Title: Simulation‐based configurations study of active millimetre‐wave imaging system for personal security
Abstract: The Journal of EngineeringVolume 2019, Issue 19 p. 6130-6133 IET International Radar Conference (IRC 2018)Open Access Simulation-based configurations study of active millimetre-wave imaging system for personal security Yan You, Yan You Nuctech Company Limited, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorLingBo Qiao, LingBo Qiao Nuctech Company Limited, Beijing, 100084 People's Republic of China Department of Engineering Physics, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorZiRan Zhao, Corresponding Author ZiRan Zhao [email protected] Department of Engineering Physics, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this author Yan You, Yan You Nuctech Company Limited, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorLingBo Qiao, LingBo Qiao Nuctech Company Limited, Beijing, 100084 People's Republic of China Department of Engineering Physics, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorZiRan Zhao, Corresponding Author ZiRan Zhao [email protected] Department of Engineering Physics, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this author First published: 24 July 2019 https://doi.org/10.1049/joe.2019.0462Citations: 4AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Active millimetre-wave (mm-wave) imaging techniques and systems have been widely investigated and developed, due to their ability to detect personal carried concealed weapons and other contraband, at airports and other secure locations. Up to date, several imaging configurations have been tested or proposed for active mm-wave personal security imaging system by different institutes, and each one focuses mainly on one configuration. To have visualised data or figures of different imaging system configurations, here, the authors build the simulation model of each imaging system for Feko software. Comparisons of different configurations to image the same object are provided based on simulation. The transmitting antenna number, receiving antenna number, and the reconstruction images are presented. 1 Introduction In response to an increasing threat of terrorism, personnel screening at secure checkpoints, such as airports, is becoming increasingly timely and indispensable. Mm-wave imaging systems are under wide investigation [1-3], due to their ability to detect concealed weapons, and other contrabands at airports and other secure locations. Active mm-wave imaging can be used to achieve images with high dynamic range and signal-to-noise ratio. Real-time operation and high-quality image with high signal-to-noise ratio, dynamic range, and resolution and without misrepresenting artifacts are the ultimate goals of the future active mm-wave imaging system. To realise these goals, up-to-date, several configurations have been tested or proposed for personal security imaging system by different institutes. In 1997, Pacific Northwest National Laboratory (PNNL) first proposed the monostatic system, or single input single output configuration (SISO) [3], and the corresponding imaging algorithm, the SAR-holographic algorithm [4]. An imaging system operating at 27–33 GHz, with a linear sequentially switched antenna array and a high-speed mechanical scanner, has been developed. This technique has been licensed to L3-communication Company and succeeded in business promotion. Limitations of SISO imaging system are mainly related with the appearance of reconstruction artifacts such as the dihedral effects and mis-presenting sudden indentations and protrusions [5]. Besides a mechanical servo-control scanner is usually required, which leads to a longer screening time. Therefore, multiple-input multiple-output (MIMO) configurations are useful options to improve the imaging quality and shorten the screening time. R&S Company is pioneering in developing MIMO active mm-wave imaging system, and they have developed a sparse MIMO system [6], the QPASS system, operating at 72–80 GHz. Fully electronic scanning has been achieved in this system, so that quick screening is realised. To increase the sparseness of the antenna panel further, plus array has been tested by Wei Xiao group [7]. A test system operating at 36 GHz has been used to measure a small iron ball and a rectangular iron frame placed at 1.5 m far away. Reliable results have been achieved. The number of transmitter antennas (Tx) and receiver antennas (Rx) are dramatically reduced because of the high sparseness of the Tx/Rx. Therefore, the cost and complexity of this kind of system are highly reduced. In 2016, Northeastern University proposed a fully grid MIMO imaging architecture [2]. Rx are fully located on a plane sampling at , while only nine Tx are placed off the receiving aperture panel. A reduced number of Tx allows for fast imaging. Simulation results are presented in their paper. Active mm-wave imaging systems have been developed by various research groups; and each group has reported its imaging system ability in security checking. To have a better understanding of different imaging configurations, and their ability of personal security screening, it is useful to image the same object using different configurations. In this paper, we perform the simulation modelling of the different imaging configurations by Feko software [8], and use the simulation data to reconstruct the image of the detected objects. The Tx/Rx number and the reconstruction images are presented. Visualised data and figures have also been presented to illustrate the merits and drawbacks of different configurations. 2 Active mm-wave imaging systems In Fig. 1, the antenna geometries for different imaging system configurations of an aperture size of 0.5 m × 0.5 m are shown, following the configuration design principles of PNNL SISO system, RS MIMO-cell system, Wei Xiao' group MIMO-plus system, and Northeastern University MIMO-grid scheme, respectively. The base spacing between antennas is 2 mm for SISO, MIMO-plus, and MIMO-grid configurations, and 2.9 mm for MIMO-cell pattern. Fig. 1 a shows the SISO configuration. A vertical antenna array is used to scan the whole aperture. The Tx and Rx are located at the same position. Fig. 1 b shows the Tx/Rx distribution as MIMO-cell pattern. Red horizontal points represent Tx positions, and blue vertical points represent Rx positions. In order to achieve an array aperture of 0.5 m × 0.5 m, 4 × 4 clusters are used. Each cluster contains of 24 antennas with side length 6.96 cm; and the distance between each cluster is 13.92 cm. Fig. 1 c shows the Tx/Rx distribution as an MIMO-plus pattern. Fig. 1 d is the full Rx geometry with nine Tx, MIMO-grid pattern. Tx and Rx number of different cases are summarised in Table 1. The MIMO-grid configuration has the maximum antenna, while the SISO and MIMO-plus configurations have much less antennas. The antenna number ratio of MIMO-plus panel to MIMO-grid panel is ∼0.8%. Fig. 1Open in figure viewerPowerPoint Antenna topologies of different active mm-wave imaging system configurations (a) SISO, (b) MIMO-cell, (c) MIMO-plus, (d) MIMO-grid Table 1. Different system configuration parameters Tx.N Rx_N SISO 250 250 MIMO-cell 768 768 MIMO-plus 250 250 MIMO-grid 9 62,500 To characterise different active mm-wave imaging system configurations, one needs to perform an image reconstruction. For active mm-wave imaging systems, reconstruction algorithm is used to recover scattering geometry. Here, we just summarise the equations. Under Born approximation, the general imaging algorithm for MIMO system is the well-known SAR-back-propagation technique [2]. This algorithm is also true for SISO system. The coordinate is defined as follows: x -axis and y -axis are the cross range, and z -axis is rang axis (depth), as shown in Fig. 2. Given the reflection function of an object is r (x, y, z), and the field scatted on a planar receiving aperture located at z = Z 0 is , then the SAR-back-propagation imaging algorithm formula is (1) where and are the positions of Tx and Rx, respectively, k is the wavenumber, and indicates complex conjugate operation. Fig. 2Open in figure viewerPowerPoint Geometrical description in the Cartesian coordinates for a general imaging problem with planar/array antenna aperture. The imaged object is located around the coordinate origin, whereas the Tx/Rx are both located in the (z = Z 0) plane The antenna panel is placed at Z 0 = 0.5 m (for point target and torso-sheet) or 1 m (for gun). The simulation models of different imaging system configurations are built by Feko software. We first construct the geometry of the imaged object, and then use an ideal dipole antenna for Tx/Rx. In this way, the antenna topologies are constructed (shown in Fig. 1). We load single frequency, 80 GHz or wide-band, 70–80 GHz mm-waves onto the antenna. Emitting from Tx, waves inject into the imaged object, and then are scattered by the object and collected by Rx. Physical optics method is applied due to its low requirement on computer memory and sufficient accuracy for the electrically large scattering problem. Scattering power and phase corresponding to each Rx are recorded. A workstation with 96 2.5 GHz-frequency cores and 1 T random access memory is used to run the process. Once we obtained the scattering signals, by using SAR-back-propagation algorithm, the reconstructed images of the scattering object can be presented. Both single frequency, 80 GHz, and wideband frequencies, 70–80 GHz, are investigated by different imaging configurations. First, we use a point target to test the resolution ability of each configuration at single frequency 80 GHz. The point spread function (PSF) of a point target can be used to evaluate how well is the imagining system to detect a point target. PSF and its amplitude along x -axis of the four imaging systems are shown in Figs. 3 and 4 (a) SISO, (b) MIMO-cell, (c) MIMO-plus, and (d) MIMO-grid. To distinguish the −3 dB bandwidth, the reflectivity amplitude at y = 0 is shown as a function of x in Figs. 4 a –d in dB scale. Each system presents different PSF style. In SISO configuration, the grating lobes of PSF are greatly cancelled, resulting in the lowest amplitude. The − 3 dB bandwidth of PSF (or equivalently the lateral resolution) is 2.3 mm, which is about 9.5% larger than the theoretical limit, 2.1 mm. The −3 dB bandwidths of MIMO-cell and MIMO-grid system are the same, 2.8 mm, which is worse than the SISO. The MIMO-plus system yields the worst result with a 4.7 mm −3 dB bandwidth. Fig. 3Open in figure viewerPowerPoint PSF of different active single frequency 80 GHz mm-wave imaging system configurations (a) SISO, (b) MIMO-cell, (c) MIMO-plus, (d) MIMO-grid Fig. 4Open in figure viewerPowerPoint PSF's amplitudes along x-axis of different active single frequency 80 GHz mm-wave imaging system configurations (a) SISO, (b) MIMO-cell, (c) MIMO-plus, (d) MIMO-grid Figs. 5 and 6 show the reconstructed images of a torso-sheet by using single frequency 80 GHz, and a metal gun by wideband frequencies 70–80 GHz for different imaging configurations, respectively. The geometries of the torso-sheet and gun model are given in Fig. 7. By comparing the images of the torso-sheet and gun to the prototype, we find that some parts of the torso-sheet and gun are mis-presented due to the limited antenna aperture in the MIMO-plus system. To gain a fully recovered image, the antenna aperture should be twice larger than the size of the gun. For planar imaging object, like torso-sheet, SISO system has a better image quality than that of all MIMO configurations. However, for the non-planar object, like gun, the best imaging quality is obtained by MIMO-cell case. The indentation and protrusion part of the gun, such as the handle and magazine, are best presented by the MIMO-cell system. Scattering data are weighted for a smooth effective aperture following a Kaiser window for MIMO-cell case. Although MIMO-grid system has a larger number of antenna than MIMO-cell case, but the recovered imaging quality is worse. Each imaging configuration has its merit and drawback: SISO system has better lateral resolution, but not good at imaging item with uneven parts, and also it needs mechanical scanning; least number of antenna is required for MIMO-plus system with a sacrifice on imaging resolution and effective antenna aperture. MIMO-grid system with only nine Tx means a shorter screening time compared the other MIMO imaging systems. Fig. 5Open in figure viewerPowerPoint Reconstructed torso-sheet images of different active single frequency 80 GHz mm-wave imaging system configurations (a) SISO, (b) MIMO-cell, (c) MIMO-plus, (d) MIMO-grid Fig. 6Open in figure viewerPowerPoint Reconstructed gun images of different active wideband 70–80 GHz mm-wave imaging system configurations (a) SISO, (b) MIMO-cell, (c) MIMO-plus, (d) MIMO-grid Fig. 7Open in figure viewerPowerPoint Geometries of (a) a torso-sheet and, (b) a typical metal gun 3 Conclusion In this work, we use Feko software to perform the simulation of different active mm-wave imaging systems. Four different imaging configurations operating at single frequency and wideband are used to screen the same object and images are recovered by reconstruction algorithms. Considering the application prospect of the active mm-wave imaging system for personal security screening, our simulation modeling can help to provide an economic and efficient way to efficiently find a novel imaging configuration and predict its characteristics. It can also help us to design the future more advanced imaging system. Moreover, new imaging reconstruction algorithms can be also investigated based on the simulation data. 4 Acknowledgments This work was supported by the National Natural Science Foundation of China and the Civil Aviation Administration of China under Grant No. U1633202 5 References 1Ahmed S.S., Schiessl A., Schmidt LP.: 'A novel fully electronic active real-time imager based on a planar multistatic sparse array', IEEE Trans. Microw. Theory Tech., 2011, 59, p. 3567 2Gonzalez-Valdes B., Alvarez Y., Mantzavinos S. et al.: 'Improving security screening: a comparison of multistatic radar configurations for human body imaging', IEEE. Antennas. Propag. 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