| In recent years,multiple-input multiple-output(MIMO)terahertz scattering imaging technology has been widely used in production and life due to its high-speed and high-efficiency advantages,such as anti-terrorism operations,security inspections,and medical inspections.Three types of MIMO arrays that work in the terahertz band are designed and the design rules for each type of array are showed.By designing the structure of MIMO arrays reasonably,the imaging with smaller error can be realized with only 15 transmitters and receivers,which greatly reduce the complexity of the antenna system.Then,based on the array structure designed in this paper,the back projection(BP)algorithm,range migration(RM)algorithm,pseudo inverse(PI)algorithm and orthogonal matching pursuit(OMP)algorithm are used in the imaging simulation.The imaging performance of each imaging algorithm is compared and analyzed in detail in terms of the quality of the reconstructed image and the imaging time.The specific work content of this paper is as follows.1.Firstly,the research background and current status of MIMO terahertz scattering imaging technology are investigated.The technology methods,including radar cross section(RCS)imaging technology,MIMO radar technology,terahertz imaging technology and microwave correlation imaging technology,as well as the imaging performance characterization parameters,including point spread function(PSF),peak side lobe ratio(PSLR),signal-to-noise ratio(SNR)and relative error(RE)are introduced.2.Three types of MIMO arrays working in the terahertz band are designed based on the phase center approximation(PCA)principle.The design rules are deduced and the equivalent phase center error(EPCE),the two-way phase error(TWPE),the average EPCE and the average TWPE are calculated.The equivalent error of each array and the effect of the position of the target to be imaged on the equivalent error are explored.The 7x8 array(array 3)composed of 15 transmitter-receivers has almost the same equivalent error as the 2×26 array(array 2)composed of 28 transmitter-receivers.When the error remains unchanged,the simplification of the antenna system can be realized.The calculation results of the average EPCE and the average TWPE provide a reference and basis for the determination of the relative position of the test system and the target to be imaged in space.3.In order to make the traditional scattering imaging algorithm applicable to the MIMO array designed in this paper,the improved BP algorithm based on monostatic scanning and the equivalent monostatic RM algorithm are used in the imaging simulation.By calculating the PSF,PSLR,SNR and RE parameters,the imaging quality of the two algorithms based on different MIMO array-structures is explored.Finally,by performing phase calibration on the RM algorithm based on the array 2 and array 3 structures,the image reconstruction with high SNR can be realized by using only 15 transmitter-receivers and the imaging time is very short.This strongly shows that the RM algorithm has the potential for real-time detection and high-quality imaging.4.In this paper,the imaging equation is constructed by using the parameterized method.The image reconstruction of the PI algorithm and the OMP algorithm are carried out by comparing the array 3 with the simple one-dimensional multiple input single output(MISO)array commonly used in the traditional microwave correlation imaging.The number of the imaging channels in the MIMO array has been greatly increased,which increases the rank of the reference matrix in the imaging equation,so both the PI algorithm and the OMP algorithm can obtain higher-quality reconstructed images.Especially in the OMP algorithm,the SNR of the reconstructed image is more than twenty orders of magnitude higher than that in other imaging algorithms and the RE of the reconstructed image is more than ten orders of magnitude higher than that in other imaging algorithms.The OMP algorithm provides the possibility to realize high-resolution MIMO terahertz scattering imaging. |
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