Huan Li

By leveraging the relation between sampling and sparsity, compressive sensing (CS) has been developed and applied in many fields, including electromagnetics. In this article, we propose a CS-based method (CSM) to quantitatively image unknown objects under test (OUTs) within a real-time period. Throughout the resolve of inverse scattering problem (ISP), the CS scheme is used to estimate the sparse induced current and contrast profiles of the OUTs, respectively. An experimental system assisted with switching network was established to acquire the scattered data without any mechanical movement. The simulations and experiments verified that the proposed method is able to reconstruct the quantitative images when solving ISPs for sparse objects. In comparison, it is found that the constructed images by CSM are overall superior to the three typical traditional inverse methods, while providing a clean background with less artifacts. Based on CSM, the images of four testing experiments can be reconstructed in nearly 0.35 s with a minimum mean square error of 0.051, 0.085, 0.093, and 0.131, respectively. In terms of computational efficiency, the millisecond-level inversion of CSM implies promising potential in the application of real-time imaging.

Multi-beam antenna is an essential subsystem for concurrent wireless communication systems in satellite and massive multi-input multi-output (MIMO) applications. In this brief, we presented the first experimantal demonstration of a concurrent two-dimension (2D) steerable multi-beam antenna (SMBA) based on multi-feed reconfigurable reflectarray. The features and practicality of the 2D SMBA are verified with theoretical analysis, full-wave simulation and experimental measurements based on a prototype working at 9.3 GHz with around 300 MHz bandwidth. With element’s tunable range of more than 290°, the SMBA can be steered continuously up to θ=±49.4° (φ = 0 360°) in two-dimension. Meanwhile, with multiple beams steered simultaneously, the 2D SMBA can eliminate the crossover regions between adjacent beams of conventional multi-beam systems. The architecture would be potentially used in many wireless communication applications, such as satellite communications.

Inspired by animals’ long-distance migration behaviors, we proposed a novel and reliable long-distance true navigation method based on the measurement of geomagnetic fields. By establishing a two-dimensional (2D) gradient approaching coordinate plane with geomagnetic intensity and inclination, long-distance true navigation can be achieved from any starting spots in this area. Without other calibration and auxiliary information, this navigation method is highly independent, which can be used in electromagnetic shielding circumstances, such as deep ocean. The geomagnetic intensity and inclination gradients are estimated with real-time measurement of the geomagnetic field along the navigation trajectory. With estimated local gradients, 2D gradient approaching algorithm can be applied to predict the heading direction, and the agent can press on towards the destination step by step. Theoretical analysis and Monte Carlo simulations verified the feasibility and practicality of this method, proving its robustness in the presence of interference and different measurements errors. It can be a promising candidate in the design of navigation systems used in autonomous underwater vehicles or platform.

Multi-beam system is a core part for wireless concurrent communication and massive MIMO applications. Traditional concurrent communication techniques, like phased-array or digital beam-forming (DBF) system, suffer from massive RF frontends or power-hungry high speed ADCs, which makes them complex and expensive in fabrication. Inheriting the advantages of reflectarray and passive multi-beam system, we reported in this letter a novel and cost-effective concept of steerable simultaneous multi-beam antenna (SMBA). New features and the practicality of it are fully verified with theoretical analysis and experimental implementation of a prototype working at 7GHz with 300MHz bandwidth. With multiple beams steered simultaneously, the SMBA can eliminate the crossover regions between adjacent beams of passive multi-beam antenna. Experimental results indicate its potential in the design of diverse concurrent communication systems such as base stations and point-to-point links.

In this communication, a wideband reconfigurable reflectarray based on reflector-backed active second-order bandpass frequency selective surface (FSS) is presented. The reflector is composed of periodic short-circuited parallel plate waveguide (PPW) and the FSS is composed of stacked non-resonant metallic elements separated by thin dielectric substrates. By integrating microwave varactors in the capacitive layers of FSS, more than 270° continuous phase tunability is achieved within a fractional bandwidth of 14%. A one-dimensional reflectarray prototype operating at C band is fabricated and measured. The experimental results show that it can achieve ±55° beam scanning coverage. Symmetric beam steering is observed due to the center-fed configuration. With advantages of low cost and simple structure, the proposed reflectarray can be potentially used in wideband wireless communication and radar systems.

Wireless power transfer (WPT) is a promising technique used in many applications, like smart terminals, IoT equipment, etc. For wireless power transfer systems based on RF rectifying, working in very low power density situation, high gain rectenna system is of great essential. According to the antenna theory, for the same physical size, higher gain means higher aperture efficiency, which means more power will be received by the antenna instead of being scattered. However, high gain rectenna usually means narrow beam width and high directivity, which is contradictory to a power harvesting system who need to have wide spatial coverage. In order to realize high aperture efficiency and wide spatial coverage at the same time, a 2-D multi-beam rectenna based on gain enhanced patches and hybrid coupler is proposed, which can improve the 2-D spatial coverage and achieve the maximum 81.5% power receiving efficiency. Simulation and experimental results indicate that the power receiving efficiency is more than 50% with 100° coverage in main beam cutting plane, 70° in H-plane, and 90° in E-plane. The practicality of this letter is fully verified, which can help to provide a guidance for the design of rectenna.

Layer-stacked metasurfaces or frequency selective surfaces (FSSs) have been employed as a cost-effective approach to design reconfigurable lenses. Previous work showed that the intrinsic technical drawback, i.e. the transverse field interaction effect, significantly limits the beamforming performance of these electrically thick lenses. In this paper, an active metasurface lens loaded with parallel plate waveguide (PPW) is experimentally validated. By inserting parallel plate waveguide (PPW) into electrically thick lenses, the transversely propagating field inside the lenses can be completely blocked, based on which the beam steering range and the side-lobe level (SLL) can be remarkably improved. A linear PPW lens array operating around 5.75 GHz is fabricated. Experimental results show that the scanning coverage of lens antenna can reach 60o, and the side-lobe level (SLL) are below -15.7 dB within 50 degrees.

Real-time imaging is of significant importance for practical applications. To achieve the goal of real-time operation, the noniterative inversion or pretrained strategy is used in the existing methods. However, both rough reconstruction and high-required computation resources prevent the rapid implementation of the real-time imaging system. In this letter, a modified back-propagation method is proposed for real-time microwave imaging based on the two-step iterative shrinkage/thresholding (TwIST) algorithm. Making use of the prior information, the TwIST method is applied as effective $\ell _{1}$ -norm regularization to improve the solution of the ill-posed linear equations in inverse scattering problem. The experimental measurements were conducted by a single-input multiple-output system. The simulation and experimental results verified that based on the proposed method, improved reconstruction can be retrieved from the measured data in millisecond-level imaging time. Moreover, it also provides a preliminary nice result for other optimization methods.

Besides the prohibitive cost, complexity, and baseband scale, the development of millimeter-wave (mmWave) digital-beamforming fifth-generation (5G) frontends is facing a physical-layer challenge of thermal dissipation. This is caused by the massively integrated power amplifiers (PAs) and the extremely compact volume due to millimeter-scale wavelengths. In this work, we theoretically analyze and experimentally demonstrate a reconfigurable planar Fresnel lens, based on which a new-concept mmWave beamforming frontend can be implemented with significantly reduced number of PAs. It overcomes the inherent shortcomings of analog beamforming solutions relying on massive use of power dividers and phase shifters and can replace the convex lens used in the recently proposed hybrid beamforming solution. The implemented Ka-band lens is simple-structured, cost-effective, and easy to control, providing multiple modes for beamspace massive-multiple-input multiple-output (MIMO) communications. Experimental results verified its potential in different mmWave 5G systems such as base stations and point-to-point links.