A type of modulation of electromagnetic waves in wired and radio communication channels for data transmission. The main difference between QPSK and modulation types such as AM and FM is that the information density per channel bandwidth (per symbol, per hertz) is greater than one. Quadrature Phase Shift Keying (QPSK) provides four phase states (Q — quadruple), meaning 4 states correspond to 2 bits per symbol.
From radio communication theory, it is known that binary phase modulation BPSK has the highest noise immunity. However, in some cases, by reducing the noise immunity of the communication channel, its throughput can be increased. Moreover, when using error-correcting coding, it becomes possible to more accurately plan the coverage area of a mobile communication system.
In QPSK, four phase values of the carrier wave are used. In this case, the phase y(t) of the signal described by expression (25) should take four values: 0°, 90°, 180°, and 270°. However, more often, alternative phase values are applied: 45°, 135°, 225°, and 315°.
Quadrature phase modulation is widely used in formats such as CDMA cellular systems (Code Division Multiple Access), wireless local area networks, the Iridium satellite voice/data system, and DVB-S (broadband satellite television system).
Related articles: BPSK, Modulation, FDMA, TDMA, dPMR.