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Quadrature phase-shift keying (QPSK)
Sometimes known as quaternary or quadriphase PSK, 4-PSK, or 4-QAM, QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray coding to minimize the BER — twice the rate of BPSK. Analysis shows that this may be used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed.
As with BPSK, there are phase ambiguity problems at the receiver and differentially encoded QPSK is used more often in practice.

{play}{/play} QPSK-31 LSB test sample
{play}{/play} QPSK-31 USB test sample
{play}{/play} QPSK-63 LSB test sample
{play}{/play} QPSK-63 USB test sample
{play}{/play} QPSK-125 LSB test sample
{play}{/play} QPSK-125 USB test sample

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