| In recent years,with the rapid development of informatization and intelligence,the global telecommunications industry has an increasing demand for wireless spectrum resources.How to achieve efficient sharing of wireless spectrum between radar and communication systems has gained considerable attentions from both academia and business.To this end,the passive bistatic radar(PBR)can complete the “green” situational awareness of the monitoring target without occupying additional time-frequency resources,which is considered to be a new type of spectrum sharing technology.The digital television terrestrial multimedia broadcasting(DTMB),located in the U-band,is intended for broadband wireless communications.Due to its advantages of large bandwidth,stable power,waveform-like noise,and good low-altitude coverage,this signal becomes the first choice for PBR.However,since the particularity of the modulation characteristics of the physical frame of the DTMB signal and the uncontrollable characteristics of the station deployment method,there are still many challenges for target detection of DTMB PBR.From the point of view of solving the engineering application problem in DTMB-based PBR,this paper conducts detailed research work on the problems of reference signal purification,complex clutter cancellation,ambiguity sidelobe suppression,weak target detection.The research contents of this dissertation can be summarized as follows:1.To solve the problem of impure DTMB reference signal,a novel reference signal purification method based on the low-rank and sparse feature is proposed in this paper.In the field of PBR,the reference signal is used to assist in the suppression of complex clutter/interference in the surveillance channel and the extraction of moving target parameters.However,due to the influence of the fading effect of the actual wireless channel scale,the reference channel will inevitably be doped with multipath clutter,limiting the detection performance of the target.Aiming at this problem,a novel reference signal purification method based on the low-rank and sparse feature is proposed in this paper.A time-frequency synchronization method based on multi-signal frame accumulation is firstly explored,which can accurately separate the received signal PN frame header and OFDM frame body.Then,by innovatively exploiting the inherent low-rank structure of DTMB signals,the noise component in PN sequences is reduced.After that,based on the sparse characteristics of the time-domain channel response,the multipath channel response estimation is completed by taking the denoised PN sequence and the time-domain autocorrelation result of the reference signal as the input of the algorithm.Simulation and measured data show that the proposed method can effectively remove multipath clutter,thereby improving the detection performance of the system.2.Aiming at the problem of the complexity of the actual clutter scenarios(stationary,timevarying and fractional-order clutter)in DTMB-based PBR,a distributed complex clutter suppression method based on subband processing is proposed.Since DTMB signals work in the form of continuous waves,the influence of complex clutter sidelobes will spread to the entire accumulation plane,seriously deteriorating the target detection performance of the PBR system.Aiming at the actual distribution characteristics of clutter,an adaptive clutter suppression method based on subband processing is proposed in this paper.In this method,the received signal is first divided into short batches in frequency domain to coarsen range resolution,and then the clutter is removed over each batch via extensive cancellation algorithm(ECA).The method can be seen as the batch version of ECA in the frequency domain,denominated as ECA-FB.Further,in order to suppress the time-varying clutter,an improved ECA-FB method is proposed,which is equivalent to adding Doppler frequency shift to the clutter subspace formed by ECA-FB,so it is called ECA-FBD.Since the filter weights are updated in each subband,the proposed method will obtain a wider notch in the range dimension,which can quickly and effectively suppress large-scale complex clutter.The effectiveness of the above methods is verified by simulation and measured data.3.For the non-ideal characteristics of ambiguity function of the DTMB signal,a joint twodimensional ambiguity sidelobe suppression method is proposed.The original intention of the DTMB system is to carry out broadcasting services.In PBR system,the periodic sequence and power difference characteristics of the DTMB signal lead to a series of undesired sidelobes in its ambiguity function,which will cause an increase in the probability of false alarms and missed alarms.In this paper,based on the physical frame structure of the DTMB signal,we firstly analyze and mathematically deduces the mechanism of ambiguity sidelobes in different modes.Then,a joint two-dimensional ambiguity sidelobe suppression method is proposed.In this method,the time-frequency parameter estimation method based on multi-signal frame accumulation is firstly used to obtain the synchronization processing of the received reference signal,and thus realizing the separation of the PN frame header and the data frame body;Then,the range and Doppler dimension ambiguous sidelobes are removed by the pre/post amble selective zeroing and energy equalization of the PN periodic sequence,respectively;Finally,the unambiguous detection results of the target in the range and Doppler dimension are jointly processed to obtain a unique detection peak.This method can not only effectively avoid the appearance of additional peaks,but also has a small loss of mainlobe.Simulation and measured data verify the effectiveness and feasibility of the proposed method.4.Focusing on the problem of weak target detection in different motion modes,this paper studies the related long-time coherent integration method for DTMB-based PBR.In the PBR field,increasing the signal accumulation time is an effective way to improve the energy of weak target signal.Unfortunately,as the high range resolution of DTMB signals,long-time integration will cause range and Doppler migration effect.Firstly,for the uniform motion model,this paper analyzes and deduces the generation mechanism of the PBR range migration,and then proposes a long-time coherent integration method based on stretch processing in the frequency domain.The core of the method is to correct the range migration effect by stretching the reference signal according to the prior information of Doppler frequency.Since the method performs range migration correction and target detection in the frequency domain,the complexity of the algorithm is greatly reduced.Secondly,aiming at the problems of range and Doppler migration in the uniformly accelerated motion model,this paper further proposes a long-time coherent integration method based on intra-partition range-Doppler processing.In this method,the range and Doppler migration effects caused by target velocity and acceleration are eliminated by means of intra-partition preaccumulation and inter-segment multi-stage filter processing,which effectively improves the long-time coherent performance of uniformly accelerated moving targets.Finally,the simulation and measured data results demonstrate the long-time coherent performance of the proposed method. |
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