Research On Doppler Estimation And Compensation Method Based On OFDM Underwater Acoustic Communication System

With the development of wireless communication technology and the increase of human demand for marine development,research in the field of underwater acoustic communication has received more and more attention.As a multi-carrier modulation technology,Orthogonal Frequency Division Multiplexing has been widely used in underwater acoustic communications in recent years due to its good anti-multipath capability and high frequency band utilization.Compared with land,the underwater channel environment is more complex and changeable,and has a stronger Doppler effect,which will cause the orthogonality between subcarriers in the OFDM block to be destroyed,making it difficult for the receiving end to demodulate the information correctly..Therefore,in the OFDM underwater acoustic communication system,how to accurately estimate the Doppler frequency shift and reasonably compensate for its impact has attracted more and more attention.This thesis is the research on this issue.Traditional Doppler estimation methods usually add a known preamble signal at the beginning or end of the data frame,such as hyperbolic frequency modulation(HFM),OFDM block with cyclic prefix(CPOFDM),etc.,and use these at the receiving end The correlation calculation of the known signal detects the "strongest peak" in the correlation calculation result,and calculates the overall Doppler based on the time difference of such peaks.However,since underwater acoustic channels often have a large number of multipaths,and sometimes multipaths are even distributed in clusters.In fact,the Doppler sizes corresponding to different paths are also different,and only the Doppler of the corresponding path with the "strongest peak" is used for the measurement.As the only Doppler of the channel,it is difficult to completely eliminate Doppler interference.Based on the above problems,this paper proposes a new sub-path Doppler estimation.For multipath channels,based on the idea of matching pursuit(MP),the Doppler information of each sub-path is estimated.This MP-based sub-path Doppler estimation method can not only estimate the respective Doppler offsets of multiple paths,but also has a higher Doppler resolution than existing methods.In addition,this paper also studies the sub-path Doppler compensation method for OFDM system.Since the improved MP-based sub-path estimation method can obtain the Doppler size of multiple sub-paths,the original Doppler compensation method of resampling and carrier frequency offset compensation is no longer applicable.The time flip mirror technology is used here,and the sub-path information obtained by the above method is used,thereby indirectly eliminating the interference caused by the Doppler effect.And combined with Page Test detection technology,the clustered channel impulse response is accurately divided into blocks,so as to realize the block-based Doppler compensation in the clustered channel environment.The research verifies the improved Doppler estimation method and Doppler compensation method through simulation,Qiandao Lake experiment and MACE10 marine experiment.Experiments show that the improved Doppler estimation and compensation scheme can effectively reduce the bit error rate of the OFDM system,which shows that the improved sub-path Doppler estimation method can effectively improve the anti-Doppler capability of the OFDM communication system,and thus improve System reliability.