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Quadrature Amplitude Modulation

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Quadrature Amplitude Modulation
Quadrature amplitude modulation (QAM) ([pic] /ˈkwɑːm/ or /ˈkæm/ or simply "Q-A-M") is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or (in the analog case) of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digitaltelecommunication systems. Spectral efficiencies of 6 bits/s/Hz can be achieved with QAM.[1]

QAM modulation is being used in optical fiber systems as bit rates increase – QAM16 and QAM64 can be optically emulated with a 3-path interferometer.[2]

|Contents |
| [hide] |
|1 Digital QAM |
|2 Analog QAM |
|2.1 Fourier analysis of QAM |
|3 Quantized QAM |
|3.1 Ideal structure |
|3.1.1 Transmitter |
|3.1.2 Receiver |
|4 Quantized QAM performance |
|4.1 Rectangular QAM |
|4.1.1 Odd-k QAM |
|4.2 Non-rectangular QAM |
|5 Interference and noise |
|6 See also |
|7 References
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