Reseach On Demodulation Algorithm Of QPSK Communication System Under Low SNR Environment

Deep space channel is typical power-limited channel with high bandwidth efficiency but extremely low SNR caused by far transmission distance and channel noise. In addition, even small frequency offset has great influence on the decoding performance at low signal-to-noise ratio environment, how to recovery signal accurately under low SNR environment is already in urgent need of research. Focusing on these problems, synchronization of Quadrature Phase Shift Keying (QPSK) signals under low SNR is researched in this paper.The main research work can be summarized:Firstly briefly introduce the digital transmitter and the receiver model of QPSK communication system, On the base of the basic principle of Turbo encoding and decoding algorithms, the influence factor of Turbo are discuss by simulation results; Secondly several kinds of symbol timing synchronization algorithm are introduced, based on these algorithms, an improved symbol timing synchronization algorithm is proposed; Thirdly, CA carrier synchronization is researched, combing with DA synchronization, a CA iterative algorithm auxiliary joint soft decoding algorithm is given.The contributions of this paper can be summarized as follows:(l)Using the gradient algorithm and group variance algorithm proposed by C. Richard Johnson, an improved time synchronization algorithm is presented, the simulation results shows the improved time synchronization algorithm has fast convergence speed as well as low complexity.(2)According to the CA/DA carrier synchronization algorithms which are already been published, a joint pilot and iterative decoding carrier synchronization is introduced. The synchronization are consisting of coarse synchronization part using phase deviation algorithm and fine synchronization using time-frequency correlation and maximum likelihood estimation algorithm, simulation results show that the improved algorithm can correct the carrier frequency offset with faster convergence speed.(3)Two kinds of soft synchronization joint with iterative soft decoding models are put forward and analyzed. The simulation results show that both schemes achieve decoding requirements under low signal-to-noise ratio.The joint processing receiver systems in this paper have properties of high precision, low error rate, good stability and low implementation complexity, which have a good application at low signal-to-noise ratio environment.