Terahertz (THz) band will play an important role in enabling sixth generation (6G) envisioned applications. Compared with lower frequency signals, THz waves are severely attenuated by the atmosphere temperature, pressure, and humidity. Thus, designing a THz communication system must take into account how to circumvent or diminish those issues to achieve a sufficient quality of service. Different solutions are being analyzed: intelligent communication environments, ubiquitous artificial intelligence, extensive network automation, and dynamic spectrum access, among others. This survey focuses on the benefits of integrating intelligent surfaces (ISs) and THz communication systems by providing an overview of IS in wireless communications with the scanning of the recent developments, a description of the architecture, and an explanation of the operation. The survey also covers THz channel models, differentiating them based on deterministic and statistical channel modeling. The IS-aided THz channels are elucidated at the end of the survey. Finally, discussions and research directions are given to help enrich the IS field of research and guide the reader through open issues.

In this work, we have applied the multipole expansion method to analyze the transmission/reflection properties of allmetallic metasurfaces operating in the microwave regime. This analytical technique provides enhanced physical insights into the electromagnetic properties of these metasurfaces. We show that the sharp electric-dipole resonance in a metasurface composed of metallic squares can be resonantly coupled with the sharp magnetic-quadrupole resonance from a metasurface made of metallic circles. This coupling yields a significantly broadened operating band for the metasurface. Specifically, the metallic-square based metasurface operates under a fractional bandwidth of around 61%, while the operational bandwidth of a metallic-circle based metasurface is ca. 22%. In contrast, the resonantly-coupled electric dipole-magnetic quadrupole metasurface exhibits a fractional bandwidth of approximately 96% operating at the central frequency 5.7 GHz. The feasibility of the design was proven by fabricating metasurfaces over an FR4 substrate, whose electromagnetic properties were as predicted qualitatively using the analytical multipole expansion method.

This letter demonstrates that an inexpensive, easy-to-deploy, and portable 3D-printed all-dielectric corner-like Bragg reflector can boost antenna gain. Notably, the concept shown here works by placing the reflector around the antenna's near-field without changing its design. The idea behind this work stems from the combination of quarter-wave stack (QWS) Bragg mirrors and electric image theories, providing further gain enhancements from previous metal corner-like and 3D-printed Bragg reflectors. In particular, we considered an operating frequency of 5.7 GHz, for which a 3D-printed prototype was manufactured and characterized at microwave frequencies. As a result, an aperture efficiency of 40.96% with the gain enhanced up to 12.96 dB were observed, without disturbing the primary source impedance bandwidth, in excellent agreement with full-wave numerical analysis.

This work introduces a cost-effective photonics-based approach for fast-implementing horn antennas operating in millimeter-wave frequencies (mm-waves). Instead of using conventional metallic guiding structures, we employed an all-dielectric quarter-wave stack Bragg mirror photonic design. As a proof-of-concept, we used five semi-spherical air-polylactic acid bilayers stacked with a conical (horn-like) aperture fabricated through a one-step 3D printing process. The prototype, with a bandwidth of 2.6 GHz (from 24.96 to 27.50 GHz), was fed by the WR28 standard waveguide mechanism with measured gain ranging from 10.6 to 13.9 dBi (between 25 and 27 GHz). These outcomes demonstrate our idea's suitability for alternative design of high-frequency antennas for future 5G and 6G wireless communications systems, overcoming the precision constraints of traditional manufacturing methods.

We demonstrate a concept for a large enhancement of the directivity and gain of readily available cm- and mm-wave antennas, i.e., without altering any property of the antenna design. Our concept exploits the high reflectivity of a Bragg reflector composed of three bilayers made of transparent materials. The cavity has a triangular aperture in order to resemble the idea of a horn-like, highly directive antenna. Importantly, we report gain enhancements of more than 400% in relation to the gain of the antenna without the Bragg structure, accompanied by a highly directive radiation pattern. The proposed structure is cost-effective and easy to fabricate with 3D-printing. Our results are presented for frequencies within the conventional WiFi frequencies, based on IEEE 802.11 standards, thus, enabling easily implementation by non-experts and needing only to be placed around the antenna to improve the directivity and gain of the signal.

his paper presents a frequency-selective metasurface that provides strong linear-polarization selectivity through the suppression of the Cartesian electric and toroidal dipole moments, with weak toroidal–electric dipole interference further enhancing the transparency response. The structure is a two-dimensional periodic array of Tai-Chi-shaped metallic unit cells that exhibits a transparency frequency under normal incidence. Using a multipole-based analytical model together with full-wave simulations and microwave measurements, we show that this transparency frequency is characterized by a weakened radiating total electric-dipole contribution, yielding near-unity transmission. Measurements in both transmission and reflection geometries confirm the predicted polarization-dependent spectra, and additional measurements up to 45° incidence demonstrate that the transparent-metasurface response remains largely angle-robust. Bistatic radar cross-section measurements at the corresponding frequency (4.75 GHz) further indicate low backscattering around the backward direction.Waveform integrity was evaluated in 256-QAM experiments which show that the error vector magnitude (EVM) remains below 1% for both polarizations (0.96% for TE and 0.81% for TM) for the transparency frequency, while polarization-dependent EVM degradation was observed at the transmission minima (e.g., 0.81%!3.58% at 3.4 GHz and 1.06%!11.74% at 9.2 GHz). The metasurface produced here therefore represents a compact platform for polarization-selective RF filtering and transparent operation at resonance, with straightforward scalability to millimeter-wave and terahertz bands via geometric scaling and re-optimization.his paper presents a frequency-selective metasurface that provides strong linear-polarization selectivity through the suppression of the Cartesian electric and toroidal dipole moments, with weak toroidal–electric dipole interference further enhancing the transparency response. The structure is a two-dimensional periodic array of Tai-Chi-shaped metallic unit cells that exhibits a transparency frequency under normal incidence. Using a multipole-based analytical model together with full-wave simulations and microwave measurements, we show that this transparency frequency is characterized by a weakened radiating total electric-dipole contribution, yielding near-unity transmission. Measurements in both transmission and reflection geometries confirm the predicted polarization-dependent spectra, and additional measurements up to 45° incidence demonstrate that the transparent-metasurface response remains largely angle-robust. Bistatic radar cross-section measurements at the corresponding frequency (4.75 GHz) further indicate low backscattering around the backward direction.Waveform integrity was evaluated in 256-QAM experiments which show that the error vector magnitude (EVM) remains below 1% for both polarizations (0.96% for TE and 0.81% for TM) for the transparency frequency, while polarization-dependent EVM degradation was observed at the transmission minima (e.g., 0.81%!3.58% at 3.4 GHz and 1.06%!11.74% at 9.2 GHz). The metasurface produced here therefore represents a compact platform for polarization-selective RF filtering and transparent operation at resonance, with straightforward scalability to millimeter-wave and terahertz bands via geometric scaling and re-optimization.T