Research On Near-Field OFDM Source Localization Based On Uniform Circular Array

Orthogonal Frequency Division Multiplexing(OFDM)is widely used in military and civilian fields,because this signal form has high spectrum utilization,can reduce the transmission rate of sub-channel symbols and can resist multipath effects.In recent years,OFDM signals are gradually used in underwater acoustic communication.Underwater equipment utilizes the wideband characteristics of OFDM signals to make it easier for target detection,feature extraction and parameter estimation.The need for passive detection of such underwater equipment is increasing urgent.The high-precision fast localization algorithm of underwater targets is the core content of underwater acoustic array detection system.At present,the positioning of OFDM sources is mainly concentrated in the far field,while the research on the near field is relatively rare,and these studies do not focus on the specific form and characteristics of OFDM sources.This paper mainly researches the near-field OFDM source under uniform circular array and the high-precision fast localization algorithm of mixed near-field and far-field OFDM sources.The research results will provide theoretical and algorithm support for system design and development.The main work and research results of this paper are as follows:The first chapter is the introduction,which introduces the research background and significance of the subject,compares the propagation mode and the working distance between the near-field source and the far-field source,and emphasizes the advantages of a uniform circular array relative to other array structures.In addition,the domestic and foreign research status of near-field radiation source parameters estimation,mixed near-field and far-field source parameter estimation and wide-band signal parameter estimation under uniform circular array are elaborated.It is pointed out that there is little literature on the research of wideband OFDM source location.Therefore,it is urgent to propose a reasonable and effective near-field OFDM source and mixed near-field and far-field OFDM source localization algorithms.In the second chapter,the basic principle of OFDM and the array structure of uniform circular array are analyzed.The mathematical model of the near-field OFDM source receiving signal under uniform circular array is established.Since there is an error in estimating the subcarrier frequency by employing Fourier transform,we propose to estimate the subcarrier frequency of an OFDM source by sparse representation.The principle and the whole process of the algorithm are introduced in detail,and the performance of the sparse representation is analyzed by simulation experiments.It can be noticed that the sparse representation method can realize the high-resolution processing of the sub-carriers of the OFDM source in the frequency domain,and then obtain the super-resolution phase corresponding to the sub-carriers by the frequency.The third chapter mainly studies the three-dimensional position parameters such as azimuth,elevation angle and range of near-field OFDM source by adjacent array element phase difference inversion algorithm.The contents of this chapter are mainly divided into the following two parts.Under the condition that the frequency of OFDM source subcarrier is used as a priori information,the first part introduces the phase difference inversion algorithm of multiple near-field source three-dimensional position parameter estimation to the three-dimensional position parameter estimation of near-field OFDM source subcarriers.The three-dimensional position parameters of the sub-carriers are averaged to realize the localization of the near-field OFDM source.The parameter estimation of the single sub-carrier and the average positioning performance of the sub-carrier parameters are analyzed by simulation experiments.The second part introduces the method of phase difference inversion based on uniform circular array in the non-cooperative scenario where the subcarrier frequency is unknown.The OFDM source subcarriers are discriminated with high precision in the frequency domain by the sparse representation method,and the phase difference of the subcarrier frequencies in the adjacent array elements is obtained.The sub-carriers are averaged in the phase difference of adjacent array elements,and the three-dimensional position parameters such as azimuth,elevation and range of the near-field OFDM source are inversely combined with the least squares method,which realizes more accurate three-dimensional position parameter estimation of the near-field OFDM source.The performance of the phase difference averaging method and the subcarrier averaging method are compared by simulation experiments.The fourth chapter mainly studies the mixed near-field and far-field OFDM source location methods.First,the algorithm constructs a matrix containing only the azimuth and elevation angle information of the subcarriers by calculating the phase difference of the diagonal elements of the uniform circular array.The azimuth and elevation angles of all subcarriers in the mixed near-field and far-field OFDM sources are estimated using a phase difference inversion algorithm.Then,the one-dimensional MUSIC method is used to obtain the distance spatial spectrum of each sub-carrier,The near-field signal and the far-field signal are identified according to whether the appears peak or not.The distance of the near-field OFDM source can be obtained from the spatial spectrum peak.Finally,the performance of the proposed algorithm is analyzed through simulation experiments.The simulation results show that the proposed algorithm can estimate the azimuth and elevation of the near-field and far-field hybrid OFDM sources simultaneously,which can realize the recognition and localization of mixed near-field and far-field OFDM sources under a uniform circular array.Moreover,compared with the TSMUSIC method,the positioning accuracy is high and the computational complexity is significantly reduced.The fifth chapter is mainly the summary and outlook of the full text.The main work and innovations of the full text are summarized,and the problems of coherent near-field OFDM source location and motion OFDM source location are prospected.