Tese de mestr.. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 1998

Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 2008

Desde su nacimiento, la evolución de las redes de sensores inalámbricos se ha traducido en un aumento de la complejidad y heterogeneidad de las mismas, constituyendo este marco, la base del despliegue de una infraestructura experimental (en la que se fundamenta esta Tesis Doctoral), compuesta por un elevado número de nodos de la Internet de las cosas (superior a 10.000), en un entorno urbano. En este sentido, dentro de esta Tesis Doctoral, se han abordado los aspectos relativos a la gestión uniforme de sensores heterogéneos; la descripción de una arquitectura que cubra los requerimientos de los planos de experimentación, gestión y servicio del despliegue masivo; el diseño, desarrollo y validación de un protocolo de reconfiguración remota de los nodos constitutivos, así como el soporte de nuevos experimentos y servicios. Finalmente, entre las líneas futuras, se destacan la federación de infraestructuras de experimentación y la simbiosis entre el paradigma de la Internet de las cosas y las redes sociales. / From its birth, the evolution of Wireless Sensor Networks has translated into an increase in terms of complexity and heterogeneity associated to them, constituting this framework the basis for the deployment of an experimental infrastructure (on which this Thesis is based), composed of a high number of Internet of Things nodes (higher to 10000), in a urban environment. In this sense, within this Thesis, they have been tackled the aspects relating to the uniform management of heterogeneous sensors; the description of an architecture that fulfills the requirements associated to experimentation, management and service planes associated to the massive deployment; the design, development and validation of a remote reconfiguration protocol of the constitutive nodes, as well as the support of new experiments and services. Finally, among the future lines, they can be highlighted the federation of experimentation infrastructures and the symbiosis between the IoT paradigm and the social networks.

Dissertação apresentada para obtenção do grau de Mestre em Engenharia Electrotécnica e de Computadores na Faculdade de Engenharia da Universidade do Porto, sob a orientação do Prof. Doutor António Carlos Sepúlveda Machado e Moura da FEUP e do Prof. Doutor José Américo Dias Pinto do Instituto Superior de Engenharia de Coimbra

Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores (Major Energia). Faculdade de Engenharia. Universidade do Porto. 2009

Dissertação apresentada para obtenção do grau de Doutor em Engenharia Electrotécnica e de Computadores a Faculdade de Engenharia da Universidade do Porto sob a orientação do Prof. Dr. Fernando Pires Maciel Barbosa

Recently, there has been an increasing interest in the electronics world for those aspects related to semiconducting gas sensor (SGS) materials. In view of the increasingly strict legal limits for pollutant gas emissions, there is a great interest in developing high performance gas sensors for applications such as controlling air pollution and exhaust gases. In this way, semiconductor gas sensors offer good advantages with respect to other gas sensor devices (such as spectroscopic and optic systems), due to their simple implementation, low cost and good reliability for real-time control systems. In the present work, we have been especially interested in the study of the different ways of metal additive distribution in the most common SGS materials used nowadays and furthermore in the physical and chemical sensing properties they can achieve.

This doctoral dissertation deals with the study of nanocrystalline tungsten oxide-based powders for gas sensing applications. Gas monitoring is receiving increasing attention due to environmental and safety reasons. Gas sensors based on heated semiconducting metal oxides are appreciated as they can detect low concentrations of certain gases by a variation of conductance of the metal oxide layer. Among the different metal oxides proposed, tungsten oxide (WO3) is considered one of the most promising materials for the detection of ammonia (NH3), hydrogen sulphide (H2S) and nitrogen dioxide (NO2). The first chapter of this dissertation presents the general framework where this investigation is placed. It includes a brief overview of chemical sensors, metal oxide-based gas sensors and the reported properties of WO3 for gas sensing applications. Finally, motivation and main targets of this investigation, as well as the organisation of this dissertation, are presented and argued. Chapter 2 presents the experimental details of this study. It discusses the preparation of WO3 nanocrystalline powder, the experimental techniques used to analyse the structural properties of the WO3 powders and the implementation and test of gas sensor devices. For this work, XRD, Raman spectroscopy, TEM-EELS, XPS, EPR, TPD and DRIFTS have been used. Thick-film screen-printed gas sensors based on WO3-powders were tested.The target of Chapter 3 is to present an investigation into the structural and spectroscopic properties of pure and catalysed nanocrystalline WO3 powder. Firstly, the characterisation of pure nanocrystalline WO3 powder is considered. The main parameter analysed is the influence of the annealing treatment on the structural properties. Structural and spectroscopic properties of catalysed WO3 are also reported here. In this case, the emphasis lays on the characterisation of catalytic centres, rather than on bulk WO3. The catalytic additives introduced were copper (Cu), vanadium (V) and chromium (Cr). Results concerning gas sensors based on WO3 nanocrystalline powders for the detection of the previously mentioned gases are reported in Chapter 4. The purpose of this investigation was to evaluate the sensing properties of thick-film gas sensors based on WO3 obtained from tungstic acid and to determine the effect of different additives on sensor response to NH3, H2S and NO2. Interference of humidity on the detection of these gases was also evaluated. A tentative interpretation of the reported results based only on the test data presented is also provided.The aim of Chapter 5 is to explore the implementation of real condition characterisation techniques to WO3-based nanopowders in order to study surface species and reactions involved in gas sensing. By real condition, we refer to a characterisation under controlled conditions of temperature and gas concentration on the sample, as similar to test conditions as possible. These studies, which are standard in the field of catalysis, are not so common for gas sensors. This is still a key point in the field of gas sensors at the moment: to obtain a deeper understanding of what occurs on the surface of the sensing material. The chapter has two main parts, corresponding to the results of DRIFTS and TPD techniques. By means of DRIFTS it is possible to identify surface species that present infrared vibrations. By TPD, the desorption of adsorbed target gases is analysed. Finally, Chapter 6 aims at discussing the results previously presented as a whole, as well as presenting the main conclusions that can be drawn from this investigation and some proposals for a future research grounded on the present work.

La interferencia supone el mayor factor limitante de las redes de comunicaciones inalámbricas. En contraposición a las técnicas de gestión actual de la misma, consistentes en repartir los recursos disponibles entre los usuarios de manera ortogonal, esta tesis proporciona un amplio estudio de técnicas de cooperación entre usuarios, que, sustentándose en resultados de teoría de la información, tienen como objetivo gestionar la interferencia de manera activa y, en consecuencia, más eficiente. La tesis se centra en el diseño de algoritmos y empleo de técnicas de optimización para diseñar las estrategias de transmisión en tres escenarios cooperativos distintos: el canal de interferencia, el modelo de radio cognitiva subyacente y el canal de repetición bidireccional. Cada uno de estos escenarios presenta diferentes necesidades y objetivos en cuanto a cooperación y tratamiento de interferencias, y recogen gran parte de las características y problemas prácticos que rodean la cooperación multiusuario. / Interference is one of the major limiting factors in wireless communication networks. Contrarily to current interference management techniques, consisting in an orthogonal resource sharing among users, this thesis provides a wide study on multiuser cooperation, which, based upon information theory results, aims at an active treatment of interference and, consequently, more efficient. This thesis focuses on algorithm design and optimization techniques to design transmission strategies in three different cooperative scenarios: the interference channel, the underlay cognitive radio model and the two-way relay channel. Each of these scenarios presents varying cooperation needs and interference management goals, and they capture some of the main features and practical issues of multiuser cooperation.

La Tesis realiza una contribución a la problemática de la selección de acceso, proponiendo una metodología que permite analizar, desde una perspectiva global e integral, los diferentes escenarios que puedan presentarse en las futuras redes de comunicación inalámbricas. Para ello hace uso de dos enfoques diferentes, pero complementarios entre sí. El primero modela el proceso de selección de acceso desde la perspectiva del usuario, en base a los eventos que se producen en su contexto, proporcionando en todo momento la mejor alternativa de acceso. Aunque esto permite dotar al usuario con la mejor calidad de experiencia en sus comunicaciones, incluso estando en movilidad, no se puede asegurar si dicha solución contribuye a alcanzar el rendimiento óptimo que se podría obtener en todo el sistema bajo estudio. Debido a ello se puede decir que este modelo tiene una naturaleza “local”, esto es, limitada al usuario, en tanto en cuanto éste no puede conocer las posibles consecuencias de su decisión sobre los demás usuarios o elementos de la red. Con el objetivo de responder apropiadamente a la anterior incógnita, se propone un segundo modelo, que permite estudiar el mismo escenario mediante un análisis discreto en el tiempo, para establecer el límite superior del rendimiento que puede alcanzar el sistema, mediante el uso de técnicas de optimización. A diferencia del anterior, este enfoque no busca la mejor alternativa para un usuario específico, sino que se centra en el rendimiento óptimo de todo el sistema, por lo que se podría decir que este modelo hace uso de una perspectiva más “global”. Los resultados obtenidos al aplicarlo se utilizarán para determinar en qué medida el rendimiento de las estrategias analizadas con el enfoque anterior es mejorable, evitando las comparaciones con otros estudios con objetivos similares, pero claramente diferentes en los escenarios utilizados. Ambas aproximaciones proponen un modelado que pretende reflejar las características más relevantes de los despliegues de red reales, mimetizando su comportamiento en la mayor medida posible, mediante entidades que emulan las propiedades y funcionalidades de los elementos correspondientes: estaciones base, operadores, usuarios, terminales, servicios, entre otros. Todos ellos constituyen el escenario sobre el cual se pueden realizar diversos análisis en función de un conjunto de parámetros, como las preferencias del usuario, características del servicio, o los requisitos provenientes de la red y de los operadores que la gestionan. En base a los dos modelos propuestos, se realiza el diseño e implementación de sendas herramientas de simulación, correspondientes a cada uno de ellos. Se desarrollan entornos flexibles, escalables y de fácil configuración, con el objetivo de acometer la evaluación de los procesos de selección de acceso en entornos heterogéneos. A diferencia de otras plataformas similares, las herramientas de simulación que se describen en este documento permiten realizar despliegues de red altamente configurables, facilitando la tarea de reflejar la diversidad que caracterizará las redes inalámbricas en un futuro cercano. Finalmente, se lleva a cabo la evaluación de un conjunto de escenarios, analizando diversas estrategias de selección de acceso, mediante las herramientas de simulación implementadas. En cada uno de ellos se plantean objetivos concretos, buscando en todo momento proporcionar la mejor alternativa de acceso para el usuario en entornos de red multi-acceso, multi-interfaz, multi-servicio y multi-operador. / This PhD Thesis contributes to the problem of access selection, proposing a methodology that allows the global and generic analysis of the various scenarios that can spring in the future wireless communication networking realm. For that, it fosters two different, yet complementary, approaches. The first one models the access selection process from the user perspective, based on the events that may happen within her environment, providing the best access alternative. Although this provides the user with the best quality of experience for her own communications, even when she is moving, it such solution cannot be said to contributes to the global optimum behaviour (considering the whole system under analysis). We can therefore state that this model has a ``local'' character, i.e. focused on the end-user, since she could not be aware of the potential consequences of her decision over the remaining users or network elements. In order to appropriately answer the aforementioned question, this PhD proposes a second model, which allows studying the very same scenario by means of a discrete-time analysis, to establish the upper performance limit that can be provided by the whole system, using optimization techniques. As opposed to the previous model, this approach does not seek the best alternative for a specific user, but it focuses on the overall system performance; it can be therefore said that this second model fosters a more ``global'' approach. The results that are obtained with this model will be used to determine whether the performances of the strategies studied by means of the previous approach might be improved or not, avoiding the comparison with other studies with the same objectives, but carried out over rather distinct scenarios. Both approaches propose a model that aims at reflecting the most relevant characteristics of real network deployments, mimicking their behaviour as closely as possible, by using entities that emulate the properties and functionalities of the corresponding elements: base stations, operators, users, terminals, services, etc. All of them jointly create a scenario over which different analysis can be carried out, considering a number of different parameters, such as user preferences, service characteristics, or the requirements from the network and the operators managing it. Based on the proposed models, the PhD also tackles the design and implementation of two simulation tools. The corresponding frameworks are flexible, scalable and enable an easy configuration, to foster the analysis of multiple scenarios and access selection strategies. As opposed to other similar platforms, the two simulation tools that are described within this document allow the deployment of highly configurable network scenarios, mimicking the large heterogeneity of the forthcoming wireless networks. Last, the PhD tackles the evaluation of a number of access selection strategies, exploiting the two aforementioned simulation platforms. Each of them would address particular objectives, aiming at offering the end-user with the best access alternative, considering multi-access, multi-interface, multi-service and multi-operator network scenarios.