With the strengthening of global anti-terrorist measures, it is increasingly important to conduct security checks in public places to detect concealed objects carried by the human body. Research in recent years has shown that deep learning is helpful for detecting concealed objects in passive terahertz images. However, previous studies have failed to achieve superior accuracy and performance for real-time labeling. Our research aims to propose a novel method for accurate and real-time detection of concealed objects in terahertz images. To reach this goal we trained and tested a promising detector based on deep residual networks using human image data collected by passive terahertz devices. Specifically, we replaced the backbone network of the SSD (Single Shot MultiBox Detector) algorithm with a more representative residual network to reduce the difficulty of network training. Aiming at the problems of repeated detection and missed detection of small targets, a feature fusion-based terahertz image target detection algorithm was proposed. Furthermore, we introduced a hybrid attention mechanism in SSD to improve the algorithm’s ability to acquire object details and location information. Finally, the Focal Loss function was introduced to improve the robustness of the model. Experimental results show that the accuracy of the SSD algorithm is improved from 95.04 to 99.92%. Compared with other current mainstream models, such as Faster RCNN, YOLO, and RetinaNet, the proposed method can maintain high detection accuracy at a faster speed. This proposed method based on SSD achieves a mean average precision of 99.92%, an F1 score of 0.98, and a prediction speed of 17 FPS on the validation subset. This proposed method based on SSD-ResNet-50 can provide a technical reference for the application and development of deep learning technology in terahertz smart security systems. In the future, it can be widely used in some public scenarios with real-time security inspection requirements.

In this article, a novel 3-D imaging method based on the chirp scaling algorithm (CSA) is proposed for millimeter-wave near-field imaging. We deduce the theoretical formula of CSA applied to multiple-input–multiple-output (MIMO)/single-input–single-output (SISO) array imaging and then analyze the computational complexity and phase error. Compared to the classical Fourier-based algorithm, such as range migration algorithm (RMA), they can replace the redundant interpolation with the phase multiplication. Meanwhile, it can handle the large relative bandwidth system compared with frequency scaling algorithm (FSA). It greatly improves imaging speed and broadens the application scenarios. The rearrangement operation of the MIMO wavenumber spectrum is also accelerated by the sparse matrix multiplication. To further reduce the computation time, we perform the direct rearrangement of the MIMO wavenumber spectrum before phase compensation. It works well but also sacrifices part of the signal-to-noise ratio (SNR), so it may not be suitable for the weakly scattering targets. The simulation data of a 2-D uniform array with the large number of antennas and a published dataset are used to verify the effect of the proposed algorithm. The simulation and experiment results show that the proposed method can effectively improve the computational efficiency.

This article proposes a novel method for eliminating the first grating lobe of wideband MIMO millimeter-wave imaging array. It exploits the position shift phenomenon of the first grating lobe and can achieve grating lobes suppression without additional calculation. The far-field formula for the first grating lobe is derived and the cause of its shift phenomenon in both azimuth- and range-direction is analyzed. The proposed multi-apodization (MA) method can be used to cut off the part of the shifted grating lobes while maintaining the resolution. However, non-sparse targets still have the far-field residual grating lobes after MA. To solve the problem, the position formula and detailed derivation of the residual grating lobes is given. Therefore, a pixel search (PS) method can be used to remove them. The proposed MA can also be utilized to suppressed the residual grating lobes of the near-field MIMO array. Both simulated and experimental data in different system regimes and frequencies verify its effectiveness.

To realize magnetic induction (MI) communication through seawater, a rotating permanent magnet array (RPMA) operating in the ultralow frequency (ULF) band is proposed. The array consists of 5×5 cylindrical Nd-Fe-B magnets driven by servo motors. To maximize the transmitting magnetic moment, the motors are controlled to rotate synchronously through network communication. The design of the magnet and array configuration is implemented by analyzing the torque and magnetic moment. Magnetic forces between the array magnets are analyzed by using the equivalent static magnetic dipole model. Permalloy sleeves are used to reduce the influence of magnetic force on the synchronization of magnets. The total transmitting magnetic moment of the RPMA is 360A∙m ² , and its power efficiency is 14.4 times higher than the conventional transmitting system using coils. Binary frequency shift keying (BFSK) modulation is realized by switching the rotating speed of the magnets and verified by an indoor test. The equivalent orthogonal horizontal magnetic dipole (HMD) model of the array is used, and the magnetic field distribution in layered media is simulated using FEKO. The calculated magnetic induction is 100fT in the position with a distance of 1km and a depth of 5m in the sea. The results of the through-the-sea experiment show that the magnetic field of the RPMA can be detected in a range of 500m by magnetic inductive sensors immersed at a depth of 5m in seawater, and agree well with the simulated results. Transmission bit rates of 10bps and 5bps can be achieved in the positions of 60m and 110m.

Multiple-input-multiple-output synthetic aperture radar (MIMO-SAR) is being studied and applied in more and more scenarios. However, there is still a certain distance away from real-time imaging using advanced algorithms. The traditional backpropagation algorithm (BPA) multi-accumulation integration is unsuitable for dealing with large-size scanning data, and the wavenumber domain algorithm requires the array to satisfy Nyquist sampling law in azimuth to avoid aliasing in imaging reconstruction. Based on these issues, a novel 3D imaging method is proposed for MIMO-SAR. An appropriate transformation and inverse Fourier transform (FT) is carried out for the frequency domain; thus, accumulation in the wavenumber domain is not required, which is easy to implement. The computational complexity of the algorithm is much lower than BPA and has better generalizability than the wavenumber domain algorithm. Coherence factor (CF) is also introduced to achieve sidelobe suppression. Proof-of-principle experiments were also carried out in the 92.5 GHz band based on the MIMO-SAR prototype system. Both simulation and experimental results of different distributed targets show good performance of imaging and do not lose the quality of image reconstruction.

This paper proposes a rotating shutter antenna that can directly generate 2FSK signals in the ULF band and it is expected to be used as the transmitter for magnetic induction (MI) underground communication systems. The antenna was modeled using ANSYS Maxwell and the magnetic field distribution was simulated. The results show that the interaction between the high-permeability shutter and the mutual cancellation of magnets decreased the transmitting magnetic moment of the antenna. A prototype antenna was manufactured and the time and frequency properties of the measured B${}_z$ field were the same as the simulated results, while the magnitude of the measured signal was larger. The propagation characteristics of the antenna in air–soil–rock were simulated using FEKO and the results show that the signal strength was greater than 1 fT at a depth of 450 m from the antenna whose magnetic moment as 1 Am${}^2$. The relationship between different magnetic components and azimuth could be used to enhance the signal strength. The formula of the B${}_z$ field was derived using the measured magnitude versus distance and the path loss was also analyzed. Finally, the 2FSK modulation property of the antenna was verified by indoor communication experiments with a code rate of 12.5 bps in the ULF band.