arXiv subjects

Juan F. Valenzuela-Valdés

Publications and source records attributed to Juan F. Valenzuela-Valdés.

13 recordsLinked to original sources

arXiv2026

Time-Controlled Resonances in 2-D Metasurfaces via Equivalent Circuits

This work introduces a semi-analytical frequency-domain framework for the analysis of two-dimensional, time-modulated (2+1)-D metasurfaces controlled by PIN diodes. The formulation focuses on the unit-cell level, modeled as a waveguide discontinuity problem, where the space-time periodicity of the structure enables the representation of scattered fields via Floquet expansions. After appropriate mathematical treatment, these expansions lead to an equivalent circuit description of the metasurface, providing physical insight into its spatiotemporal scattering behavior and facilitating the design of reconfigurable electromagnetic devices. The model is employed to explore key phenomena present in space-time systems, such as frequency mixing and spatiotemporal scattering. In addition, dynamic tuning is explored in resonant metasurfaces, where time becomes an additional degree of freedom for the design. The dynamic control of resonances opens a new way to explore multi-band and wideband behaviors from very thin metasurfaces under temporal coupling.

physics.app-ph↗

arXiv2026

Enabling 6G Wireless Communications: UWB Characterization of Corridors within the H-Band

Future sixth-generation of wireless system is expected to provide data-rates in the order of 1 Tbps and latencies below 1 ms. Among others, one of the most promising strategies to meet these requirements is to operate at higher frequencies than millimeter wave bands: the THz bands. Nevertheless, because of the higher losses and the detriment of classical propagation mechanisms, deploying systems operating at these frequencies becomes a real challenge. Consequently, short-range scenarios are of special interest since these effects of THz bands can be managed. This work conducts an extensive campaign within corridors at frequencies within the H-band in the range from 250 GHz to 330 GHz. For the first time in literature, an ultra wideband of 80 GHz is studied simultaneously. Large scale effects are assessed by estimating and modeling path gain. The path gain exponent varies between -2.1 and -1.6, which is explained by a guiding effect also observed at millimeter wave bands. Small scale effects are also evaluated in terms of parameters such as rice $K$-factor, root mean squared delay spread and coherence bandwidth. Additionally, an analytical approximation based on the classical N-rays model is proposed obtaining an accurate representation of the wireless channel which is coherent with the empirical analysis. The full analysis reveals the suitability of these THz bands for deploying point-to-point links due to the predominance of the line-of-sight contribution respect to the reflected components.

eess.SP↗

arXiv2024

Empirical Validation of a Class of Ray-Based Fading Models

As new wireless standards are developed, the use of higher operation frequencies comes in hand with new use cases and propagation effects that differ from the well-established state of the art. Numerous stochastic fading models have recently emerged under the umbrella of generalized fading conditions to provide a fine-grain characterization of propagation channels in the mmWave and sub-THz bands. For the first time in literature, this work carries out an experimental validation of a class of such ray-based models in a wide range of propagation conditions (anechoic, reverberation and indoor scenarios) at mmWave bands. These models allow to characterize the communication channel with a reduced number of physically interpretable parameters. In specific, we show that the independent fluctuating two-ray (IFTR) model has good capabilities to recreate rather dissimilar environments with high accuracy and only four parameters. We also put forth that the key limitations of the IFTR model arise in the presence of reduced diffuse propagation, and also due to a limited phase variability for the dominant specular components.

eess.SP↗

arXiv2024

Observations on Large-Scale Attenuation Effects in a 26 GHz Urban Micro-Cell Environment

This letter presents a measurement campaign carried out in an FR2 urban outdoor environment in a live experimental network deployment. The radio propagation analysis from a physical perspective at 26 GHz is essential for the correct deployment and dimensioning of future communication networks. This study performs a walk test emulating realistic conditions under which a pedestrian may be affected, summarizing and evaluating some of the typical effects encountered in a communications scenario such as penetration losses in a building, losses due to vegetation or the human body, or diffraction/scattering propagation around corners in street canyon-like environments. The operational conditions of the 5G network, the urban micro-cell scenario, and the use of omnidirectional antennas on the UE side validate the channel conditions from a perspective closer to a realistic scenario for a pedestrian within a FR2 live network.

eess.SP↗

arXiv2024

Space-time Metallic Metasurfaces for Frequency Conversion and Beamforming

This paper details a class of metal-based space-time metasurfaces for application in wireless communications scenarios. Concretely, we describe space-time metasurfaces that periodically alternate their properties in time between three spatial states: "air", "conductor" and "grating". We analyze the physics of these metastructures via a computationally-efficient analytical technique based on the use of Floquet-Bloch series, integral equations and circuit models. By doing so, we reveal important features of these spatiotemporal metasurfaces: scattering parameters, field profiles, diffraction angles and nature of the space-time harmonics. The results, corroborated with a self-implemented numerical FDTD approach, show the potential application of these space-time metasurfaces as beamformers acting in reflection, in transmission or both. The amplitude and direction of the diffracted orders can be electronically controlled with the paramaters of the metasurface. Moreover, the intrinsic ability of time-modulated diffractive metasurfaces to mix and multiply frequencies is tested. We show how two different modulations can lead to the same diffraction angle but with different mixed output frequencies.

physics.app-ph↗

arXiv2024

FR2 5G Networks for Industrial Scenarios: Experimental Characterization and Beam Management Procedures in Operational Conditions

Industrial environments constitute a challenge in terms of radio propagation due to the presence of machinery and the mobility of the different agents, especially at mmWave bands. This paper presents an experimental evaluation of a FR2 5G network deployed in an operational factory scenario at 26 GHz. The experimental characterization, performed with autonomous mobile robots that self-navigate the industrial lab, leads to the analysis of the received power along the factory and the evaluation of reference path gain models. The proposed assessment deeply analyzes the physical layer of the communication network under operational conditions. Thus, two different network configurations are assessed by measuring the power received in the entire factory, providing a comparison between deployments. Additionally, beam management procedures, such as beam recovery, beam sweeping or beam switching, are analyzed since they are crucial in environments where mobile agents are involved. They aim for a zero interruption approach based on reliable communications. The results analysis shows that beam recovery procedures can perform a beam switching to an alternative serving beam with power losses of less than 1.6 dB on average. Beam sweeping analysis demonstrates the prevalence of the direct component in Line-of-Sight conditions despite the strong scattering component and large-scale fading in the environment.

eess.SP↗

arXiv2024

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.

eess.SP↗

arXiv2023

Analytical Equivalent Circuits for Three-dimensional Metamaterials and Metagratings

In recent times, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison to their 1D and 2D counterparts due to the intricate geometries involved. In this paper, we present a fully-analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared to full-wave solutions and also works under oblique incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in commercial software CST. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarizations states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated.

physics.app-ph↗

arXiv2023

Analytical Framework to Model Reconfigurable Metasurfaces including Lumped Elements

This paper presents an analytical framework, based on Floquet modal expansions of the electromagnetic fields and equivalent circuits, to model reconfigurable metasurfaces loaded with generic lumped elements (resistors, capacitors, inductors, varactors, etc.). The analytical approach is computationally efficient compared to full-wave solvers. Additionally, it works under oblique-incidence conditions in a wideband range of frequencies, even far beyond the onset of the first grating lobe (diffraction regime). The analytical framework is validated with some numerical examples in the commercial software CST Studio Suite, demonstrating its potential for analyzing and designing RF and microwave devices, including lumped elements, such as absorbers, polarizers, and reflectarray/transmitarray cells.

physics.app-ph↗

arXiv2023

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.

eess.SP↗

arXiv2022

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 designed

physics.app-ph↗

arXiv2022

Artificial Intelligence and Dimensionality Reduction: Tools for approaching future communications

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 (DR) 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 (VAE) 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.

eess.SP↗

arXiv2021

Dispersion and Filtering Properties of Rectangular Waveguides Loaded With Holey Structures

This paper analyzes thoroughly the dispersion and filtering features of periodic holey waveguides in the millimeter-wave frequency range. Two structures are mainly studied depending on the glide and mirror symmetries of the holes. A parametric study of the dispersion characteristics of their unit cells is carried out. Glide-symmetric holey waveguides provide a higher propagation constant and a low dispersion over a wide frequency range regarding hollow waveguides. This property is particularly useful for the design of low-loss and low-dispersive phase shifters. We also demonstrate that glide-symmetric holey waveguides are less dispersive than waveguides loaded with glide-symmetric pins. Furthermore, we perform a Bloch analysis to compute the attenuation constants in holey waveguides with mirror and broken glide symmetries. Both configurations are demonstrated to be suitable for filter design. Finally, the simulation results are validated with two prototypes in gap-waveguide technology. The first one is a 180$^{o}$ phase shifter based on a glide-symmetric holey configuration that achieves a flat phase shift response over a wide frequency range (27.5\% frequency bandwidth). The second one is a filter based on a mirror-symmetric holey structure with 20-dB rejection from 63 GHz to 75 GHz.

physics.app-ph↗