Response Analysis And Suppression Methods Wireless Communication System To Doppler Frequency Offset

In high dynamic scenario communication,Doppler frequency offset is produced due to the high-speed movement of both transmitters and receivers(usually more than Mach 1),which reduces the reliability of communication.In this paper,the influence of Doppler frequency offset on the performance of communication system is studied as follows:Firstly,the causes of Doppler frequency offset are analyzed,the mathematical model of Doppler frequency offset is established,and the influence of Doppler frequency offset on coherent demodulation of different single carrier modulation modes is quantitatively analyzed.On the basis of qualitative analysis and lack of quantitative analysis,the mathematical models of MFSK modulation signal,MASK modulation signal,MPSK modulation signal and MQAM modulation signal in white Gaussian noise channel with Doppler frequency offset are established,and the symbol error rate formula is deduced.The results of MATLAB simulation prove the correctness of the mathematical formulas of the error rate of the four signals mentioned above.Then,based on single carrier,the response mechanism of OFDM system to Doppler frequency offset is studied.The general model of OFDM system under Doppler frequency offset is established.Based on practical engineering,the influence of subcarrier number on the performance of the whole OFDM system is analyzed when the bandwidth and Doppler frequency offset are fixed.The theoretical and experimental results show that the anti-Doppler frequency offset capability of OFDM system is not affected when the number of subcarriers is large.Finally,we compare the two most commonly used modulation modes in OFDM system: 16 PSK modulation and 16 QAM modulation,and prove the correctness of the derivation by MATLAB simulation.The theoretical and experimental results show that the 16 PSK modulated signal has better anti-Doppler frequency offset ability than the 16 QAM signal under the condition of high signal-to-noise ratio.Then,several methods to resist Doppler frequency offset for single-user OFDM systems in white Gaussian noise channel are studied.The basic principles of these methods are introduced in detail and the performance simulation is carried out.Finally,the above algorithms are compared.The simulation results show that the quantized frequency offset algorithm has a good ability to adapt to Doppler frequency offset,but its complexity is thehighest;the phase factor rotation method has low anti-Doppler frequency offset ability but low anti-noise ability;while the adjacent symbol inversion method has strong anti-noise ability,it wastes half of the spectrum resources and has very poor anti-Doppler frequency offset ability.Secondly,aiming at the deficiency of the phase factor rotation method which can not select the optimal rotation factor,the method of combining frequency offset estimation with phase factor rotation method is used to find the optimal rotation phase factor for the system performance,and then an improved algorithm of phase factor is proposed.The theoretical and simulation results show that the anti-Doppler frequency offset ability of the improved algorithm is improved by 2 dB.To solve the multi-user problem commonly used in data link,the subspace decomposition method is used to estimate the Doppler frequency offset of each user,and the performance of the algorithm is analyzed.Experiments show that the performance of the system after Doppler frequency offset suppression is better than that before suppression.