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Quadrature phase-shift keying
Quadrature phase-shift keying (quadriphase, quaternary phase-shift keying) is a form of modulation in which a carrier is sent in four phases, 45, 135, 225, and 315 degrees, and the change in phase from one symbol to the next encodes two bits per symbol. In QPSK, the four angles are usually out of phase by 90°. This is a significant advantage over the normal phase-shift keying of two values effectively doubling the bandwidth.
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