Guanying Sun’s PhD Dissertation Defense

“Optimizing Reconstruction for Mm-Wave Body Scanner Imaging”

Committee:

Prof. Carey Rappaport (Advisor)

Prof. Edwin Marengo

Prof. Jose Martinez-Lorenzo

Abstract:

In the past decades, due to evolving threats, passenger screening has become an important secure measure at airport and other secure locations. Numerous passenger screening techniques have been developed by researchers in both academia and industry to detect threats from explosives and weapons. Among these developments, the multistatic mm-wave radar Advanced Imaging Technology (AIT) system was developed at Northeastern University. A problem with this system is the sidelobes from its physical limitations, such as the finite aperture extent and the violation of the Nyquist sampling criterion by the sparse array. Therefore, it is important to suppress the sidelobes so that to improve the quality of the reconstruction image. In this proposal, we investigate two categories of methods, one is based on post-processing, and the other is based on system configuration optimization. In the former category four methods are developed, while in the latter two methods are proposed.

In the first category, the first method is the phase coherence method which is designed to weight the coherent sum based on the phase diversity of the reconstructed solutions for different transmitters. In this method, two ways are considered to construct the Phase Coherence Factor (PCF). The first way is to use the information of wrapped phase, and the second way is to use the information of unwrapped phase, which is more intuitive than the first way. The second method is the coherence factor related method. Three coherence-factor based methods are analyzed and then incorporated into the imaging procedure of our nearfield millimeter-wave radar security scanning system. The third method is the SNR-dependent coherence factor method, which takes SNR into consideration when forming the coherence factor. This method can generate better results than the pure coherence-factor based methods by choosing a proper set of parameters. The fourth method is the block-weighting algorithm where the neighbor weight amplifies bright areas and attenuates dark areas, while the block keeps the influence local. The effectiveness of these methods has been verified with both simulation and measurement data.

In the second category, the first method is optimizing receiver positions via PSF-based multi-objective optimization. Two metrics for measuring image quality of the PSF are proposed and defined as objective functions. The solution-selection metric is introduced to select the desired solution from the numerous Pareto-optimal solutions. Simulation shows that the optimized receiver design generates images with lower sidelobe level than the uniform receiver design. The second method is the dual-frequency radar design, where a dual frequency, wideband antenna array is designed by combining a high frequency subarray with a low frequency subarray. The image of the dual frequency array is obtained by multiplying the images of the two subarrays. We analyzed the amplitude of the PSF theoretically and proposed a criterion for the selection of dual frequency array design. The system imaging simulation shows that the grating lobes are significantly reduced for the dual frequency array with fewer radar modules/elements than the conventional array. This design will make the new generation system superior to the conventional scanning system.