可快速精确重建的毫米波MIMO近距离成像机制研究 下载: 538次
1 Introduction
Due to the characteristics of nonionizing radiation,high resolution and good penetrability,active MMW imaging applied to personnel surveillance has been extensively studied over the past decades [1-4]. The most classical MMW imaging mechanism is what we called SISO in which transmitting antenna(T)and receiving antenna(R)are co-located,and a two-dimensional(2D)aperture is achieved by scanning electrically or mechanically[5-6]. Researchers have developed various fast imaging algorithms for SISO mechanism in the past decades [1-2,7-9]. Range migration algorithm(RMA)is one of them. And this FFT-based imaging method is thought to be a standard imaging algorithm for SISO mechanism because of both high accuracy and rapid computation speed. When imaging a volume that consists of
Therefore,some researchers have set their sights on the mechanism of MIMO in which transmitting and receiving antennas are not co-located. MIMO antenna array operates sequentially or simultaneously,when one transmitting antenna emits signal,multiple receiving antennas receive the echo signal. Compared with the SISO imaging system,the MIMO imaging system requires fewer antennas,has faster scanning speed,and the antennas can be sparsely arranged. We noticed that there are some research focusing on MIMO array topology design [12-16]. However,MIMO mechanism faces a main challenge of fast image reconstruction. Most of researches usually use the back-projection algorithm(BPA)to reconstruct images. While BPA is accurate and suitable for any MIMO array configuration,its computational complexity is too tremendous. Especially in 3D cases,the computational complexity of BPA can reach
In this study,a MIMO short-range imaging mechanism of MMW for fast and accurate reconstruction is presented. The proposed MIMO imaging mechanism adopts a mechanical scanning MIMO linear array in order to balance the system cost and scanning speed. Through the sparse design of antenna array and control technology,the data acquisition speed and the utilization rate of antenna can be greatly improved. Meanwhile,the antennas in the MIMO array are arranged sparsely,which helps to couple suppression and hardware complexity reduction. More importantly,the MIMO mechanism could use accurate and fast imaging algorithms developed for SISO mechanism,such as RMA,to reconstruct images,which significantly improves the speed of reconstruction. It must be noted that RMA is just one type of fast imaging algorithms,and we choose it to verify the effectiveness of our mechanism for its generality. In essence,the proposed MIMO mechanism is suitable for a variety of fast imaging algorithms developed based on SISO mechanism,such as phase shift migration algorithm(PSMA)[7],range resolution enhancement algorithm(RREA)[8]. The applicability of the proposed MIMO mechanism will not be affected by the change of algorithm.
1 Method
1.1 Background: SISO imaging mechanism
Before discussing the MIMO mechanism,we first review the SISO imaging mechanism and its image reconstruction algorithms. As shown in
where
Typically,the antennas are evenly distributed in 2D aperture. According to the sampling requirement,the interval between antennas should satisfy the Nyquist criterion [19]:
where
1.2 Dimension Reduction for MIMO imaging mechanism
For MIMO imaging mechanism,transmitting and receiving antennas are not co-located. The process of scattering data acquirement can be written as:
where
Obviously,the scattering data set is a 5-D matrix in MIMO imaging system. To reconstruct a 3-D image by RMA,the operation of dimension reduction must be done. Equivalent phase center(EPC)principle is a simple and effective method for spatial dimension reduction [20]. Based on the principle,each pair of transmitter-receiver form an EPC,which lies on the midpoint of the two antennas. Therefore,in Cartesian coordination,the coordinate of EPC can be expressed as:
Then,the 5D data
Nevertheless,the EPC principle is valid only under the far-field condition,i.e.,the imaging distance is much larger than the size of MIMO array,which can be expressed as Rayleigh’s far-field criterion [21]:
where
1.3 MIMO linear array topology design
The MIMO linear array adopted by the presented MIMO mechanism is depicted as
As shown in the
图 2. 新型MIMO体制线性天线阵列配置示意图
Fig. 2. Diagram of MIMO linear array configuration adopted by the proposed new MIMO mechanism
We define the number of cells as
Hence,compared with SISO array,the antenna utilization rate in this MIMO array can be expressed as:
Since the interval of EPC usually was set as
In addition,the traditional MIMO imaging mechanism in Ref.[22]is introduced for comparation. As shown in
图 3. 传统MIMO体制线性天线阵列配置示意图
Fig. 3. Diagram of MIMO linear array configuration adopted by the traditional MIMO mechanism
2 Verification process
In this section,we did both simulations and experiments to verify the MIMO imaging mechanism. As shown in
2.1 Numerical Simulations
The adopted MMW source is 70~82 GHz with a sampling interval of 0.25 GHz. The imaging range is 0.3 meter and the holographic data are obtained by(4). Based on the design method in section II,we design a set of MIMO linear arrays with different
图 6. 分辨率测试板数值仿真成像结果 (a) 采用RMA的SISO阵列 (b) 采用RMA的传统MIMO阵列 (c) 采用BPA的传统MIMO阵列 (d)采用RMA的1:4:69 MIMO阵列 (e) 采用RMA的2:3:47 MIMO阵列 (f) 采用RMA的1:8:35 MIMO阵列 (g) 采用RMA的1:10:28 MIMO阵列
Fig. 6. Reconstructed images (maximum projection) of resolution chart via numerical simulattons (a) SISO array with RMA (b) traditional MIMO array with RMA (c) traditional MIMO array with BPA (d) 1:4:69 MIMO array with RMA (e) 2:3:47 MIMO array with RMA (f) 1:8:35 MIMO array with RMA (g) 1:10:28 MIMO array with RMA
To quantitatively compare the imaging quality of different imaging mechanism,we adopt the index of peak side-lobe ratio(PSLR). An ideal scattering point that is located at(0,0,0.3)serves as the imaging target. After imaging,3D imaging result is projected in x-y plane,then the PSFs in the array direction are plotted in
图 7. 不同成像体制或不同阵列的点扩散函数数值仿真结果 (a) 不同阵列 (b)不同成像体制Besides, by imaging with RMA, the computational time of proposed MIMO mechanism is approximately same with that of SISO mechanism. Compared with the traditional MIMO mechanism, the proposed MIMO mechanism not only gets better imaging performance but also faster reconstruction speed. The computational time of proposed MIMO mechanism is just about 0.00052% that of the traditional MIMO mechanism. It verifies the superiority of the proposed MIMO mechanism.
Fig. 7. PSFs of different arrays or different mechanisms for numerical simulations. (a) different arrays. (b) different mechanisms
表 1. 数值仿真中不同成像体制或不同阵列的天线利用率、计算时间及峰值旁瓣比
Table 1. Comparation on Antenna utilization rate, computational time and PSLR of different Imaging mechanisms or arrays, in numerical simulations
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From
2.2 Electromagnetic Simulations
We used the electromagnetic simulation software FEKO with the Physical-Optics(PO)method to obtain the results of electromagnetic simulations. The mannequin is placed between Z=0.15 m and Z=0.35 m. We use MMW between 70-82 GHz with an interval of 0.25 GHz to illuminate the target. One electric dipole is used as an antenna. Similarly,a 1-meter-long MIMO array and the SISO array whose elements located at the equivalent phase center are chosen for comparation. Based on
图 8. 人体模特电磁仿真成像结果 (a) 采用RMA的SISO阵列 (b)采用RMA的1:4:69 MIMO阵列
Fig. 8. Reconstructed images (maximum projection) of mannequin via electromagnetic simulations. (a) SISO array with RMA. (b)1:4:69 MIMO array with RMA
图 9. 不同成像体制的点扩散函数电磁仿真结果
Fig. 9. PSFs of different mechanisms for electromagnetic simulation
The computational time of the two mechanisms for imaging a 1m×1m×0.2 m volume with an interval of 1.85 mm and the PSLR corresponding
表 2. 电磁仿真中不同成像体制的天线利用率、计算时间及峰值旁瓣比
Table 2. Comparation on Antenna utilization rate, computational time and PSLR of different mechanism in electromagnetic simulations.
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2.3 Experiment Results
In this section,the experiments are designed to cross-validate with the simulations. As shown in
图 11. SISO成像系统原理示意图及光学图像 (a) 原理示意图 (b)光学图像
Fig. 11. Schematic diagram and photograph of SISO imaging system (a) Schematic diagram (b) Photograph.
A metal resolution test chart,which is very similar to that in numerical simulations,is used to test the performance of the MIMO imaging system and the SISO imaging system. The resolution test chart is placed 0.4 m in front of the scanning aperture and the scanning aperture is set as 1m×1m with a 1.85 mm interval in both the MIMO imaging system and the SISO imaging system.
图 12. 分辨率测试板测量结果 (a) 光学图片 (b)采用RMA的1:4:69 MIMO阵列重建结果 (c) 采用RMA的SISO阵列重建结果
Fig. 12. Measurement of the resolution test chart (a) Photograph (b) Reconstructed image of 1:4:69 MIMO array with RMA (c) Reconstructed image of SISO array with RMA
Since the parameter configurations are the same with electromagnetic simulations,and the size of data and the number of antennas are the same. Hence,the computational time and antenna utilization rate of the proposed MIMO imaging mechanism and the SISO imaging mechanism are also the same with the results in
In order to further test the performance of the proposed MIMO imaging mechanism in practical application,we choose a child mannequin as a 3D target to represent the subjects most at risk. Moreover,detection of a child is harder than an adult for its smaller size. The child mannequin is located in 0.2~0.4 m in front of scanning aperture and the scan length along vertical direction is set to 1m and the interval is 1.85 mm. The photograph and reconstructed image of the child mannequin are shown in
图 13. 儿童人体模特测量结果 (a) 光学图片 (b)采用RMA的1:4:69 MIMO阵列重建结果
Fig. 13. Measurement of the child mannequin (a) Photograph (b) Reconstructed image of 1:4:69 MIMO array with RMA
From the above imaging results,we see that the experimental results are highly consistent with the simulation results. For the proposed MIMO imaging mechanism,both 2D and 3D targets can be completely focused and accurately imaged by using RMA,which means the proposed MIMO mechanism is suitable for fast imaging algorithms developed for SISO mechanism. In addition,compared with the SISO imaging mechanism,the proposed MIMO imaging mechanism has a comparable imaging resolution,but higher antenna utilization and lower speckles noise due to the advantages of MIMO mechanism. In other words,the proposed MIMO imaging mechanism has the advantages of both SISO mechanism and MIMO mechanism.
3 Conclusion
A MIMO short-range imaging mechanism of MMW that can achieve fast and accurate reconstruction is presented. The applicable conditions of the mechanism are given quantitatively. The feasibility of the method is verified by simulations and experiments. The results demonstrate that the MIMO imaging mechanism presented in the paper is suitable for RMA and the applicable conditions of the mechanism are effective. When imaging with RMA,the quality and the computational time of reconstructed images formed by our MIMO mechanism are similar to those formed by the SISO mechanism but the antenna utilization rate and antenna spacing are much better than traditional MIMO mechanism. Compared with the traditional MIMO imaging mechanism,although the proposed MIMO mechanism requires more antennas,it can achieve higher imaging quality and its applicability to various fast imaging algorithms greatly improves the imaging speed of the MIMO system.
Article Outline
于洋, 游燕, 陈旭东, 乔灵博, 赵自然. 可快速精确重建的毫米波MIMO近距离成像机制研究[J]. 红外与毫米波学报, 2021, 40(5): 638. Yang YU, Yan YOU, Xu-Dong CHEN, Ling-Bo QIAO, Zi-Ran ZHAO. Research on the MIMO short-range imaging mechanism of millimeter wave for fast and accurate reconstruction[J]. Journal of Infrared and Millimeter Waves, 2021, 40(5): 638.