To solve the problems of high computational complexity and unstable image quality inherent in the compressive sensing (CS) method, we propose a complex-valued fully convolutional neural network (CVFCNN)-based method for near-field enhanced millimeter-wave (MMW) three-dimensional (3-D) imaging. A generalized form of the complex parametric rectified linear unit (CPReLU) activation function with independent and learnable parameters is presented to improve the performance of CVFCNN. The CVFCNN structure is designed, and the formulas of the complex-valued back-propagation algorithm are derived in detail, in response to the lack of a machine learning library for a complex-valued neural network (CVNN). Compared with a real-valued fully convolutional neural network (RVFCNN), the proposed CVFCNN offers better performance while needing fewer parameters. In addition, it outperforms the CVFCNN that was used in radar imaging with different activation functions. Numerical simulations and experiments are provided to verify the efficacy of the proposed network, in comparison with state-of-the-art networks and the CS method for enhanced MMW imaging.
Millimeter-wave Holography is a promising technique for security screening to detect concealed weapons. However, a crucial disadvantage of this technique is that the close-range large-aperture operation result in a very short depth of focus. In this paper, a W-band auto-focus holographic imaging system is presented. By calculating and comparing the amplitude integral value of holographic imaging results reconstructed at different focusing distances, the algorithm can assess focusing quality of each imaging result, choose the optimal focusing distance, and extract the optimum viewing image from the imaging results. The scheme of imaging system is described in detail. Both simulation and experimental results are provided to demonstrate that the focusing performance of new auto-focus imaging system is much better than conventional holographic system.
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