Quadrature phase-shift keying
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Quadrature phase-shift keying is a form of modulation in which a carrier is sent in four phasess, 45, 135, 225, and 315 degrees, and the change in phase from one symbol to the next encodes two bits per symbol.

The original article can be found at: http://en.wikipedia.org/wiki/Quadrature_phase-shift_keying.
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