirect|QAM|the digital television standard|QAM (television)|other uses|QAM (disambiguation)}}

{{Technical|date=June 2020}}
{{Modulation techniques}}

'''Quadrature amplitude modulation''' ('''QAM''') is the name of a family of [[digital modulation]] methods and a related family of [[analog modulation]] methods widely used in modern [[telecommunications]] to transmit information. It conveys two analog message signals, or two digital [[bit stream]]s, by changing (''modulating'') the [[amplitude]]s of two [[carrier wave]]s, using the [[amplitude-shift keying]] (ASK) digital modulation scheme or [[amplitude modulation]] (AM) analog modulation scheme. The two carrier waves are of the same frequency and are [[out of phase]] with each other by 90°, a condition known as [[orthogonality]] or [[Quadrature phase|quadrature]]. The transmitted signal is created by adding the two carrier waves together. At the receiver, the two waves can be coherently separated (demodulated) because of their orthogonality. Another key property is that the modulations are low-frequency/low-bandwidth waveforms compared to the carrier frequency, which is known as the [[In-phase and quadrature components#Narrowband signal model|narrowband assumption]].

[[Phase modulation]] (analog PM) and [[phase-shift keying]] (digital PSK) can be regarded as a special case of QAM, where the amplitude of the transmitted signal is a constant, but its phase varies. This can also be extended to [[frequency modulation]] (FM) and [[frequency-shift keying]] (FSK), for these can be regarded as a special case of phase modulation. {{citation needed|date=September 2024}}

QAM is used extensively as a modulation scheme for digital [[communications system]]s, such as in [[802.11]] Wi-Fi standards. Arbitrarily high [[Spectral efficiency|spectral efficiencies]] can be achieved with QAM by setting a suitable [[Constellation diagram|constellation]] size, limited only by the noise level and linearity of the communications channel.<ref>{{cite web|title=Digital Modulation Efficiencies|url=http://www.barnardmicrosystems.com/L4E_comms_2.htm|publisher=Barnard Microsystems|archive-url=https://web.archive.org/web/20110430132506/http://www.barnardmicrosystems.com/L4E_comms_2.htm|archive-date=2011-04-30}}</ref>&nbsp; QAM is being used in optical fiber systems as bit rates increase; QAM16 and QAM64 can be optically emulated with a three-path [[interferometer]].<ref>{{cite web
 | url = http://www.lightwaveonline.com/topics/16-qam.htm
 | title =Ciena tests 200G via 16-QAM with Japan-U.S. Cable Network
 | date = April 17, 2014
 | publisher = lightwave
 | access-date = 7 November 2016
}}</ref><ref>[http://kylia.com/QAM.html Kylia products] {{webarchive |url=https://web.archive.org/web/20110713175309/http://kylia.com/QAM.html |date=July 13, 2011 }}, dwdm mux demux, 90 degree optical hybrid, d(q) psk demodulatorssingle polarization</ref>