Working principle of phase shift keying

In PSK modulation, the phase of the carrier varies with the state of the modulated signal. If two carriers with the same frequency start to oscillate at the same time, and the two frequencies reach a positive maximum, a zero value and a negative maximum at the same time, they are in an "in-phase" state; If one reaches a positive maximum and the other reaches a negative maximum, it is called inversion. The signal oscillates once at 360 degrees. If the phase difference between one wave and the other wave is half a period, the phase difference between the two waves is 180 degrees, that is, they are out of phase. When transmitting digital signals, the "1" code controls the transmission phase of 0 degree, and the "0" code controls the transmission phase of 180 degree.

PSK phase shift keying modulation technology has been widely used in data transmission, especially in medium-speed and high-speed digital transmitters (2400 bit/s ~ 4800 bit/s). Phase shift keying (PSK) has good anti-jamming performance, and can also achieve good results in fading channels. This paper mainly discusses two-phase and four-phase phase modulation, and there are eight-phase and sixteen-phase phase modulation in practical application.

PSK can also be divided into binary PSK(2PSK or BIT/SK) and multilevel PSK(MPSK). In this modulation technique, the carrier phase has only two values, 0 and π, which correspond to "0" and "1" of the modulation signal respectively. When transmitting the "1" signal, transmit the carrier with the initial phase of π; When a "0" signal is transmitted, a carrier with a starting phase of 0 is transmitted. Binary modulation signals represented by "0" and "1" are level converted into bipolar NRZ signals represented by "-1" and "1", and then multiplied by the carrier to form a 2PSK signal.

QPSK (Quadrature Phase Shift Keying) is the most commonly used technology in MPSK. QPSK modulation is adopted when transmitting digital TV signals on satellite channels. It can be seen as consisting of two 2PSK modulators. After serial-parallel conversion, the input serial binary information sequence is divided into two sequences with half the rate. The level shifter generates bipolar two-level signals I(t) and Q(t) respectively, and then modulates the carriers Acos2πfct and Asin2πfct, and then adds them to obtain QPSK signals.

PSK signal can also be represented by vector diagram, which usually takes zero carrier phase as reference phase. Four-phase phase shift modulation is a kind of four-phase phase shift keying, which uses four different phase differences of the carrier to represent the input digital information. QPSK is a phase modulation technology when M=4, which specifies four carrier phases, namely 45, 135, 225 and 3 15. The data input by the modulator is a sequence of binary digits. In order to match the carrier phase of quaternary, it is necessary to convert binary data into quaternary data, which means that every two bits in the binary digit sequence need to be divided into one group. * * * There are four combinations, namely 00,065,438+0,654,38+00,654,38+065,438+0. Each binary symbol consists of two bits of binary information, which respectively represent one of the four symbols in the quaternary. In QPSK, each modulation can transmit two information bits, which are transmitted through four phases of the carrier. The demodulator judges the information bits sent by the sender according to the constellation and phase of the received carrier signal.