Do We Really Need Frequency‐Selective Surface and Metasurface Reflectors for Antenna Gain Enhancement, or Are Metallic Reflectors Enough?

A frequency selective surface (FSS) with a unit cell containing of an L-notched rectangular metallic ring pattern that can be adopted for dual polarization applications is proposed and analyzed using the wave concept iterative procedure (WCIP). The parametric study of the FSS provides three FSS dimensions where their variation independently or simultaneously results in resonant frequencies tuning. They are the two dimensions of the L part of the FSS and its notch penetration and changing their values in the FSS allowed intervals gives rise to charts and equations based on polynomials determined by the use of least mean square method. Thus no need to software to determine the FSS resonant frequencies once the application is restricted to a specific FSS such as the L- notched rectangular metallic ring FSS for low cost FSS use. The presented synthesis approach is extended to the determination of the equivalent FSS structures based on non-coupled parallel metallic strips for the complex FSS structures of difficult comprehensive resonance sources. One FSS is manufactured and characterized. A good agreement is recorded when the WCIP results for the transmission coefficient are compared with the results of COMSOL Multiphysics software and measurement.

Metasurface reflectors and reconfigurable intelligent surfaces (RISs), which are key technologies for robust, high-speed, and high-capacity communications in millimeter-wave bands, can eliminate shadowed areas caused by buildings and moving objects by artificially shaping the propagation environment. Metasurface reflectors are passive devices that can generate reflected waves in desired directions and are expected to be installed on buildings and windows as low-cost, power-free solutions. However, since their reflection angles cannot be adjusted after installation, the improvement in coverage area may be limited. To address this limitation, we propose the concept of building a multipath environment by deploying a massive number of metasurface reflectors on windows and building surfaces. When scaling up such deployments, each metasurface reflector must be individually designed due to variations in the distance and angle relative to the base station and the user equipment, leading to increased design and manufacturing costs. To mitigate this issue, we also propose a method for simplifying the design of the incident and reflection angles for metasurface reflectors. We fabricated prototype metasurface reflectors, each measuring one square meter, and designed them to exhibit specific incident and reflection angle characteristics. These metasurface reflectors were deployed continuously to form a ten-meter-long array, and we experimentally evaluated their angle-of-arrival characteristics in a large-scale setting. We confirmed that the reflected waves arrived from the designed reflection angle at each point within the measurement area.

The prospects of using a Reconfigurable Intelligent Surface (RIS) to aid wireless communication systems have recently received much attention from academia and industry. Most papers make theoretical studies based on elementary models, while the prototyping of RIS-aided wireless communication and real-world field trials are scarce. In this paper, we describe a new RIS prototype consisting of 1100 controllable elements working at 5.8 GHz band. We propose an efficient algorithm for configuring the RIS over the air by exploiting the geometrical array properties and a practical receiver-RIS feedback link. In our indoor test, where the transmitter and receiver are separated by a 30 cm thick concrete wall, our RIS prototype provides a 26 dB power gain compared to the baseline case where the RIS is replaced by a copper plate. A 27 dB power gain was observed in the short-distance outdoor measurement. We also carried out long-distance measurements and successfully transmitted a 32 Mbps data stream over 500 m. A 1080p video was live-streamed and it only played smoothly when the RIS was utilized. The power consumption of the RIS is around 1 W. Our paper is vivid proof that the RIS is a very promising technology for future wireless communications.

Reconfigurable Intelligent Surface (RIS) has become a popular technology to improve the capability of a THz multiuser Multi-input multi-output (MIMO) communication system. THz wave characteristics, on the other hand, restrict THz beam coverage on RIS when using a uniform planar array (UPA) antenna. In this study, we propose a dynamic RIS subarray structure to improve the performance of a THz MIMO communication system. In more details, an RIS is divided into several RIS subarrays according to the number of users. Each RIS subarray is paired with a user and only reflects beams to the corresponding user. Based on the structure of RIS, we first propose a weighted minimum mean square error - RIS local search (WMMSE-LS) scheme, which requires that each RIS element has limited phase shifts. To improve the joint beamforming performance, we further develop an adaptive Block Coordinate Descent(BCD)-aided algorithm, an iterative optimization method. Numerical results demonstrate the effectiveness of the dynamic RIS subarray structure and the adaptive BCD-aided joint beamforming scheme and also show the merit of our proposed system.

The wave concept iterative procedure (WCIP) is used to analyze a quasi-square open metallic ring frequency selective surface (FSS). The quasi-square open metallic ring FSS is dual-polarized. When the incident plane wave is polarized in a direction parallel to the FSS' coupled parallel strips, it shows two rejecting bands. Moreover, another rejecting band can be obtained if the source plane wave is perpendicularly polarized with respect to the FSS' coupled parallel strips. The three resonant frequencies are inversely proportional to the length of the FSS' coupled strips to provide an easy fine tuning of the FSS structure. The simulated results obtained using WCIP are compared to the measured results, and a good agreement is reported.

This work presents a transfer learning approach for the forward design of composite unit cells of metasurfaces and frequency-selective surfaces (FSS). As a specific application, we target reconfigurable intelligent surfaces (RIS) for wireless communication. We demonstrate that forward models for dual-dipole unit cells can be trained using significantly fewer data by reusing pre-trained models of simpler, single-dipole structures. Our architecture reduces the required training data by up to a factor of 25 while maintaining a mean squared error (MSE) on the order of $$10^{-2}$$ . After establishing the forward model for the dual-dipole RIS, we use it in an inverse design framework to synthesize a composite 2-bit RIS unit cell operating at 26.5 GHz. The resulting RIS provides phase modulation in a 270 $$^\circ$$ range by tuning the reverse bias voltage of integrated varactor diodes. Numerical simulations confirm the validity of the proposed approach, establishing transfer learning as a data-efficient and practical method for the design of composite unit cell structures.

In this paper, we deploy a full-wave FDTD paradigm to investigate the effect of reconfigurable intelligent surface (RIS) – switchable frequency-selective surfaces (FSS) – on generic massive MIMO uplink channel’s eigenspace structure. We place an RIS based on two switchable FSS layers in the vicinity of a 64-element massive MIMO base-station (BS) array, serving a cluster of four fixed user equipment (UE) units. Utilizing an electromagnetic tool based on time-averaged Poynting flow developed recently by the authors, we demonstrate how the illumination of BS-array aperture can be controlled by the intentional deployment of various switching states in the RIS placed near the BS. We show that such supplementary RIS structures may assist the wireless link engineer in deterministically “customizing” the uplink channel behaviour by selectively enhancing/suppressing certain channel eigenvalues.

This document describes a study of a tunable frequency selective surface (FSS) for applications in reconfigurable intelligent surface (RIS) with millimeter wave beamforming, for applications in the Fifth generation of mobile networks (5G). The work developed a square loop that changes its reflection coefficient and reflection phase shift controlled by PIN diodes. The study was based on the simulation of two tunable unit cells, the 1-bit FSS and the 2-bit FSS, designed by the ANSYS HFSS. Simulated results obtained a phase shift difference of 123.10° for the 1-bit FSS and 177.48° for the 2-bit FSS at 26.25 GHz.

A novel parallel coupled Metallic strips frequency selective surface (FSS) is presented. The design is based on a filter composed of horizontal and vertical metallic strips. The structure has the advantage of simplicity and gives multi resonant frequencies easily controlled by a simple variation in the strips length. The proposed FSS rejects frequencies at 7GHz, 9.4GHz with bandwidths of 0.646GHz, 0.793GHz respectively when the structure is excited with an x polarized plane wave, and two frequencies at 9.1GHz and 11.2 GHz with bandwidth of 280.3MHz, 63MHz respectively when the structure is excited with a y polarized plane wave. To suppress frequencies ideal diode on reverse bias are inserted. The simulated results obtained using WCIP (Wave Concept Iterative Method) are compared to the COMSOL Multiphysics 4.3b software results, and measurements, a good agreement is observed. Then, an FSS synthesis approach based on non coupled parallel metallic strips is presented. It allows the synthesis of FSS for resonant frequencies ranging from 4.5GHz and 12.25 GHz for strips lengths inversely varying from 5mm to 19mm. To validate the synthesis approach, measured resonant frequencies are used as desired resonant frequencies to determine the metallic strips lengths. As a result the metallic strips FSS dimensions are extracted. Then the WCIP method is used to characterize the synthesized FSS. Three FSSs are synthesized based on the fabricated FSS and good agreement between measurements and synthesized FSSs results is recorded.

The rapid expansion of Internet of Things (IoT) networks has intensified the demand for high data throughput, energy efficiency, and reliable connectivity under resource-limited and interference-prone conditions. This research presents a comprehensive study on adaptive beamforming algorithms integrated with Reconfigurable Intelligent Surfaces (RIS) to optimize wireless communication performance in IoT environments. The proposed adaptive framework combines model-based and data-driven optimization, jointly adjusting base station beamforming weights and RIS reflection parameters to achieve dynamic control of the propagation environment. A simulation model with varying RIS element densities was implemented to evaluate system performance in terms of sum-rate, energy efficiency, convergence time, and robustness under channel uncertainty and mobility. Results demonstrate that the adaptive algorithm achieves near-optimal throughput comparable to full semidefinite programming (SDP) methods while substantially reducing computational complexity and signaling overhead. Energy efficiency improved by more than 20%, and convergence speed increased fourfold relative to conventional optimization techniques. The system maintained stability under imperfect channel state information and mobility conditions, confirming the robustness of the proposed framework for practical IoT deployment. Statistical analysis validated the significance of performance differences among methods, with p-values <0.001 and large effect sizes. The integration of adaptive beamforming and RIS effectively transforms static wireless channels into intelligent, reconfigurable environments capable of self-optimization. Based on these findings, the study recommends incorporating RIS-assisted adaptive control in future IoT network designs, developing low-complexity learning-based algorithms for edge devices, and standardizing RIS control protocols to ensure interoperability. The proposed approach paves the way for energy-efficient, scalable, and resilient communication systems in next-generation IoT ecosystems, supporting sustainable connectivity for diverse real-world applications such as smart cities, industrial automation, and wireless sensor networks.

ABSTRACTIn this paper, a compact wideband circularly polarized slot antenna consisting of two L-shaped resonators is proposed. The slot antenna is loaded with a band-stop frequency selective surface (FSS) reflector to improve the gain of the antenna and to reduce the profile of the antenna. The radiation and scattering characteristics of FSS loaded slot antenna are compared with the metallic reflector loaded slot antenna, and the proposed antenna shows improved impedance bandwidth of 1.75–2.65 GHz. The measured results show that the peak gain of the slot antenna with FSS is enhanced by 4 dB. The proposed antenna with FSS also exhibits 3-dB ARBW of 2.2–2.65 GHz, and the scattering performance shows an average RCS reduction of 7.34 dB is achieved by slot antenna with FSS as compared to slot antenna with metallic reflector with peak RCS reduction of 24 dB at 1.2 GHz.

Proposed is a concept of a directional multi‐band antenna employing frequency selective surfaces (FSSs). To confirm the feasibility of the concept, the proposal is implemented by combining a metal reflector, two FSSs that act as frequency filters, and a multi‐band radiator. The proposed triple‐band antenna can radiate at 800 MHz (the metal reflector or FSS 1), 2 GHz (FSS 2), and 4 GHz (FSS 3). FSS 2 passes waves at one frequency band (800 MHz) and reflects all other bands, and FSS 3 passes waves at two frequency bands (800 MHz/2 GHz) and reflects all other bands. Beam control is easy since all that is needed is to change FSS size and/or the distance between the radiator and metal reflector/FSS. Electromagnetic field simulations and measurements demonstrate good directivity in the frequency bands of 800 MHz, 2 GHz and 4 GHz.

We consider a scenario in which an unmanned aerial vehicle (UAV) performs transmit antenna selection (AS) and is assisted by a reconfigurable intelligent surface (RIS) for integrated sensing and communication. We optimize the antenna subset and transmit beamformer at the UAV and phase shift at RIS to maximize the beampattern gain towards the target under a specific rate constraint for the communication. To solve this, we propose an alternating optimization based algorithm in which the optimization problem is split into a sequence of sub problems and presented as semidefinite programs. Our numerical results show that an improved average beampattern gain and outage performance is obtained by increasing number of transmit antennas or number of RIS elements. We show that by increasing the RIS elements, we can obtain the savings of radiated power while keeping the average beampattern gain fixed, a system with subset AS can obtain improved performance compared to a multi antenna system. Our results also show that an improved average beampattern gain and outage performance can be obtained even when UAV goes away from the user and comes closer to the RIS and target. A discussion on real-time feasibility challenges and potential directions for future work is also presented.

A modified rectangular metallic ring Frequency Selective Surface (FSS) with single layer for multiband applications is proposed in this article. By using the wave concept iterative process method WCIP, the resonant characteristics of the proposed FSS are derived. This structure is sensitive to polarization of the incident wave. The proposed FSSs elements are composed by development of a basic rectangular metallic ring FSS. The tuning of the ring dimensions allows the adjustment of the FSS resonant frequencies. The results found by the WCIP method and compared with those simulated by COMSOL Multiphysics software and a good agreement is recorded.

The rapid evolution of terahertz (THz) and millimeter-wave (mmWave) technologies is driving nextgeneration wireless systems such as 5 G and emerging 6 G, but these bands suffer from severe propagation loss, limited coverage, and dynamic channels; beam-steering metasurfaces and recon- figurable intelligent surfaces (RIS) offer energy-efficient, cost- effective alternatives to phased arrays. This paper reviews recent advances in RIS- and metasurface-based beam steering, covering design methods, reconfiguration mechanisms, and deployment strategies, with emphasis on conformal beam-steerable metasur- face antennas for mmWave, cylindrical RIS for integrated sensing and communication (ISAC), and real-time hardware prototypes. The study analyzes trade-offs in system integration, fabrication complexity, scalability, bandwidth, and radiation efficiency, and highlights open challenges and future research directions to accelerate RIS-assisted wireless systems for 6 G and beyond.