Phase Shifter for Millimeter-Wave Frequency Range Based on Glide Symmetric Structures

The use of glide symmetry in radiofrequency devices to introduce dispersive effects has been recently proposed and demonstrated. One of these effects is to control the propagation constant of the structure. Here, we propose a mm-wave phase shifter whose elements have a glide-symmetric configuration to achieve a greater phase shift in the same waveguide space than the non-glide-symmetric case. The glide-symmetric phase shifter is implemented in waveguide technology and is formed by rows of metallic pins that produce the desired phase shift. To assess the better performance of the glide-symmetric phase shifter, it is compared to its non-glide-symmetric version whose metallic pins are located only in one of the broad sides of the waveguide. The operating frequency range of the phase shifter is 67 to 75 GHz. Results show a 180 degree phase shift in regard to the reference waveguide without pins, and 50 degrees more than the non-glide-symmetric version.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Analysis of Metallic Space-Time Gratings using Lorentz Transformations

This paper presents an analytical framework for the study of scattering and diffraction phenomena in spacetime-modulated metallic gratings. Using a Lorentz transformation, it is shown that a particular class of spacetime-modulated gratings behave effectively as moving media. We take advantage of this property to derive a closed analytical solution for the wave scattering problem. In particular, using our formalism it is possible to avoid spacetime Floquet-Bloch expansions, as the solution of the problem in the original laboratory frame (grating parameters are periodic in space and time) is directly linked to a co-moving frame where the metallic grating is time-invariant (grating parameters are periodic only in space). In this way, we identify a fundamental connection between moving metallic gratings and spacetime-modulated metamaterials, and exploit this link to study the nonreciprocal response of the structure. Some limitations and difficulties of the alternative nonrelativistic Galilean approach are discussed and the benefits of the Lorentz approach are highlighted. Finally, some analytical results are presented in order to validate the formalism. The results include scenarios involving TM(p) and TE(s) normal and oblique incidence, even beyond the onset of the diffraction regime. Furthermore, we show how the synthetic Fresnel drag can tailor the Goos-H\"anchen effect and create a specular point shifted towards the direction of the synthetic motion, independent of the sign of the incidence angle.Comment: Published in Physical Review Applied, 20, 014063, 202

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

This work was funded in part by the Predoctoral Grant FPU18/01965 and in part by the financial support of BBVA Foundation through a project belonging to the 2021 Leonardo Grants for Researchers and Cultural Creators, BBVA Foundation. The BBVA Foundation accepts no responsibility for the opinions, statements, and contents included in the project and/or the results thereof, which are entirely the responsibility of the authors.Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques.Predoctoral Grant FPU18/01965BBVA Foundatio

Joint Direction-of-Arrival and Time-of-Arrival Estimation with Ultra-wideband Elliptical Arrays

This paper presents a general technique for the joint Direction-of-Arrival (DoA) and Time-of-Arrival (ToA) estimation in multipath environments. The proposed ultra-wideband technique is based on phase-mode expansions and the use of nearly frequency-invariant elliptical arrays. New possibilities open with the present approach, as not only elliptical, but also circular and linear (highly flattened) arrays can be considered with the same implementation. Systematic selection/rejection of signals-of-interest/signals-not-of-interest in smart wireless environments is possible, unlike with previous approaches based on circular arrays. Concentric elliptical arrays of many sizes and eccentricities can be jointly considered, with the subsequent improvement that entails in DoA and ToA detection. This leads to the realization of pseudo-random array patterns; namely, quasi-arbitrary geometries created from the superposition of multiple elliptical arrays. Some simulation and experimental tests (measurements in an anechoic chamber) are carried out for several frequency bands to check the correct performance of the method. The method is proven to give accurate estimations in all tested scenarios, and to be robust against noise and position uncertainty in sensor placement.Comment: Published in IEEE Transactions on Wireless Communication

Gain-Reconfigurable Hybrid Metal-Graphene Printed Yagi Antenna for Energy Harvesting Applications

This paper presents a hybrid metal-graphene printed Yagi antenna with reconfigurable gain that operates in the 5.5-GHz band. The balun and the driven elements are made of copper, while the directors are made of graphene. The graphene acts as a tunable material in the design. By switching the conductivity of the graphene, it is achieved a similar effect to adding or subtracting directors in the antenna. Hence the gain of the printed Yagi can be easily controlled. This could be of special interest in RF energy harvesting in the design of reconfigurable harvesting elements.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Artificial Intelligence and Dimensionality Reduction: Tools for Approaching Future Communications

ACKNOWLEDGMENT The authors would like to thank the Fraunhofer-Heinrich- Hertz-Institut for acquiring and sharing the data associated to the rooftop and auditorium communication scenarios, the NextG Channel Model Alliance for creating a space to share public databases of propagation measurements, José Francisco Cortés-Gómez for the graphical support, Carmelo García-García for his help in the measurements acquisition, and Sohrab Vafa, Pablo Padilla and Francisco Luna-Valero for their valuable comments.This article presents a novel application of the t-distributed Stochastic Neighbor Embedding (t-SNE) clustering algorithm to the telecommunication field. t-SNE is a dimensionality reduction algorithm that allows the visualization of large dataset into a 2D plot. We present the applicability of this algorithm in a communication channel dataset formed by several scenarios (anechoic, reverberation, indoor and outdoor), and by using six channel features. Applying this artificial intelligence (AI) technique, we are able to separate different environments into several clusters allowing a clear visualization of the scenarios. Throughout the article, it is proved that t-SNE has the ability to cluster into several subclasses, obtaining internal classifications within the scenarios themselves. t-SNE comparison with different dimensionality reduction techniques (PCA, Isomap) is also provided throughout the paper. Furthermore, post-processing techniques are used to modify communication scenarios, recreating a real communication scenario from measurements acquired in an anechoic chamber. The dimensionality reduction and classification by using t-SNE and Variational AutoEncoders show good performance distinguishing between the recreation and the real communication scenario. The combination of these two techniques opens up the possibility for new scenario recreations for future mobile communications. This work shows the potential of AI as a powerful tool for clustering, classification and generation of new 5G propagation scenarios.Spanish Program of Research, Development, and Innovation under Project RTI2018-102002-A-I00Junta de Andalucía under Project B-TIC-402-UGR18 and Project P18.RT.4830Ministerio de Universidades, Gobierno de España under Predoctoral Grant FPU19/0125

Time-periodic Metallic Metamaterials defined by Floquet Circuits

In this paper, we study the scattering and diffraction phenomena in time-modulated metamaterials of metallic nature by means of Floquet equivalent circuits. Concretely, we focus on a time-periodic screen that alternates between "metal" and "air" states. We generalize our previous approaches by introducing the concepts of "macroperiod" and "duty cycle" to the time modulation. This allows to analyze time-periodic metallic metamaterials whose modulation ratios are, in general, rational numbers. Furthermore, with the introduction of the duty cycle, perfect temporal symmetry is broken within the time modulation as the time screen could remain a different amount of time in metal and air states. Previous statements lead to an enrichment of the diffraction phenomenon and to new degrees of freedom that can be exploited in engineering to control the reflection and transmission of electromagnetic waves. Finally, we present some analytical results that are validated with a self-implemented finite-difference time-domain (FDTD) approach. Results show that the scattering level and diffraction angles can be controlled independently by means of the duty cycle and the modulation ratio, respectively. Thus, novel time-based pulsed sources and beamformers can be efficiently designe

Diffraction Phenomena in Time-Varying Metal-Based Metasurfaces

This paper presents an analytical framework for the analysis of time-varying metal-based metamaterials. Specifically, we particularize the study to time-modulated metal-air interfaces embedded between two different semi-infinite media that are illuminated by monochromatic plane waves of frequency ω0. The formulation is based on a Floquet-Bloch modal expansion, which takes into account the time periodicity of the structure (Ts = 2π/ωs) and integral-equation techniques. It allows us to extract the reflection and transmission coefficients as well as to derive nontrivial features about the dynamic response and dispersion curves of time-modulated metal-based screens. In addition, the proposed formulation has an associated analytical equivalent circuit that gives a physical insight into the diffraction phenomenon. Similarities and differences between space- and time-modulated metamaterials are discussed via the proposed circuit model. Finally, some analytical results are presented to validate the present framework. Good agreement is observed with numerical computations provided by a self-implemented finite-difference time-domain (FDTD) method. Interestingly, the present results suggest that time-modulated metal-based screens can be used as pulsed sources (when ωs ω0), beam formers (ωs ∼ ω0) to redirect energy in specific regions of space, and analog samplers (ωs ω0).Spanish Government ID2020-112545RB-C54 RTI2018-102002-A-I00Junta de Andalucia B-TIC-402-UGR18 A-TIC-608- UGR20 PYC20-RE-012-UGR P18.RT.4830Leonardo Grant of the BBVA FoundationBBVA Foundatio

Joint Ultra-wideband Characterization of Azimuth, Elevation and Time of Arrival with Toric Arrays

In this paper, we present an analytical framework for the joint characterization of the 3D direction of arrival (DoA), i.e., azimuth and elevation components, and time of arrival (ToA) in multipath environments. The analytical framework is based on the use of nearly frequency-invariant beamformers (FIB) formed by toric arrays. The frequency response of the toric array is expanded as a series of phase modes, which leads to azimuth-time and elevation-time diagrams from which the 3D DoA and the ToA of the incoming waves can be extracted over a wide bandwidth. Firstly, we discuss some practical considerations, advantages and limitations of using the analytical method. Subsequently, we perform a parametric study to analyze the influence of the method parameters on the quality of the estimation. The method is tested in single-path and multipath mm-wave environments over a large bandwidth. The results show that the proposed method improves the quality of the estimation, i.e., decreases the level of the artifacts, compared to other state-of-art FIB approaches based on the use of single/concentric circular and elliptical arrays.Comment: 10 pages, 8 figure