The Full Form of QPSK is‍ Quadrature Phase Shift Keying.

The Quadrature Phase Shift Keying QPSKQPSK is a variation of BPSK, and it is also a Double Side Band Suppressed Carrier DSBSCDSBSC modulation scheme, which sends two bits of digital information at a time, called as bigits.

Instead of the conversion of digital bits into a series of digital stream, it converts them into bit pairs. This decreases the data bit rate to half, which allows space for the other users.

The QPSK Modulator uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit. Following is the block diagram for the same.

At the modulator’s input, the message signal’s even bits (i.e., 2^nd bit, 4^th bit, 6^th bit, etc.) and odd bits (i.e., 1st bit, 3^rd bit, 5^th bit, etc.) are separated by the bits splitter and are multiplied with the same carrier to generate odd BPSK (called as PSK_I) and even BPSK (called as PSK_Q). The PSK_Q signal is anyhow phase shifted by 90° before being modulated.

The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs.