Quadrature Phase Shift Keying (QPSK) is one of the most popular digital modulations. The term "Quadrature" implies, the carrier can have one of the four possible phases, at a given time slot.
The phase shift of 90º in each time period, gives four possible states in the complete cycle of 360º. In this type of digital modulation, each phase shift represents 2-bits of information, called ‘Symbol’.
Hence it has four symbols in total and each stands for two bits.
The means of conveying information through phase variations i.e. the cosine carrier is varied in phase by keeping the amplitude and frequency as constant is called Quadrature Phase Shift Keying (QPSK).
Although, this process may seem to be insignificant at initial stage, but this modulation scheme enables the carrier to transmit the 2-bits of information instead of 1-bit. This doubling makes the bandwidth of the carrier, more effective
Simulation: ModelSim XE III 6.4b.
Synthesis: XiLinx ISE 10.1.