Quadrature amplitude modulation

Quadrature amplitude modulation (QAM) encodes digital symbols by varying the amplitudes of two carrier components separated by 90 degrees in phase. Each symbol corresponds to a point in a two-dimensional constellation.

I/Q representation: A representation of a signal using in-phase and quadrature components, whose values specify a point in a plane. QAM sets these two component values to encode each transmitted symbol.
Amplitude modulation: A modulation method that conveys information by varying a carrier’s amplitude. QAM extends amplitude modulation by controlling two orthogonal carrier components.
Orthogonal frequency-division multiplexing: A multicarrier transmission method that sends data over many mutually orthogonal subcarriers. Each subcarrier commonly carries QAM symbols in broadband wireless and wired systems.
Binary phase-shift keying: A phase-shift keying scheme that represents two symbols using opposite carrier phases. Its two widely separated symbols tolerate noise better than dense QAM constellations.
Spectral efficiency: The amount of information transmitted per unit bandwidth, commonly measured in bits per second per hertz. Increasing QAM’s bits per symbol can raise spectral efficiency without widening the channel.
Constellation diagram: A plot of a digital modulation scheme’s symbols as points in a plane of signal components. Its points show the amplitudes and phases QAM uses to distinguish symbols.
Phase-shift keying: A digital modulation method that encodes symbols through discrete changes in carrier phase. QAM varies both amplitude and phase, unlike fixed-amplitude phase-shift keying.
Wi-Fi: A family of wireless local-area networking standards based on IEEE 802.11. Wi-Fi uses different QAM orders to adapt data rates to channel conditions.
Quadrature phase-shift keying: A phase-shift keying scheme with four carrier phases representing two bits per symbol. QPSK keeps a fixed symbol amplitude, while higher-order QAM adds amplitude variation.
Signal-to-noise ratio: The ratio of signal power to noise power in a communication channel. Higher-order QAM needs greater signal-to-noise ratio to separate its more closely spaced points.

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