Design of new radiating systems and phase shifters for 5G communications at millimeter-wave frequencies

With the arrival of the new generation of communications, known as 5G, the systems that constitute it must offer better performance in terms of data speed, latency and connection density than the previous generation of communications. For 5G, an allocation of the frequency ranges that will support future wireless communications has been established. This allocation is formed by a range of frequencies corresponding to bands below 6 GHz and the other range of frequencies includes bands above 24 GHz. In the latter frequency range, which includes part of the millimeter-wave frequency band (from 30 GHz to 300 GHz), the development of new radio frequency (RF) components is necessary because their design and manufacture is a technological challenge. As the frequency that supports wireless communications increases, propagation losses also increase. Therefore, these losses must be compensated by the radiating systems in 5G to make these communications possible. The RF devices that make up these new systems must provide high antenna gain, be power efficient and offer spatial reconfigurability of the radiated signal. In this thesis, the main objective is the design of both guided and radiating RF devices to provide design solutions for future 5G systems at millimeter-wave frequencies. In particular, the contributions made have been to the design of phase shifters and antenna arrays. To improve efficiency at millimeter-wave frequencies, these devices have been designed in waveguide technology. Phase shifters are essential RF devices to control the phase shift of the electromagnetic wave that will be radiated to a certain spatial direction by an antenna array. The design of beamforming networks requires the implementation of phase shifters that produce a fixed or variable phase shift value. However, the design and fabrication of these devices at millimeter-wave frequencies is a complex task. In this thesis, four designs of waveguide phase shifters that produce both fixed and variable phase shift are presented. For phase shifters that provide a fixed phase shift, the value of this phase shift along the frequency is tuned in a desired manner by using periodic structures with higher symmetries. These types of configurations provide both flexibility in the design process and improved electromagnetic performance such as greater operating bandwidth. All the phase shifters have been implemented in gap-waveguide technology to demonstrate its effectiveness in these devices for millimeter-wave frequencies. Regarding the radiating systems, two feeding strategies have been considered in the design process. First, the design of a 70 GHz centered antenna array implemented in gap-waveguide technology combined with the use of separate waveguides in E-plane is proposed. In this design, the feed is guided through a waveguide corporate-feed network. Second, the design of a reflectarray whose unit cells are formed using three-dimensional geometries is presented. In this case, the feeding is done in free space by radiation from a source antenna. In the previous designs, the fabrication of the prototypes was done by 3D printing based on stereolithography. Finally, using unit cells with three-dimensional geometries, the design of radiating devices with more complex functionalities such as reflection/transmission with high directivity and reconfiguration of the reflected radiation by means of graphene structures are proposed.Con la llegada de la nueva generación de comunicaciones, denominada 5G, los sistemas que la conforman deben ofrecer unas mejores prestaciones en términos de velocidad de datos, latencia y densidad de conexiones respecto a la generación de comunicaciones anterior. Para 5G se ha establecido una asignación de los rangos de frecuencia que van a soportar las futuras comunicaciones inalámbricas. Esta asignación se compone por un rango de frecuencias correspondiente a las bandas por debajo de los 6 GHz y el otro rango de frecuencias engloba a las bandas por encima de los 24 GHz. En este ´ultimo rango de frecuencias, en el cual están incluidas parte de la banda de las frecuencias milimétricas (desde 30 GHz a 300 GHz), es necesario el desarrollo de nuevos componentes de radiofrecuencia (RF) ya que su diseño y fabricación supone un reto tecnológico. Al aumentar la frecuencia que soporta las comunicaciones inalámbricas, las pérdidas por propagación también aumentan. Es por ello por lo que estas pérdidas deben ser compensadas por los sistemas radiantes en 5G para que las comunicaciones sean posibles. Los dispositivos de RF que componen estos nuevos sistemas deben proporcionar una alta ganancia de antena, ser eficientes en términos de potencia y ofrecer reconfigurabilidad espacial de la señal radiada. En esta tesis, el objetivo principal es el diseño de dispositivos de RF tanto guiados como radiantes para ofrecer soluciones de diseño a los futuros sistemas 5G en frecuencias milimétricas. De manera particular, las contribuciones realizadas han sido al diseño de desfasadores y agrupaciones de antenas. Para mejorar la eficiencia en frecuencias milimétricas, estos dispositivos han sido diseñados en tecnología en guía de ondas. Los desfasadores son dispositivos RF esenciales para controlar el desfase de la onda electromagnética que será radiada hacia una cierta dirección espacial por una agrupación de antenas. Las redes de beamforming tienen la necesidad de implementar en su diseño desfasadores que producen un valor de desfase fijo o variable. Sin embargo, el diseño y fabricación de estos dispositivos en frecuencias milimétricas resulta una tarea de alta dificultad. En esta tesis se presenta cuatro diseños de desfasadores en guía de onda que producen un desfase tanto fijo como variable. Para los desfasadores que proporcionan un desfase fijo, el valor de este desfase a lo largo de la frecuencia es ajustado de manera deseada mediante el uso de estructuras periódicas con simetrías superiores. Este tipo de configuraciones proporcionan tanto flexibilidad en el proceso de diseño como una mejora de las características electromagnéticas como puede ser un mayor ancho de banda de operación. Todos los desfasadores realizados han sido implementados en tecnología gap waveguide para demostrar su efectividad en estos dispositivos para frecuencias milimétricas. Respecto a los sistemas radiantes, se han considerado dos estrategias de alimentación en el proceso diseño. En primer lugar, se propone el diseño de un array centrado a 70 GHz implementado en tecnología gap waveguide combinado con el uso de guías de onda separadas en plano E. En este diseño, la alimentación es guiada a través de una red de alimentación corporativa en guía de onda. En segundo lugar, se presenta el diseño de un reflectarray cuyas celdas unitarias son formadas mediante geometrías tridimensionales. En este caso, la alimentación se hace en el espacio libre mediante la radiación de una antena fuente. En los anteriores diseños, la fabricación de los prototipos se realizó mediante impresión 3D basado en estereolitografía. Finalmente, a través del uso de celdas unitarias con geometrías tridimensionales, se proponen el diseño de dispositivos radiantes con funcionalidades más complejas como la reflexión/transmisión con alta directividad y la reconfiguración de la radiación reflejada mediante estructuras con grafeno.Tesis Univ. Granada

Ray-Tracing Model for Generalized Geodesic-Lens Multiple-Beam Antennas

Geodesic lenses are a compelling alternative to traditional planar dielectric lens antennas, as they are low loss and can be manufactured with a simple mechanical design. However, a general approach for the design and analysis of more advanced geodesic-lens antennas has been elusive, limiting the available tools to rotationally symmetric surfaces. In this article, we present a fast and efficient implementation built on geometrical optics and scalar diffraction theory. A numerical calculation of the shortest ray path (geodesic) using an open-source library helps quantify the phase of the electric field in the lens aperture, while the amplitude is evaluated by applying ray-tube power conservation theory. The Kirchhoff-Fresnel diffraction formula is then employed to compute the far field of the lens antenna. This approach is validated by comparing the radiation patterns of a transversely compressed geodesic Luneburg lens (elliptical base instead of circular) with the ones computed using commercial full-wave simulators, demonstrating a substantial reduction in computational resources. The proposed method is then used in combination with an optimization procedure to study possible compact alternatives of the geodesic Luneburg lens with size reduction in both the transverse and vertical directions

V-Band Fully Metallic Geodesic Luneburg Lens Antenna

Antennas in emerging millimeter-wave (mm-wave) appli- cations are often required to have low losses and produce a steerable directive beam. These properties are achievable with fully metallic geodesic Luneburg lens antennas. In this communication, we report the first experimental verification of a geodesic Luneburg lens antenna in the V-band. The designed lens antenna is fed with 13 waveguides providing beam switching capability in a 110◦ range. The lens is implemented in the parallel plate waveguide (PPW) technology. The antenna is manufactured in two pieces, and a tolerance analysis indicates that gaps between the pieces can cause a severe performance degradation. Based on this tolerance analysis, two measures are taken to alleviate the manufacturing tolerances for the prototype. First, electromagnetic band gap (EGB) structures are placed around the feeding waveguides. Second, the electrical contact between the two pieces is improved in critical regions. Two prototypes are manufactured, one without and one with the extra measures implemented. The measured radiation patterns of the prototype without these measures have high side lobes and low realized gain compared with the simulation. The measurements of the robust version of the prototype agree well with the simulations and demonstrate the applicability of geodesic Luneburg lens antennas for applications in the V-band.Strategic Innovation Program Smarter Electronics System under Project High-Int 2019-02103European Space Agency European Commission 4000125905/18/NLVR Project 2019-0393

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

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

A 1-to-8 Fully Modular Stacked SIW Antenna Array for Millimeter-Wave Applications

This paper presents a vertically stacked SIW antenna array that enables different array configurations with the minimum number of SIW layers. This achievement lies in the modular feature offered by the proposed design. Specifically, 4 distinct array configurations can be produced with only 3 different design of SIW layers. Depending on the number of SIW layers employed in the stacked antenna, the directivity in the E-plane is modified. To obtain an equal and in-phase power distribution among the array elements, H- and E-plane corporate feeding networks are efficiently implemented in each array configuration. Array configurations of 1, 2, 4 and 8 radiating layers are offered by the proposed modular array, where each radiating layer is formed by 8 H-plane horn antennas. The simulated directivity for the array configurations ranges from 15.8 dBi to 23.8 dBi and the main beam direction remains fixed along the operating frequency range. The array design has been manufactured and agreement between simulated and measured results are observed. The measured impedance bandwidth in all the array configurations is from 35 GHz to 41 GHz (15.79% bandwidth) with a reduction in the E-plane beamwidth as the number of radiating layers increases.Spanish GovernmentEuropean Commission PID2020-112545RB-C54 Junta de Andalucia B-TIC-402-UGR18 A-TIC-608-UGR20 P18.RT.4830 PYC20-RE-012-UGR FPU20/00256 FPU18/0196

Optimized Varactor Parasitic Modelling in the Millimeter-Wave Band.

This work explores the utilization of varactors in the millimeter-wave (mm-wave) band, specifically focusing on their application in voltage-controlled reconfigurable devices. Varactors, or variable capacitors, can adjust capacitance through voltage control, making them ideal for creating rapidly variable systems for radiofrequency (RF) applications. However, parasitic elements in varactors can significantly affect their performance when moving up to the millimeter-wave band and limit their efficiency. Therefore, this work aims to present an optimization approach that accurately calculates the parasitic model for these varactors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Holey SIW Horn Antenna Based on an H-plane Lens-wise Wavefront Collimation

This paper presents an H-plane SIW horn antenna whose directivity is enhanced using holey unit cells along the horn flaring. By wisely drilling the horn antenna, it is possible to reduce the phase error in the aperture which is a common problem in horn antennas if the optimum dimensions are not employed. An analysis of the distribution of the unit cells along the horn antenna has been carried out to achieve the desired equivalent refractive indices. By changing the hole radius, different equivalent refractive indices can be tuned with a wideband performance. This fact enables the implementation of a collimation zone inside the horn antenna which transforms the pseudo-circular wavefront into a quasiplanar one in the radiating aperture. The produced directivity is similar to the horn antenna with the optimum dimensions but a longitudinal reduction of 53.7% and a higher realized gain are achieved. A holey SIW horn antenna is designed and manufactured. The measured results show an impedance bandwidth performance below -10 dB from 34.3 GHz to 44.5 GHz (25.9%) with a realized gain above 10 dBi. The gain difference regarding a SIW horn antenna without the collimation zone is about 2-3 dBi in the operating frequency range

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

Cross-Polarization Control in FSSs by Means of an Equivalent Circuit Approach

This work was supported in part by Spanish Research and Development National Program under Project TIN2016-75097-P, Project RTI2018-102002-A-I00, Project TEC2017-84724-P, Project PID2020-112545RB-C54, and Project EQC2018-004988-P; in part by the Predoctoral under Grant FPU18/01965; and in part by Junta de Andalucia under Project P18-RT-4830 and Project B-TIC-402-UGR18.This paper presents an ef cient equivalent circuit approach (ECA), based on a Floquet modal expansion, for the study of the co- and cross-polarization in frequency selective surfaces (FSS) formed by periodic arrays of patches/apertures in either single or stacked con gurations. The ECA makes it possible the derivation of analytical expressions for the generalized scattering parameters associated with the proposed circuit networks. Furthermore, the proposed circuit approach is an ef cient surrogate model that can be combined with optimization techniques and arti cial intelligence algorithms for the ef cient design of FSS structures, saving efforts in the computation compared to time-consuming full-wave simulators and tedious synthesis (simulation-assisted) techniques. Due to the simplicity of the topology of the involved networks, the ECA can also be advantageously used to gain physical insight. The proposed approach is applied and validated in different FSS con gurations where the cross-pol component plays a fundamental role in the design, as in circular polarizers, polarization rotators, and reflectarray cells.Spanish Research and Development National Program TIN2016-75097-P RTI2018-102002-A-I00 TEC2017-84724-P PID2020-112545RB-C54 EQC2018-004988-P FPU18/01965Junta de Andalucia P18-RT-4830 B-TIC-402-UGR1