Reconfigurable intelligent surfaces (RISs) have promising coverage and data-rate gains for wireless communication systems in 5G and beyond. Prior work has mainly focused on analyzing the performance of these surfaces using simulations or lab-level prototypes. To draw accurate insights about the actual performance of these systems, this paper develops an RIS proof-of-concept prototype and extensively evaluates its potential gains in the field under realistic wireless communication settings. In particular, a 160-element RIS, operating at a 5.8 GHz band, is designed, fabricated, and accurately measured in the anechoic chamber. This surface is then integrated into a wireless communication system and the beamforming gains, pathloss, and coverage improvements are evaluated in realistic outdoor communication scenarios. When both the transmitter and receiver employ directional antennas, the developed RIS achieves 15-20 dB gain in the signal-to-noise ratio (SNR) in a range of ±60∘ beamforming angles. In terms of coverage, and considering a far-field experiment with a blockage between a basestation and a grid of mobile users and with an average signal path of 35 m, the RIS provides an average SNR improvement of 6 dB (max 8 dB) within an area >75 m. Thanks to the scalable RIS design, these SNR gains can be directly increased with larger RIS areas. For example, a single 1,600-element RIS with the same design is expected to provide 26 dB SNR gain for a similar deployment (theoretical estimation). These results draw useful insights into the design and performance of RIS systems and provide an important proof for their potential gains in real-world far-field wireless communication environments.

We present a 1-bit reconfigurable intelligent surface (RIS) operating at millimeter-wave frequencies that suppresses the undesired grating lobes encountered in binary phase modulation schemes and achieves high resolution beam steering. We incorporate fixed, random phase delays at each unit cell of the surface which breaks the periodicity of the phase quantization error and suppresses side lobes. Additionally, the random phase delays reduce the beam pointing error – a limitation of binary RISs -which can be beneficial in applications that require high resolution beam steering. The proposed topology allows for scalable RIS apertures that are compatible with printed circuit board (PCB) fabrication technology. It consists of four metasurface tiles of 256 radiating elements (16 × 16) connected on a separate control board that houses the control unit and power supply. The prototype is designed to operate at 27.2 GHz and perform electronic beam steering in ±60° in both azimuth and elevations planes. A quantization lobe reduction of more than 10 dB is achieved with the proposed technique and the surface is well suited for mmWave 5G communication scenarios to enhance signal coverage and signal-to-noise ratio.

We present a novel imaging method for compact, low-profile imagers in millimeter wave (mmW) and terahertz (THz) frequencies. The method borrows the principles of computerized tomography (CT) to generate 2D or 3D high-resolution images using simplified RF-front-ends. To perform imaging the proposed scheme implements the projection-slice theorem. Specifically, it records the Radon Transform (RT) of the desired field of view (FOV) and reconstructs the image using the Fourier Slice Theorem. To achieve the illumination of the FOV the proposed imaging configuration uses a rotating linear antenna array that produces a highly directive fan-beam. The antenna elements are further reduced using multistatic sparse array techniques without compromising the image reconstruction process. By reducing dramatically the number of active elements, the new method could allow high-resolution mmW/THz imaging systems to be embedded in low-profile and lightweight platforms, including, unmanned aerial vehicles (UAVs) or CubeSats. In this paper, we present the basic concept of the novel imaging scheme and simulation results verifying the proof of concept. Finally, we present a modified imaging antenna array that alleviates interference from objects and sources outside the FOV.

In this work, we show how active terahertz (THz) imaging systems can exploit the unique propagation properties of THz waves to reconstruct images of non-line-of-sight (NLoS) scenes. Most building surfaces' material properties and roughness allow for a unique combination of diffuse and strong specular scattering. As a result, most surfaces behave as lossy mirrors that enable propagation paths between a THz camera and the NLoS scenes. We propose a mirror folding algorithm that tracks the multireflection propagation of THz waves to 1) correct the image from cluttering and 2) see around occlusions without prior knowledge of the scene geometry and material properties. To validate the feasibility of the proposed NLoS imaging approach, we carried out a numerical analysis and developed two THz imaging systems to demonstrate real-world NLoS imaging experiments in sub-THz bands (270-300 GHz). The results show the capability of THz radar imaging systems to recover both the geometry and pose of LoS and NLoS objects with centimeter-scale resolution in various multipath propagation scenarios. THz NLoS imaging can operate in low visibility conditions (e.g., night, strong ambient light, smoke) and uses computationally inexpensive image reconstruction algorithms.

We propose a novel technique for efficient and robust modeling of multistatic, multi-dimensional, large-format millimeter-wave/terahertz (mmW/THz) imaging systems. The proposed method significantly reduces the necessary computational resources for the design of electrically large systems of multiple sensors that acquire multi-frequency images, such as imaging radars used in 3D mapping. To alleviate the problem, the radiated and scattered fields are individually computed for the sensor and the target, respectively, using full-wave numerical solvers. Afterwards, the fields are properly combined using conjugate field coupling, thus minimizing the necessary computational resources, without compromising solution accuracy. In this letter, the theoretical background of the technique is presented, along with numerical results for beam steering and raster scanning imaging systems.

In this work, we present a rigorous full-wave eigenanalysis for the study of nanoantennas operating at both terahertz (THz) (0.1–10 THz), and infrared/optical (10–750 THz) frequency spectrums. The key idea behind this effort is to reveal the physical characteristics of nanoantennas such that we can transfer and apply the state-of-the-art antenna design methodologies from microwaves to terahertz and optics. Extensive attention is given to penetration depth in metals to reveal whether the surface currents are sufficient for the correct characterization of nanoantennas, or the involvement of volume currents is needed. As we show with our analysis, the penetration depth constantly reduces until the region of 200 THz; beyond this point, it shoots up, requiring volume currents for the exact characterization of the corresponding radiating structures. The cases of a terahertz rectangular patch antenna and a plasmonic nanoantenna are modeled, showing in each case the need of surface and volume currents, respectively, for the antenna’s efficient characterization.