Multicarrier modulation allows for deploying wideband systems resilient to multipath fading channels, impulsive noise, and intersymbol interference compared to single-carrier systems. Despite this, multicarrier signals suffer from different types of distortion, including channel noise sources and long- and short-term fading. Consequently, the receiver must estimate the channel features and compensate it for data recovery based on channel estimation techniques, such as non-blind, blind, and semi-blind approaches. These techniques are model-based and designed with accurate mathematical channel models encompassing their features. Nevertheless, complex environments challenge accurate mathematical channel estimation modeling, which might neither be accurate nor correspond to reality. This impairment decreases the system performance due to the channel estimation accuracy loss. Fortunately, (AI) algorithms can learn the relationship among different system variables using a model-driven or model-free approach. Thereby, AI algorithms are used for channel estimation by exploiting its complexity without unrealistic assumptions, following a better performance than conventional techniques under the same channel. Hence, this paper comprehensively surveys AI-based channel estimation for multicarrier systems. First, we provide essential background on conventional channel estimation techniques in the context of multicarrier systems. Second, the AI-aided channel estimation strategies are investigated using the following approaches: classical learning, neural networks, and reinforcement learning. Lastly, we discuss current challenges and point out future research directions based on recent findings.

This work reports the development of a low-profile and high-gain slotted waveguide antenna array, aiming the FR2 5G frequency range and operating in the 26 GHz band. The proposed array relies on 27-sloped slots, positioned onto the WR28 standard waveguide broadside wall with six pairs of metal grooved-structures placed parallel to its axis. Experimental results demonstrate the achievement of 3.41% fractional bandwidth from 25.88 to 26.78 GHz, 27.7 dBi gain and half-power beamwidths in the azimuth and elevation planes equal to 10 and 3, respectively. The use of metal-grooved structure realizes a gain enhancement of 9-dB, and results in a high-gain antenna array which is based on a very simple and unique excitation.

This paper presents a comparison of two gain-enhancement techniques and the development of an extremely high-gain slotted waveguide antenna array (SWAA) aiming at applications in the millimeter-waves (mm-waves). The first technique utilizes pairs of grooved structures surrounding a given primary power source to improve its transmission properties, acting as secondary sources to re-radiate the surface wave energy, whereas the second is based on symmetrical wing-based reflectors. Those techniques and their combination have been applied to an SWAA based on an air-filled metallic rectangular waveguide. The proposed antenna array was designed for operation in the frequency range from 25.5 to 26.5 GHz and excited by a unique coaxial-to-waveguide transition with the TE ₁₀ fundamental mode. These techniques enable us to enhance the gain of the array by up to 10 dB, making it comparable to those of parabolic antennas with similar size apertures. Numerical analyses of the performance-enhanced antenna arrays are presented to validate the design strategies. Experimental results of 27- and 41-slots SWAA prototypes demonstrate the potential of the wing-based reflector technique to improve its aperture efficiency over that of the grooved structures. The 41-slots SWAA prototype provides as high as 31 dBi-gain in the 26 GHz band, ranging from 30.52 to 31.13 dBi within the array impedance bandwidth. Furthermore, since the reflector structures are easily attached to and detached from the SWAA structure, the prototype has the potential to provide both sectoral and directive radiation patterns in the $E$ -plane.

The deployment of fifth-generation mobile network (5G), beyond 5G and sixth-generation mobile network platforms encounters challenges of blockage, interference, and path loss in radio mobile environments. Metasurfaces provide a promising solution to address these limitations. In this paper, we present a methodology for developing ultrathin flexible metasurface-based frequency selective surfaces (FSSs). Our approach combines thermal evaporation for metallic thin films with a macroscopic metasurface mask (something analogous to screen-printing but using thermal evaporation instead of inks). As a proof of concept, we fabricate a sub-6 GHz metasurface-based FSS using gold deposition on a flexible polyethylene terephthalate substrate. Experimental results are validated through numerical full-wave simulations using COMSOL Multiphysics and equivalent-circuit model simulations. The metasurface operates within the primary frequency band utilized in 5G networks (3–5 GHz), indicating its potential applicability across a wide range of flexible, conformal, and wearable devices. The fabricated FSS can be installed on surfaces of any shape, such as flat or curved windows, as well as on walls or other external surfaces. This methodology offers practical solutions for wireless communications and enhancing signal transmission in diverse environments.

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.