Study Of Through Wall Imaging System Performance Based On IR-UWB

Through wall imaging (TWI) can realize an active non-invasive detection and recognization of the object behind the building wall or fraise, with a wide range of applications in military, rescue and other fields, and it has become one focus of current research. IR-UWB is one of the most widely used detecting signal in through wall imaging for its advantages such as its penetration capability, high resolution, target identification capability, resolving multipath capability, anti-interference and small blind region. The TWI system based on IR-UWB consists of detection part and imaging part. The system performance is closely dependent on detection environment, detection manner and imaging algorithm. The goal of through wall imaging system design is to make the system performance meet the performance requirements of applications. So, the study of IR-UWB TWI system performance is a key issue for the system design. Compared with traditional imaging systems, the IR-UWB through wall imaging system often has its own characteristics such as wall blocking, ultra wide band, real aperture and near field, which are needed to be fully considered in the system performance analysis. However, these characteristics have not been fully considered in the existing imaging system performance results, which can not be used to effectively predict and analyze the IR-UWB through wall imaging system performance, and can not be used to guide the corresponding system design appropriately too. Therefore, to study the IR-UWB TWI system performance in-depth is of great significance for system design in practical applications.Based on the analysis of the research status quo at home and abroad, combined with the above characteristics for IR-UWB TWI system, according to the target scattering point model, taking the two-dimensional through-wall imaging for ideal point targets as the background, and with the assumption that the target can be detected, the main performance of IR-UWB TWI system is studied intensively, by the way, the key issues of system parameters design and through wall imaging algorithm design in IR-UWB TWI system is investigated, which can provide a theoretical guidance for the TWI system design and application. Principal contents of this dissertation are summarized as follows:Firstly, the through wall imaging system and performance evaluation indicators are presented. The principles of TWI, detection environment, target scattering point model and detection manner are presented, based on which the basic structures of a two-dimension TWI system using IR-UWB and real aperture antenna array to detect is given. In order to determine the TWI system works in the near or far field, the boundary of near and far field of IR-UWB array is explored. After that, the methods to suppress the direct echoes in receiving echoes are presented, and the commonly used TWI algorithm under known wall parameters, back projection through wall imaging algorithm, i.e. BP-TWI algorithm, is investigated comprehensively, and the two-layer equivalent media model is used to reduce the computational complexity of traveling time delay of signal through wall, when there is a certain distance between wall and antenna. At last, the main performance evaluation indicators of IR-UWB TWI system are analyzed.Secondly, Researches on the correction of localization errors in IR-UWB TWI under wall ambiguities are carried out. Localization error is an important performance of IR-UWB TWI system. One of the main sources of the localization error in the case that the wall parameters are unknown in prior is the wall estimation errors in practical application. Based on BP-TWI algorithm, the effect of wall ambiguities on the localization errors of imaged targets is analyzed. Based on the result, the compensation of wall parameters estimation errors is achieved indirectly by compensating the focusing time delay errors in BP-TWI. At the same time, by taking image contrast as the focusing metrics, a TWI algorithm to automatically compensate the wall parameters errors, which is autofocusing BP-TWI algorithm, is proposed. And in order to reduce the computational complexity and make it practical, by compensating the focusing time delay with some approximation algorithms, an imaging algorithm, which is named as Identical Time Delay and Velocity Compensated Autofocusing BP-TWI (ITD-VCAF-BP-TWI) algorithm, is proposed. Numerical simulation results show that the errors of image targets under wall ambiguities are corrected well and the imaging results obtained by this algorithm is comparable to the imaging results by BP-TWI under known wall parameters. And the high cost and poor generality of existing algorithms has been solved.Thirdly, Resolution of IR-UWB TWI system is studied. When the shape of target or multiple targets behind the wall is needed to be detected, imaging resolution is another performance, which must be focused, for IR-UWB through wall imaging system. The IR-UWB TWI system presented in this dissertation works with characteristics of ultra wideband, near field, real aperture and wall blocking. The existing resolution computation methods are no longer applicable and can not effectively predict the resolution of IR-UWB TWI system. So, based on the above characteristics of the IR-UWB TWI system, a computation method of imaging resolution in free space and near field is proposed firstly, without considering the characteristics of wall blocking. After that, by taking into account the effect of wall, an approximate computation method of the resolution of through- wall imaging system is proposed, based on BP-TWI algorithm under known wall parameters. At the same time, the computation methods are verified by numerical simulations, the space distribution of the IR-UWB TWI resolution and the effect of system parameters on the resolution are analyzed, in order to guide the design of system parameters of TWI.Fourthly, Researches on suppressing the ghost image in IR-UWB TWI are carried out. The localization error and resolution is attached to the detected real target. However, the existence of ghost image will make the detection of real target difficult. In order to suppress the ghost image, the causes of ghost image in IR-UWB TWI is analyzed, and it can be seen that, a major source of the ghost image is the sidelobe of IR-UWB antenna array near field time domain beamforming. Based on this result, the principle of IR-UWB near field time domain beamforming is investigated comprehensively, and the parameters that affect the performance of beamforming are determined. The closed form of peak side lobe ratio for uniform linear array is derived, and in order to suppress the sidelobe level so as to suppress the ghost image, the effect of system parameters is analyzed, the side lobe performance of the uniform linear array and random array are compared by numerical simulations, and the results provide theoretical support for antenna array design in IR-UWB TWI system. At last, in order to reduce the cost of the system which is required for low sidelobe level in the premise of ensuring the system performance, the method to thin the antenna array by using Multiple Input and Multiple Output (MIMO) array in IR-UWB TWI system is explored.