This article describes the design, fabrication, and testing of a reconfigurable intelligent surface (RIS) operating within the millimeter-wave (mmWave) spectrum for 5G communication. Numerical simulations detail the operation of a 1-bit unit cell controlled by a p-i-n diode and biased with RF chokes within the n257-5G FR2 band (26.5-29.5 GHz) at the optimized angle of incidence of +/- 30 degrees. For designing the RIS, a theoretical model was evaluated to calculate the free-space path loss of an RIS-assisted wireless link, helping to select the most cost-effective RIS aperture area. The RIS design is based on a planar dipole reflector, interlaced to form a checkerboard lattice, supporting dual-polarization operation. A multilayer RF and electronics printed circuit board (PCB) is fabricated to evaluate design performance at both the unit cell and finite RIS array levels. Experiments are conducted to verify that the RIS performance aligns with analytical and simulated results. Finally, the RIS is tested in real-world scenarios, demonstrating its capabilities in reflected signal strength. Practical RIS performance makes it a promising candidate for reliable mmWave wireless communication systems.

This paper investigates machine learning (ML)assisted user localization in reconfigurable intelligent surface (RIS)-aided millimeter-wave (mmWave) systems using minimalbeam probing. A system model with three RIS apertures (10×10, $20 \times 20$, and $30 \times 30$) is considered, where the surface sequentially applies a limited set of probing beams and the user equipment (UE) records the corresponding received power. Using ML regression, we train the regressor models to predict the UE positions from these measurements and update the RIS phase distribution for efficient beam forming towards the intended UE. Simulations show that small RIS arrays achieve accurate predictions with limited probing, whereas larger apertures need deeper sweeps to curb outliers. The best approach attains mean errors below 1.5 dB for a 10 × 10 aperture and improves further on larger apertures with six probing beams. This underscores a trade-off among RIS size, probing overhead, and ML method choice for efficient RIS-aided localization in future sixth-generation (6G) wireless networks.

Accurate user equipment (UE) localization is critical for beam management in reconfigurable intelligent surface (RIS)-assisted millimeter-wave (mmWave) based sixth-generation (6G) networks, especially if the direct base-station-UE links are unavailable. This paper proposes a beam-domain fingerprint framework that maps the received signal-to-noise ratio (SNR) across a small set of predefined RIS reflection states to the UE azimuth angle and range, without requiring channel state information (CSI). Crucially, we extend the framework to a realistic interference-impaired scenario in which a nearby cross-link interferer (CLI) corrupts the clean SNR fingerprint, yielding a signal-to-interference-plus-noise ratio (SINR) fingerprint; an interference-to-noise ratio (INR)-constrained calibration strategy keeps the interference level physically interpretable. Four machine-learning (ML) regressors are evaluated under both conditions. Simulation results at 28 GHz with a 20x20 RIS show that k-nearest neighbors (KNN) achieves the lowest angle MAE of 0.37 degrees and range MAE of 4 cm under clean conditions, rising to 1.4 degrees and 7.6 cm under interference. A key finding is that interference degrades angle estimation substantially more than range estimation across all models, a consequence of the asymmetric encoding of location information in the beam-domain fingerprint.

Reconfigurable Intelligent Surfaces (RIS) use two dimensional periodic elements with electronically controllable reflection/transmission phase to redirect incident electromagnetic (EM) waves into a desired direction. This ability of RIS finds applications in wireless communication, in particular non-line of-sight (NLOS) scenarios, where the line-of-sight (LOS) path between transmitter and receiver is obstructed. In practical scenarios the transmitter or the receiver can be located at any angle with respect to the RIS, including high oblique angles. In this paper, the RIS is designed to handle a wide range of incidence angles (±30°) and reflection angles (±40°). Additionally, the design is polarization insensitive, i.e. the performance of the RIS stays consistent with the polarization of the incident EM wave. A p-i-n diode based one-bit reconfigurable unit cell is designed to operate in the 3.6-3.8 GHz frequency band. RIS panels of 8 × 6 unit cells are fabricated with integrated biasing control mechanism using shift registers and a microcontroller. The RIS panels are then assembled in a 2×2 configuration and the beamforming capabilities are experimentally verified.

Among the candidate spectrum bands for sixthgeneration (6 G) networks, millimeter-wave (mmWave) frequencies are widely considered for high-capacity links, yet exhaustive beam sweeping introduces significant overhead in reconfigurable intelligent surface (RIS)-assisted systems. This paper proposes a lightweight machine learning (ML) regression framework that directly maps received power measurements from a small set of sector-aligned probing beams to the user equipment (UE) elevation angle, enabling RIS phase optimization without explicit channel state information (CSI). Four classical regressors-Knearest neighbors (KNN), decision tree (DT), random forest (RF), and support vector regressor (SVR)-are evaluated under an angle-stratified protocol across different beam configurations. KNN consistently achieves the best performance, attaining a mean absolute error (MAE) of 0.79° and $R^{2}=0.99$ with six probing beams, while reducing beam-probing overhead by over 93 % compared to exhaustive search. The predicted angles are used to construct 1-bit RIS phase matrices, yielding radiation patterns within $1^{\circ}-2^{\circ}$ of the ground-truth direction, confirming that accurate angular localization translates directly into effective beamforming.

This paper summarises the design and channel measurements of a wide band Reconfigurable Intelligent Surface RIS operating within the millimeter wave spectrum for 5G and beyond communication. RIS elements are controlled by a p-i-n diode and biased via RF chokes operating within the n257 millimeter wave 5G band. The RIS operates in both vertical and horizontal polarizations. Experiments have been performed to validate the performance of the RIS in both near-field and far-field regions. It has been experimentally demonstrated that the proposed RIS can reflect incident beams towards desired locations with channel gain enhancement of up to 16.3 dB when appropriate phase profiles are applied.

This article presents an in-depth investigation into the design and experimental validation of an antenna array for a metallic casing handset device, optimized for millimeter-wave (mmWave) 5G frequencies, specifically within the 3GPP n257 band (26.5-29.5 GHz). It outlines the transformation of a single antenna element into a 4 x 1 linear array configuration. Deploying three such arrays within the mobile device, the study evaluates their performance across various scenarios mimicking real-world user interactions, with a focus on signal blockage due to hand placement. The study demonstrates that the optimized antenna locations achieve considerable gain coverage, further enhanced through the implementation of beam steering, aimed at mitigating signal blockage effects. This article also explores the diminishing returns of increasing the phase shifter resolution. Experimental results, supported by extensive simulations and far-field measurements, validate the antenna array's efficacy in providing quasi-omnidirectional radiation patterns and robust performance in the face of user-induced blockages. The practical mobile device antenna array results are used for a comprehensive analysis in the form of the cumulative distribution function (cdf) performance of the mobile device. Spectral efficiency (SE) is also assessed for direct line-of-sight (LOS) scenarios, and also in reflective intelligent surface (RIS) assisted wireless environments in which LOS is not available, which showcases the practicality of the device in B5G/6G applications. The set of conclusions provides valuable insights for next-generation mobile communication system design and deployment.

This study examines the importance of the extreme reflection angles in terms of the spectral efficiency in a Reconfigurable Intelligent Surfaces (RIS) - assisted wireless communication system. The investigation considers practical RIS unit cells with a non-ideal reflection response, which is typical, especially when the extreme angles of incidence, as well as reflection, are considered. A significant disparity was found in spectral efficiency when RIS is employed to reflect signals at extreme angles, with a notable 150% - 200% increase in the required number of unit cells to maintain the same spectral efficiency compared to scenarios with normal reflection angles. This observation underscores the importance of considering reflection angles in the design and optimization of RIS hardware. The results contribute valuable insights to the understanding of RIS behaviour in practical scenarios, paving the way for enhanced spectral efficiency and improved network performance.

As wireless communication systems advance toward 6th Generation (6G) networks, Cell-Free Massive MIMO (Multiple-Input Multiple-Output) has emerged as a promising network architecture for enhancing network performance, spectral efficiency, and user coverage. The existing Cell-Free Massive MIMO systems rely on a broad deployment of distributed Access Points (APs), each equipped with multiple antenna elements. However, when user density is low, resource allocation becomes less efficient, leading to suboptimal spectrum utilization. Moreover, future 6G AP architectures are expected to feature fewer antenna elements due to energy and design limitations. This paper explores the feasibility of replacing antenna elements in Cell-Free Massive MIMO APs with an alternative hardware structure, namely a continuous electromagnetic aperture. Specifically, this work investigates an electrically large chaotic cavity as a resonant electromagnetic structure capable of replacing hundreds of antenna elements with a single RF chain-based system, simplifying hardware complexity. This study examines potential trade-offs, performance implications, and integration of the proposed hardware into emerging 6G Integrated Sensing and Communication (ISAC) capabilities. The findings offer insights into how hardware innovations can shape the deployment of Cell-Free Massive MIMO in next-generation wireless networks.

This paper presents the design of a novel millimeterwave reflective unit cell for integration in reconfigurable intelligent surfaces (RIS). The proposed unit cell exhibits excellent reflection characteristics and angular stability throughout the 24 - 30 GHz frequency range, making it an excellent candidate for utilization in wide bandwidth current mmWave 5G and future 6G systems. The proposed unit has a total size of 3 x 2.2 mm(2). It comprises two diamond-shaped, out-of-phase patches connected via a p.i.n diode to enable reconfigurability and serve as a binary phase modulator. Full-wave simulations demonstrated an average phase difference error of only +/- 1.5 degrees at an angle of incidence of 30. off the RIS boresight throughout the 24 - 30 GHz band. The proposed unit cell also maintained good reflecting properties at angles of incidence of 40 degrees, 20 degrees, and 10 degrees. It reflects at least 77% of the energy and has an average phase difference of 180 degrees +/- 20 degrees between its two binary states. The performance of the proposed unit cell allows for minimal loss and beam steering errors when utilized in RIS development.

This article presents a 2D metasurface-augmented half-circle Maxwell fisheye lens (HMFL) designed to move its focal axis to a targeted direction with a maximum elevation angle of 45 degrees, suitable for switchers and power division networks in the millimetre wave (mmWave) band. This metasurface-augmented beamformer enables consistent analogue performance across a bandwidth of 22-32 GHz. The proposed beamformer features a reflective metasurface consisting of 2 x 40 Phoenix unit cells and is integrated with a 10-dielectric zone HMFL in the diameter segment opposite the antenna source. A horn antenna feed is used for excitation in normal and oblique incidences of phi( i) = - 15 degrees and phi (i) = - 30 degrees . The designed lens prototype was fabricated with 3D-printing technology and printed metasurfaces for several particular cases of focal displacement. The measurements of the proposed HMFL beamformer are presented, demonstrating a close correlation between simulated and experimental results.

This study presents a novel reflective unit-cell with wideband characteristics at millimeter-wave (mmWave) bands for application in Reconfigurable Intelligent Surfaces (RIS). The proposed unit-cell design demonstrates through full-wave simulations a superior bandwidth performance over the 26.50-29.45 GHz, targeting the n257 band of mmWave 5G. The design was created for dual-polarization operation, each controlled by a p-i-n diode to realise a 1-btt RIS. The design achieves impressive performance, maintaining a phase difference error within $\pm \mathbf{20}$ degrees across most of the 3 GHz bandwidth while reflecting over 80% of the energy. The increased reflectivity minimizes losses, while precise phase control improves beam pointing accuracy, crucial in low-complexity 1-bit systems. Numerical simulations also indicate that this unit-cell performs effectively in a 6-layer PCB stack-up. A comparison with state-of-the-art unit-cells for RIS design is also presented, demonstrating the advantages of the proposed design in terms of bandwidth, dual-polarization operation, and phase accuracy.

This paper reviews how recent advancements in sparse antenna arrays optimization help reduce mutual coupling, even in simple setups like 2 x 2 and 3 x 3 microstrip patch antennas Uniform Rectangular Array (URA). It discusses and compares techniques and widely used optimisation methods to enhance array analogue beamforming performance and minimise interference through side-lobe reduction. The study shows that careful positioning and use of optimisation algorithms can help not only maintain the array gain, but also reduce inter-element coupling, making it useful for very basic array configurations.

This paper investigates single-user uplink and two-user downlink channel estimation in reconfigurable intelligent surface (RIS)-aided millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) wireless communication systems. Because of the difficulty associated with the estimation of channels in RIS-aided wireless communication systems, channel state information (CSI) is assumed to be known at the receiver in some previous works in the literature. By assuming that prior knowledge of the line-of-sight (LoS) channel between the RIS and the base station (BS) is known, two compressive sensing-based channel estimation schemes that are based on simultaneous orthogonal matching pursuit and structured matching pursuit (StrMP) algorithms are proposed for estimation of uplink channel between RIS and user equipment (UE), and joint estimations of downlink channels between BS and a UE, and between RIS and another UE, respectively. The proposed channel estimation schemes exploit the inherent common sparsity shared by the angular domain mmWave channels at different subcarriers. The superiority of one of the proposed channel estimation techniques, the StrMP-based channel estimation technique, with negligibly higher computational complexity cost compared with other channel estimators, is documented through extensive computer simulation. Specifically, with a reduced pilot overhead, the proposed StrMP-based channel estimation scheme exhibits better performance than other channel estimation schemes considered in this paper for signal-to-noise ratio (SNR) between 0 dB and 5 dB upward at different instances for both uplink and downlink scenarios, respectively. However, below these values of SNR the proposed StrMP-based channel estimation scheme will require higher pilot overhead to perform optimally.

This paper presents a method for the design of a wideband two-dimensional (2D) metasurface Luneburg lens antenna optimized for millimeter-wave (mmWave) applications. The metasurface, with overall dimensions of 12 × 200 mm, operates in a reflective mode and consists of an 80-element array of Phoenix unit cells. We demonstrated that the 2D metasurface Luneburg lens antenna can steer the radiated beam with a wideband performance of 24-38 GHz. It is shown that the steering angle can be controlled to a desired direction within an angular range of 75°, contingent on the distribution of phasing elements and the position of the feeding source. In specific cases, we demonstrate beam-steering towards 0°, 30°, and 45°, achieved when the feeding source is oriented at −15 degrees as well as towards 0°, 15°, and 45° achieved when the feeding source is oriented at –30°.

The CASSIOPeiA Solar power satellite (SPS) helix antenna array concept has been shown to provide a viable concept with no mechanical rotational joints required between the photovoltaic cells and the antenna. The important feature of the CASSIOPeiA antenna is that it can provide 360 degrees azimuth electronic beam steering, pointing a highly directional beam, to a rectenna farm on earth. Modelling of the CASSIOPeiA antenna to date has only included setting up pre-defined phase/amplitude distributions across the array. These results are promising but to model the array for retrodirective operation, the phase distributions across the array need to be determined from phase conjugation of a received pilot tone. This mode of operation, to our knowledge, has never been theoretically validated within the triple dipole beam steering configuration of a large CASSIOPeiA array. In this paper we present the first simulated validation of a 10,000 unit cell (100 × 100) CASSIOPeiA array as a retrodirective antenna, providing a well-defined steerable main beam and good sidelobe suppression.

In this paper, single and cross helix array structures, for solar power satellite WPT applications, have been simulated, with full wave simulations up to 200x100 unit cell (H x W = 6m x 6m) array size. Near field simulations computed the WPT power densities at distances up to 500 m from the array. The results presented, are the first validation that a large cross helix structure can provide enhanced, 360 degree steerable, WPT beamforming capabilities compared to a similarly sized single helix. Previous work has only validated single helix structures. As an example, a 100x100 single helix array with 5W per element was shown to produce a WPT power density of 93 W/m 2 at 500m, whereas a 200x100 cross helix structure, with double the number of elements produced a power density of 315W/m 2 , which is an almost fourfold increase compared to the single helix. This clearly shows the increased aperture to be providing additional beamforming as well as the two times power increase from doubling the number of elements.

This paper investigates the incorporation of Reflective Intelligent Surfaces (RIS) into mm Wave wireless networks for enhancing user equipment (UE) performance. Specifically, it delves into the previously unexplored effect of user interaction with a steerable handheld mm-wave device on system bit rate performance in the presence of RIS, especially in non-line-of-sight conditions and when obstructed by a user's hand. The results emphasize the capacity of RIS to enhance both spectral efficiency and link reliability, contingent on the UE antenna array steering its beam towards the base stations or the RIS. Moreover, the investigation highlights the importance of practical UE for realistic network insights, crucial for effective optimization efforts.

This paper demonstrates a circular transmitarray-augmented Luneburg lens antenna for beam-forming at a millimeter-wave (mmWave) regime. The proposed structure is comprised of an augmentation of 80 dielectric cubes phase correction layer and two semi-circular 10-layer Luneburg lenses. The circular transmitarray-augmented Luneburg lens antenna is fed by a 9 dBi horn antenna at normal incidence and an oblique incidence of -15 degrees. The beam is successfully transmitted with an angle up to +45 degrees. When the feed is oriented in normal incidence, a pencil beam is transmitted at 166 degrees and 151 degrees with a realized gain of 20.3 dBi, and sidelobe level (SSL) below -10.0 dB, at 28 GHz.

A novel design of frequency-selective surface (FSS) generating a quasi-elliptic frequency response is proposed in the paper. Each unit cell of FSS comprises a centered rectangular aperture with three closely spaced equal-height posts alternatively installed in the aperture center along its broad walls. Numerical and measured data are presented for X-band FSS providing a narrowband transmission of linearly polarized wave. In particular, the measured bandwidth of 5% centered at 8.16 GHz with transmission zeros separation ratio of 1.3 is achieved for the 45-degree incidence.