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.

QPSK-31 LSB test sample
QPSK-31 USB test sample
QPSK-63 LSB test sample
QPSK-63 USB test sample
QPSK-125 LSB test sample
QPSK-125 USB test sample

Attachments:

QPSK125 LSB	[ ]	65 Kb
QPSK125 USB	[ ]	66 Kb
QPSK31 LSB	[ ]	160 Kb
QPSK31 USB	[ ]	160 Kb
QPSK63 LSB	[ ]	97 Kb
QPSK63 USB	[ ]	97 Kb