Phase shift keying(PSK)

Finally ,phase shift keying (psk) usese shifts in the phase of signal to represent the data.There is a phse shift of of 180 degress or pi as 0 foolows the 1.Same happens when 1 follows the 0.This simple scheme shifting the pahse by 180 degres each time the value of data changes,is also called binary PSK(BPSK).A simple implementation of a BPSK modulator could multiply ,a frequency f with +1 if the binary data is 1 and with -1 if the binary data is 0.
To receive the signal correctly, the receiver must synchronous in frequently and phase with transmitter.