Mahmoud Kazemi

This article focuses on dealing with aliasing artifacts in millimeter-wave imaging systems. To address the challenges and complexities of designing multistatic arrays, sparse arrangements are utilized in practical imaging systems. Depending on the arrangement of antennas, such arrays may present severe aliasing artifacts. Some techniques have already been introduced to tackle such artifacts and enhance the image quality. These techniques may distort the target image by causing some loss in parts of the image, impose high computational/system costs, or suffer from the lack of generalizability. One of these techniques uses nonuniform arrays. Such arrays are designed by optimizing the point spread function (PSF). The PSF is a suitable representative of the system’s performance provided that the system is shift-invariant. Since nonuniform arrays are shift-variant, these techniques are unreliable, especially in the case of off-center targets. As a consequence, there are no frameworks to design nonuniform arrays on a reliable basis to suppress aliasing artifacts in the whole region of interest (ROI). To address this issue, based on the k-space analysis of aliasing in multistatic arrays and nonuniform Fourier transform concept, we propose a small random displacement to be applied to antennas’ location in sparse periodic arrays (SPAs). This displacement prevents the aliasing components from coherently accumulating. Nevertheless, due to the imposed nonuniformity, it is impossible to use FFT-based algorithms in this case. A subsequent resampling or the use of nonuniform FFT can address this problem. The provided simulations and experimental measurements show that the proposed method can efficiently lower the peak level of aliasing artifacts by around 3 dB compared with SPAs. So, the proposed method can be used as an effective way to suppress aliasing artifacts in real-time multistatic millimeter-wave imaging systems.

This article focuses on analyzing the aliasing artifact in millimeter-wave imaging systems, with a special focus on multistatic arrays. The current framework to analyze the behavior of multistatic structures is based on the effective aperture concept. Based on this framework, an equivalent monostatic array, approximating the position of each transmitter-receiver pair by its midpoint, is used to quantify the response and efficiency of the system. Although this framework helps to simplify the study of the complex characteristics of multistatic arrays, it suffers from vital deficiencies. Especially, it fails to describe the aliasing artifacts, seen in the image of some sparse multistatic configurations, where the study of their equivalent monostatic arrays proves that the Nyquist sampling rate is satisfied. In order to explain such behaviors, we propose a k-space-based framework to evaluate multistatic imaging systems. This framework not only explains the reason behind the aliasing artifacts but also estimates the locations where the aliasing happens. Moreover, this analysis can be beneficial to design multistatic configurations with reduced aliasing artifacts. The provided simulations and the experimental results prove the accuracy of the proposed framework that can effectively estimate the occurrence and the place of aliasing artifacts.

This paper focuses on an ef?cient approach of designing multi-static arrays for millimeter-wave imaging, based on the k-space or Fourier-spatial domain characteristic of imaging systems. Our goal is to decrease the redundancy of the data measured by each antenna, and to improve the resolution of the reconstructed image. The proposed technique is based on determining the role of each transmitter and receiver, in collecting the data from each voxel of the target in k-space domain and then rotating transmitters’ beams to measure the desirable information. The effect of non-uniform redundant k-space domain frequency samples that act as an undesirable ?lter, is compensated using a modi?ed SAR back-projection algorithm. Experimental and simulation results are presented and compared with that of a sparse multi-static array with the same number of transmitters and receivers. Our simulations and measurements show signi?cant improvement in terms of overall quality and edge preservation in the reconstructed images. Also, the obtained results demonstrate that using the proposed structure and algorithm, the average improvement in peak-signalto-noise ratio (PSNR), structural similarity index measure (SSIM) and digital image correlation (DIC) metrics of 3.03 dB, 0.22 and 0.173, are achieved, respectively.

In this study, a thorough analysis of image reconstruction in an active millimetre‐wave (MMW) cylindrical imaging system is performed. To improve the system's performance in terms of the total cost as well as the coupling effect, sparse multi‐static arrays are desired. Nevertheless, the existing reconstruction methods fail to operate in sparse multi‐static antenna‐array systems. Therefore, to address these shortcomings, in this study, at first, the existing MMW mono‐static cylindrical image reconstruction methods are modified in both single‐frequency and wideband reconstruction platforms. Moreover, in order to further improve the image quality and the system cost, a sparse multi‐static one‐dimensional antenna array has been proposed. To use this sparse array in the cylindrical MMW imaging system, a novel multi‐static cylindrical image reconstruction method is also proposed. The performances of the proposed reconstruction methods for both mono‐static and sparse multi‐static cylindrical imaging structures are validated through numerical simulations. Through the obtained results, 83% reduction in antenna array cost along with a 25 dB decrease in coupling effect is achieved at the cost of a minor reduction in the reconstructed image quality.

This brief introduces a modified backprojection algorithm to compensate the effect of redundancy of captured data in spatial-fourier or k-space domain for multistatic millimeter-wave imaging. The data measured by each transmitter-receiver pair is explained in k-space domain and the redundancies are determined. Such redundancies act as an undesirable filter that distorts the appearance of the resulting image. Our goal is to modify SAR backprojection algorithm to address this problem, where extensive simulation and experimental results are also provided. Our measurements show a significant improvement in the overall quality and edge preservation in the reconstructed image of objects under the test through the proposed reconstruction algorithm.