QPSK is a digital modulation technique that stands for Quadrature Phase Shift Keying. It means that the carrier signal can have four different phase shifts, each representing two bits of data. For example, a phase shift of 45 degrees can represent 00, a phase shift of 135 degrees can represent 01, a phase shift of 225 degrees can represent 10, and a phase shift of 315 degrees can represent 11.
QPSK is used to transmit more information in the same bandwidth as ordinary phase shift keying (PSK), which only has two phase shifts. QPSK is also more robust against noise and interference than amplitude shift keying (ASK) or frequency shift keying (FSK), which are other common digital modulation techniques.
QPSK is widely used in wireless communication systems, such as satellite transmission, cable modems, videoconferencing, and cellular phone networks. QPSK can also be combined with other modulation schemes, such as orthogonal frequency division multiplexing (OFDM), to achieve higher data rates and better performance.