# 6.4 M-ary Phase Shift Keying (M-ary PSK)

## Quadrature Phase Shift Keying (QPSK)

#### We have already seen in the case of M-ary FSK that an orthogonal symbol set makes it possible to send two or more symbols simultaneously over a channel, without their affecting the coherent detection performance of any individual symbol. It turns out that as well as the M-ary FSK orthogonal symbol set, there is orthogonality between a cosine and sine carrier term when averaged over a full number of carrier cycles. This would imply that if we were to signal using binary PSK on the cosine of a carrier, and simultaneously send a second PSK signal using the sine of a carrier, then it would be possible to detect each one independently of the other, (as if the other were not there), providing that each detector averaged over a symbol period that contained a whole number of carrier cycles. |

#### We can thus envisage a PSK modulation scheme with four phase states, 0o, 90o, 180o and 270o, which are in phase quadrature with each other. This 4-ary PSK scheme is thus called Quadrature Phase Shift Keying (QPSK). The orthogonality property of QPSK means that it can be used to send information at twice the speed of BPSK in the same bandwidth, without compromising the detection performance over BPSK.