QPSK is short for Quadrature Phase Shift Keying or called four-phase shift keying. and it is a digital modulation. It is divided into absolute phase shift and relative phase shift Keying modulation. Due to the phase ambiguity problem in the absolute phase shift keying modulation, relative phase shift Keying modulation of QDPSK is mainly used in practice. It also has a series of unique advantages. It has been widely used in wireless communication and has become a very important modulation and demodulation method in modern communication.
The principle of QPSK is similar to the BPSK modulation, but QPSK has four different phases. It divided 360 degrees into 4 points, and the phase angles are different by 90 degrees, which is why it is called quadrature phase shift modulation. The commonly used initial phase angle can be 0 or 45 degrees. Generally, QPSK can be regarded as two orthogonal transmissions, included the I and Q branch.
The two orthogonal transmissions are shown above. QPSK uses two carriers: one in-phase and the other in quadrature. The point denoting 11 is composed of two combined signal elements, both of which have an amplitude of 1V, one element is represented by an in-phase carrier, and the other element is represented by a quadrature carrier.
Currently, QPSK is the most commonly used for digital signal modulation of satellite transmission. It has high spectrum utilization and modulation efficiency, strong anti-interference, requires a low signal-to-noise ratio of the transmission path, and is relatively simple to implement on the circuit.