1,500 research outputs found

    Undergraduate Catalog of Studies, 2023-2024

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    Undergraduate Catalog of Studies, 2023-2024

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    Coverage Performance Analysis of Reconfigurable Intelligent Surface-aided Millimeter Wave Network with Blockage Effect

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    In order to solve spectrum resource shortage and satisfy immense wireless data traffic demands, millimeter wave (mmWave) frequency with large available bandwidth has been proposed for wireless communication in 5G and beyond 5G. However, mmWave communications are susceptible to blockages. This characteristic limits the network performance. Meanwhile, reconfigurable intelligent surface (RIS) has been proposed to improve the propagation environment and extend the network coverage. Unlike traditional wireless technologies that improve transmission quality from transceivers, RISs enhance network performance by adjusting the propagation environment. One of the promising applications of RISs is to provide indirect line-of-sight (LoS) paths when the direct LoS path between transceivers does not exist. This application makes RIS particularly useful in mmWave communications. With effective RIS deployment, the mmWave RIS-aided network performance can be enhanced significantly. However, most existing works have analyzed RIS-aided network performance without exploiting the flexibility of RIS deployment and/or considering blockage effect, which leaves huge research gaps in RIS-aided networks. To fill the gaps, this thesis develops RIS-aided mmWave network models considering blockage effect under the stochastic geometry framework. Three scenarios, i.e., indoor, outdoor and outdoor-to-indoor (O2I) RIS-aided networks, are investigated. Firstly, LoS propagation is hard to be guaranteed in indoor environments since blockages are densely distributed. Deploying RISs to assist mmWave transmission is a promising way to overcome this challenge. In the first paper, we propose an indoor mmWave RIS-aided network model capturing the characteristics of indoor environments. With a given base station (BS) density, whether deploying RISs or increasing BS density to further enhance the network coverage is more cost-effective is investigated. We present a coverage calculation algorithm which can be adapted for different indoor layouts. Then, we jointly analyze the network cost and coverage probability. Our results indicate that deploying RISs with an appropriate number of BSs is more cost-effective for achieving an adequate coverage probability than increasing BSs only. Secondly, for a given total number of passive elements, whether fewer large-scale RISs or more small-scale RISs should be deployed has yet to be investigated in the presence of the blockage effect. In the second paper, we model and analyze a 3D outdoor mmWave RIS-aided network considering both building blockages and human-body blockages. Based on the proposed model, the analytical upper and lower bounds of the coverage probability are derived. Meanwhile, the closed-form coverage probability when RISs are much closer to the UE than the BS is derived. In terms of coverage enhancement, we reveal that sparsely deployed large-scale RISs outperform densely deployed small-scale RISs in scenarios of sparse blockages and/or long transmission distances, while densely deployed small-scale RISs win in scenarios of dense blockages and/or short transmission distances. Finally, building envelope (the exterior wall of a building) makes outdoor mmWave BS difficult to communicate with indoor UE. Transmissive RISs with passive elements have been proposed to refract the signal when the transmitter and receiver are on the different side of the RIS. Similar to reflective RISs, the passive elements of a transmissive RIS can implement phase shifts and adjust the amplitude of the incident signals. By deploying transmissive RISs on the building envelope, it is feasible to implement RIS-aided O2I mmWave networks. In the third paper, we develop a 3D RIS-aided O2I mmWave network model with random indoor blockages. Based on the model, a closed-form coverage probability approximation considering blockage spatial correlation is derived, and multiple-RIS deployment strategies are discussed. For a given total number of RIS passive elements, the impact of blockage density, the number and locations of RISs on the coverage probability is analyzed. All the analytical results have been validated by Monte Carlo simulation. The observations from the result analysis provide guidelines for the future deployment of RIS-aided mmWave networks

    Spectrum auctions: designing markets to benefit the public, industry and the economy

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    Access to the radio spectrum is vital for modern digital communication. It is an essential component for smartphone capabilities, the Cloud, the Internet of Things, autonomous vehicles, and multiple other new technologies. Governments use spectrum auctions to decide which companies should use what parts of the radio spectrum. Successful auctions can fuel rapid innovation in products and services, unlock substantial economic benefits, build comparative advantage across all regions, and create billions of dollars of government revenues. Poor auction strategies can leave bandwidth unsold and delay innovation, sell national assets to firms too cheaply, or create uncompetitive markets with high mobile prices and patchy coverage that stifles economic growth. Corporate bidders regularly complain that auctions raise their costs, while government critics argue that insufficient revenues are raised. The cross-national record shows many examples of both highly successful auctions and miserable failures. Drawing on experience from the UK and other countries, senior regulator Geoffrey Myers explains how to optimise the regulatory design of auctions, from initial planning to final implementation. Spectrum Auctions offers unrivalled expertise for regulators and economists engaged in practical auction design or company executives planning bidding strategies. For applied economists, teachers, and advanced students this book provides unrivalled insights in market design and public management. Providing clear analytical frameworks, case studies of auctions, and stage-by-stage advice, it is essential reading for anyone interested in designing public-interested and successful spectrum auctions

    Undergraduate Catalog of Studies, 2022-2023

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    Growth of Group IV and III-V Semiconductor Materials for Silicon Photonics: Buffer Layer and Light Source Development

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    High data transmission speeds, high levels of integration, and low manufacturing costs have established Si photonics as a crucial technology for next-generation data interconnects and communications systems. It involves a variety of components including light emitters, photodetectors, amplifiers, waveguides, modulators, and more. Because of its indirect bandgap, silicon is unable to serve as an efficient light source on a chip, hence this has been one of the formidable challenges. Within the framework of the monolithic approach, this thesis presents the study of two essential aspects of this challenge, the optimisation of buffer layers and development of light sources, by incorporating and improving different systems of Group IV thin films and III-V quantum dots (QDs) semiconductor materials. The monolithic approach focuses on the direct epitaxial growth of highly efficient light sources, usually by the epitaxy of III-V semiconductors lasers on a single Si chip. However, because of the material dissimilarities between III-V materials and Si, during the heteroepitaxy, a high density of crystalline defects such as threading dislocations (TDs), thermal cracks and anti-phase domains are introduced, severely impeding the performance and yield of the laser. For instance, TDs act as non-radiative recombination centres, while thermal cracks cause issues with the efficient evanescent coupling of the emitted light with Si waveguide. To address these defects, typically complex buffer growth techniques with micron-scale thickness are employed. The research in this thesis is divided into two parts, namely buffer layer optimisation and light source development. Each part outlines alternative strategies for overcoming the above-mentioned hurdles for monolithic growth. The first part highlights the optimisation of buffer layer growth to reduce threading dislocations for the monolithic integration of high-performance direct-bandgap III-V and group IV light sources on Si. The growth optimisation of low defect-density Ge buffer layers epitaxially grown on Si was first investigated. Defect elimination in Ge buffers with doped and undoped seed layers of increasing total thickness is studied under a variety of growth regimes, doping techniques, and annealing processes. This study demonstrates that a 500 nm thin Ge achieves the same defect level (1.3 × 108 cm -2) as 2.2 μm GaAs grown on Si, which greatly increases the thickness budget for the subsequent dislocation filter layers (DFLs) and laser structure growth before the formation of thermal cracks. Meanwhile, a low threading dislocation density of 3.3 × 107 cm -2 is obtained for 1 μm Ge grown on Si. The second part places emphasis on the development of light sources in the near-infrared wavelength range for Si photonics. 1) The development of GeSn, an emerging direct bandgap light source for Si photonics, is shown, which has wide bandgap tuneability and full compatibility with Si complementary metal-oxide semiconductor (CMOS). Growing the high Sn composition of GeSn required for efficient light generation is challenging and its growth generally severely affected by large surface roughness and Sn segregation. In this work, first, ex-situ rapid thermal annealing for the grown GeSn layer is investigated, showing that by proper annealing the strain can be relaxed by 90% without intriguing Sn segregation. This method shows its potential for both material growth and device fabrication. Besides, strain compensated layer and in-situ annealing techniques have been developed. Significantly improved surface quality has been confirmed by in-situ reflection high-energy electron diffraction (RHEED) observations and atomic force microscopy (AFM) images. Transmission electron microscopy (TEM) results reveal the high crystal quality of the multiple quantum wells (MQWs) grown on such buffer layers. 2) The final section details the development of InAs/InP QDs emitting near the strategic 1.55 μm, the lowest optical fibre loss window. The InAs/InP QDs growth is prone to inhomogeneous quantum dash morphologies which broaden the photoluminescence (PL) spectra and degrade the carrier confinement. Research has been conducted on growth parameters and techniques including deposition thickness, growth temperature and Indium-flush technique is applied to improve the uniformity of the dots, and narrow room temperature PL linewidths of 47.9 meV and 50.9 meV have been achieved for single-layer and five-layer quantum dot samples, respectively. The structures enable the fabrication of small footprint microdisk lasers with lasing thresholds as low as 30 μW

    Porous carbon fibers derived from PAN-based block copolymers

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    Mención Internacional en el título de doctorDurante años, los materiales porosos de carbono han suscitado un gran interés para su uso en innumerables aplicaciones tecnológicas, debido a sus excelentes propiedades fisicoquímicas y su elevada superficie específica. Estos materiales han demostrado tener un elevado potencial especialmente en aplicaciones relacionadas con el medioambiente y la energía debido a su alta capacidad para adsorber distintas especies, incluyendo gases, moléculas orgánicas y/o iones metálicos. En particular, las fibras de carbono porosas, PCFs por sus siglas en inglés (Porous Carbon Fibers), con una porosidad controlada y jerarquizada han captado una atención significativa en los últimos años, ya que combinan las ventajas de materiales macro−, meso− y microporosos, lo que las convierte en excelentes candidatas para aplicaciones de adsorción y energía, entre otras. Las PCFs ofrecen una estructura de carbono continua que combina alta conductividad, altos valores de áreas superficiales y densidades muy bajas. Concretamente, en lo que respecta a su uso como materiales para electrodos de supercondensadores, las PCFs presentan algunas ventajas sobre los carbones activos, que son los materiales comúnmente utilizados como electrodos en este tipo de dispositivos. Estas fibras de carbono pueden emplearse idealmente como electrodos sin el uso de aglutinantes o aditivos conductores. Además, para esta aplicación específica, el desarrollo de PCFs con una estructura de poros jerarquizada es especialmente interesante, ya que los mesoporos facilitan la difusión de iones hacia los microporos confinados en las regiones internas del material. Por lo tanto, se consigue incrementar el área de superficie accesible a los iones de electrolito, que se traduce en una mayor capacidad de almacenamiento y con ello se incrementa el rendimiento electroquímico. Destaca el uso de copolímeros en bloque, BCPs por sus siglas en inglés (Block copolymers), como materiales molde o plantilla para la obtención de PCFs, debido a su capacidad para autoensamblarse y separarse en microfases, lo que da lugar a múltiples morfologías. Mediante el empleo de un bloque de sacrificio compuesto por un polímero térmicamente degradable y otro bloque como fuente de carbono, es posible controlar las estructuras de carbono generadas, en términos de volumen, tamaño y forma de los poros. Ajustando la fracción volumétrica del bloque de sacrificio, la interacción entre los bloques y/o el grado de polimerización del copolímero es posible modificar el número, tamaño y forma de los poros generados después de la carbonización del material plantilla. Esta tesis analiza el uso de copolímeros en bloque como precursores para obtener fibras de carbono porosas. Se sintetizaron copolímeros con pesos moleculares definidos, a partir de una polimerización radical controlada (RAFT), basados en poliacrilonitrilo (PAN) y diferentes bloques de sacrificio, como poliestireno (PS) y poli(tert−butil acrilato) (PtBA), que dieron lugar a fibras con distintas propiedades fisicoquímicas. Se ha estudiado el comportamiento de autoensamblaje y separación de fases de los copolímeros durante el tratamiento térmico, así como el tamaño del poro y su distribución después de la pirólisis. La obtención de las fibras porosas se llevó a cabo mediante una técnica simple, versátil, y fácilmente escalable, el electrohilado. Mediante la pirólisis del bloque de sacrificio se obtuvieron fibras de carbono con estructuras de poro jerarquizadas, diámetros estrechos y varias formas y tamaños dependiendo de la naturaleza del bloque de sacrificio y el grado de polimerización. Asimismo, se estudiaron diferentes parámetros que influyen en las propiedades capacitivas y el comportamiento electroquímico de las PCFs. Entre ellos se consideraron la incorporación de heteroátomos (nitrógeno y azufre) y nanopartículas con actividad redox (nanopartículas de magnetita). Las PCFs derivadas de un copolímero en bloque de poliestireno y poliacrilonitrilo (PS−b−PAN), se activaron/doparon con urea y tiourea como precursores de heteroátomos de N y N/S, respectivamente. Estos procesos permitieron obtener PCFs co−dopadas sin comprometer los valores de área superficial y mostrando un aumento de la capacitancia. Por último, se presenta un estudio preliminar de la incorporación de nanopartículas de magnetita (MNPs) con actividad redox en la estructura de las fibras de carbono porosas (PCFs), y su influencia en la microestructura y la porosidad. Para ello, se introdujeron nanopartículas de magnetita en bajas concentraciones en la matriz del copolímero en bloque (PtBA−b−PAN), y posteriormente se utilizó esa dispersión como precursor de electrohilado en la fabricación de las fibras. Se obtuvieron PCFs con una porosidad y rendimiento electroquímico mejorados, en comparación con las fibras fabricadas sin la adición de nanopartículas. En resumen, este trabajo de tesis tiene como objetivo investigar el comportamiento de los copolímeros de bloque basados en poliacrilonitrilo como precursores para la producción de fibras de carbono porosas. En particular, se pretende estudiar las variaciones en los parámetros que influyen en la separación de fases, lo que a su vez permitirá explorar nuevas vías para modular el tamaño y la forma de los poros. Además, a través de la caracterización electroquímica de los materiales generados se analiza el impacto en el rendimiento electroquímico de las variaciones en el área de superficie específica, la distribución del tamaño de los poros y las funcionalidades de la superficie.For years, porous carbon materials have attracted wide interest for their use in countless technological applications, due to their excellent physicochemical properties and their high specific surface area. These materials have shown a high potential, especially for environmental and energy−related applications, due to their ability to adsorb different species, including gases, organic molecules and/or metal ions. In particular, porous carbon fibers (PCFs) with a well−controlled and hierarchical porosity have attracted significant attention recently, since they combine the advantages of macro−, meso− and microporous materials, making them excellent candidates for adsorption and energy applications, among others. PCFs offer a continuous carbon structure that combines high conductivity, high surface area values, and very low densities. Specifically, concerning their use as electrode materials for supercapacitor, PCFs present some advantages over active carbons, which are commonly used as electrodes in these devices. These carbon fibers can ideally be used as self−standing electrodes without using binders or conductive additives. Furthermore, for this specific application, the development of PCFs with a hierarchical pore structure is especially interesting, since the mesopores facilitate the ion−diffusion towards the micropores confined in the internal regions of the material. Therefore, the surface area accessible to electrolyte ions is increased, which translates into a higher storage capacity and thus increases the electrochemical performance. The use of block copolymers (BCPs) as template materials for obtaining PCFs stands out due to the ability of block copolymers to self−assemble and separate into microphases, producing miscellaneous morphologies. By using a sacrificial block composed of a thermally degradable polymer and another block as a carbon source, it is possible to control the carbon structures produced in terms of pore volume, size, and shape. Adjusting the volume fraction of the sacrificial block, the interaction between the blocks and/or the overall degree of polymerization of the copolymer allows to modify the number, size and shape of the pores generated after carbonization of the template. This thesis analyzes the use of block copolymers as precursors to obtain porous carbon fibers. Copolymers with defined molecular weights were synthesized using controlled radical polymerization (RAFT), based on polyacrylonitrile (PAN) and different sacrificial blocks, such as polystyrene (PS) and poly(tert−butyl acrylate) (PtBA), resulting in fibers with different physicochemical properties. The self−assembly and phase separation behavior of block copolymers during thermal treatments, as well as the pore size and distribution after pyrolysis, have been studied. PCFs were obtained using a simple, versatile, and easily scalable technique, electrospinning. By pyrolysis of the sacrificial block, carbon fibers with hierarchical pore structures, narrow diameters, and various shapes and sizes were obtained, depending on the nature of the sacrificial block and the degree of polymerization. Different parameters influencing the capacitive properties and electrochemical behavior of the obtained PCFs were also studied. Among them, the introduction of heteroatoms (nitrogen and sulfur) and redox active nanoparticles (magnetite nanoparticles) were considered. PCFs derived from a polystyrene and polyacrylonitrile block copolymer (PS−b−PAN) were activated/doped with urea and thiourea as N and N/S heteroatom precursors, respectively. These processes allowed to obtain co−doped PCFs without compromising surface area values and showing an increase in capacitance. Finally, a preliminary study is presented based on the addition of magnetite nanoparticles (MNPs) with redox activity in the structure of porous carbon fibers (PCFs), and its influence on the microstructure and porosity. For this purpose, magnetite nanoparticles were introduced in low concentrations into the block copolymer matrix (PtBA−b−PAN), and this dispersion was subsequently used as an electrospinning precursor to produce fibers. PCFs with improved porosity and electrochemical performance were obtained, compared to fibers produced without the addition of nanoparticles. In summary, this thesis aims to investigate the behavior of polyacrylonitrile−based block copolymers as precursors to produce PCFs. In particular, it is intended to study the variations in the parameters that influence phase separation, allowing to explore new ways of modulating the size and shape of the pores. In addition, through the electrochemical characterization of the generated materials, the impact on electrochemical performance of variations in the specific surface area, pore size distribution, and surface functionalities is analyzed.This project has been financed through PIPF scholarship (2019) from the same University and the project PID2021−125302NB−I00 from the Spanish Ministry of Science and Innovation.Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidenta: Pilar Herrasti González.- Secretaria: María Crespo Ribadeneyra.- Vocal: Juan José Vilatela Garcí
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