Quadrature Phase-Shift Keying or QPSK is a sibling of the Phase-Shift Keying or PSK family.

QPSK is a form of modulation in which the carrier signal is sent in four phases, 45°, 135°, 225°, and 315°. The change between one phase and the next encodes 2 bits per symbol. With a total of 4 symbols at a 90° seperation, this allows for 8 bits per signal.

QPSK is seen as providing the best error rate in the presence of radio noise. It's most common in IEEE 802.11b wireless equipment and satellite equipment.

Sources:
http://www.argospress.com/Resources/satellite-communications/quadraphshiftykeiin.htm
http://en.wikipedia.org/wiki/Quadrature_phase-shift_keying

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