Research On Wall Clutter Suppression Method For Spread Spectrum Through-The-Wall Radar

Through-the-wall radar(TWR)can detect and locate objects which stand behind walls and other obstacles.It is widely used in many fields,such as police enforcement,street battle,disaster relief and so on.However,due to the presence of the wall,the target echo amplitude will decrease and the time delay will increase.Meanwhile,th e weak target echo will be submerged in the strong wall's echo,resulting in poor imaging quality,inaccurate positioning,and detection failure.TWR based on spread spectrum signal has many advantages,such as high average power,strong anti-noise ability,good signal concealment and so on,so it has broad application prospects.However,the continuous wave characteristics of the spread spectrum signal make the wall clutter suppression difficult.Therefore,this paper focuses on the wall clutter mitigation in spread spectrum TWR,and some researches are carried out as follows.1.The wall echo and target echo have the multipath characteristics.Some multipath signal suppression methods are studied,and the characteristics and application scope of different multipath suppression techniques are compared with simulation results.Furthermore,the an ti multipath technology is used to estimate the time delay of echo signal accurately in the spread spectrum TWR.Moreover,Finite-differen ce time-domain(FDTD)method is used to simulate the echo of the TWR,and the NVIDIA graphics card CUDA parallel computing is used to improve the simulation speed.According to the application background of spread spectrum TWR,the autocorrelation characteristics of different pseudo-random sequences are compared.The simulation results show that the M-sequence has good self-correlation,low side lobe and suitable for TWR detecting.Finally,the target location accuracy under different spreading code frequencies is studied.2.A clutter suppression technique based on spread spectrum signal for multilevel Delay Lock Loop(M-DLL)of TWR is proposed.Aiming at the problem that weak target echo is submerged in strong clutter and can not be detected,the proposecd method can track and cancel the direct wave,the first echo of the wall and the multiple echoes of the wall respectively and effectively.It can effectively suppress the clutter without affecting the echo information of the stationary target,which is located behind the wall.Meanwhile,in order to improve the anti-noise ability of M-DLL technology,a long time coherent accumulation is used to improve the SNR(signal to noise ratio),which can still achieve a good target imaging effect under the high noise condition.Simulation results show that M-DLL clutter suppression method has the advantages of better clutter suppression effect,stronger anti-noise ability and higher positioning accuracy than other existing methods such as exponential average and spatial filtering3.Aiming at the problem of large computation and poor real-time performance of imaging point delay in TWR imaging,a fast algorithm for imaging point delay calculation is proposed At first,the complex 3-layer model of air wall air is converted into a simple 2-layer model of wall air under the condition of constant time delay.Secondly,the 1-D search is carried out at the interface between the wall and the air medium to find the optimal propagation path of the electromagnetic wave at the image point.Finally,the time delay of the image point is calculated This method avoids the complex operation of solving the quartic equation and a large amount of computation of Newton iteration.The simulation results show that the time delay of the image points obtained by this technique has high precision and low computation,which meets the requirements of real-time imaging in the TWR4.A new subspace clutter suppression technique for TWR is proposed.The distribution of the singular values of wall and target in the spread spectrum synthetic aperture radar(SAR)is studied,and the multidimensional characteristics of the echo of the wall are proved.On this basis,a wall subspace filtering method based on singular vector variance is proposed to improve the performance of wall clutter suppression5.A clutter suppression method based on stepped frequency through wall radar is proposed The multi-dimensional characteristics of wall information and target information in the echo scattering matrix are analyzed,and the influence of wall parameters,antenna location,target location and number on the distribution of singular values is given.On this basis,the wall subspace selected technique is extended to the frequency stepped TWR.The experimental results show that compared with some existing subspace clutter suppression methods,the proposed method has better clutter suppression effect and clear target image.