Network Topology and Protocol Design for Efficient Consensus in Sensor Networks

Doktorgradsavhandling ved Fakultet for teknologi og naturvitenskap, Universitetet i Agder, 2016In the new era of Internet of Things, complex sensor networks are becoming crucial to link the physical world to the Internet. These sensor networks, composed by hundreds or thousands of nodes, provide many important services to promote a heightened level of awareness about the area of interest such as in predictive maintenance, intelligent buildings, enhanced security systems, etc. In order to make these services possible, several signal processing tasks are needed to support their operation, some widely used examples of these tasks are parameter estimation, signal detection and target tracking. These tasks allow to improve the services by inferring missing data, reducing samples noise, etc., at the cost of some collaboration of the network nodes that implies their repeated communication over time. Most of these solutions are consensus-based strategies, which have recently attracted a great deal of research work because of its simplicity. These are in-network algorithms, where each node only exchanges information with its immediate neighbors and these are able to obtain global information as a function of some sensed data. A relevant example is the average consensus algorithm, which its goal is to obtain, in a distributed way, the average of the initial data. These algorithms avoid the need of performing all the computations at one or more sink nodes, thus, reducing congestion around them and incrementing the robustness of the network. In this dissertation, we focus on improving consensus algorithms in terms of different parameters and under different types of communications and network configurations. Each setting considered requires its own assumptions and methodologies, since the convergence conditions for each of them are related but different in general. In particular, in this work, all of the methodologies proposed are based on designing how the underlying communications are performed. In static networks, where the asymptotic convergence to the average value is easily ensured, a topology optimization can be a priori performed in terms of several relevant parameters. In particular, we optimize the network topology to make consensus algorithms fast and energy efficient. In this setting and for continuous systems, we derive a general framework to minimize several energy related functions under different network and nodes constraints To solve it, we propose a fractional convex-concave optimization problem with different constraints that leads to obtain the optimal topology in terms of the energy function considered. As a significant variation of the previous results, we also optimize the network topology in discrete systems. The discretization of the system introduces a weight matrix and certain step-size (related to the discrete increments of time) in the process. We show that if this step-size is small enough, the energy related problems stated before can be still casted as convex-concave fractional problems with the weight matrix as a unique optimization variable. As the step-size of the process increases in size, a discrete system requires a different approach. To solve it, we aim to find another formulation based on adding a constraint on the connectivity and solving the problem several times (for different values of the step-size). In addition, two low-complex methodologies with different computational requirements are proposed to a posteriori redesign an existing topology by the collaboration of the network nodes. On the contrary, in time-varying (random) networks, it is needed to guarantee a minimum accuracy of the algorithm, while maximizing the number of simultaneous exchanges of data to ensure fast convergence. Regarding random and asymmetric communications, we propose a novel gossip algorithm, which is based on the residual information that is generated when an asymmetric communication is performed. We exploit this information to preserve the summation of the process and accelerate it. Moreover, our proposal is useful in the case of having both unicast and broadcast communications, presenting faster convergence in both schemes than existing approaches in the related literature. When the problem of wireless interferences constraining the communications is additionally taken into account, we propose a novel and computationally efficient link scheduling protocol that correctly operates in the presence of secondary interference. Our protocol is easily implementable and does not require global knowledge of the network. The main objective of this new protocol is to be suitable for a cross-layer scheme in which the execution of the average consensus algorithm is favoured, ensuring necessary conditions for its convergence with certain accuracy. Additionally, the number of simultaneous links is additionally considered in order to make the convergence of the consensus process as fast as possible

Control predictivo basado en modelo para control lateral de vehículos

200 p.La mejora tecnológica ha supuesto el desarrollo de sistemas de seguridad avanzada en vehículos, entrelos que destacan los sistemas de guiado avanzados para vehículos autónomos y semiautónomos. Deforma general, este tipo de sistemas se basan en la resolución de diferentes subproblemas independientes:la definición de la trayectoria a seguir, el control para que el vehículo siga dicha trayectoria y la toma dedecisión para la asistencia al conductor. De estos tres subproblemas la presente tesis abarca el relacionadocon el control.Para dicho fin, se plantea el desarrollo de un control predictivo para control deseguimiento lateral de trayectoria de un vehículo, de forma que éste sea capaz de seguiruna trayectoria de referencia conocida de forma segura y confortable para todoslos pasajeros, cumpliendo las restricciones impuestas y garantizando así la permanenciadentro del carril y evitando giros bruscos de volante. Además, esta metodologíapropuesta, a diferencia de otros trabajos, puede ser aplicada para diferentes trazadosy en un amplio rango de velocidades.En primer lugar, se desarrolla un modelo del sistema para ser empleado adecuadamente como modelo depredicción en las diferentes estrategias de control. A continuación, se plantean tres propuestas de controlpredictivo: un control predictivo basado en modelo y con modelo de predicción variable, que sirve comobase para las siguientes estrategias de control; un control predictivo basado en modelo con estabilidadgarantizada y modelo de predicción variable; y un control predictivo basado en modelo robusto conestabilidad garantizada basado en tubos de trayectorias. Los tres controladores propuestos han sidovalidados experimentalmente

An agent-based visualisation system.

This thesis explores the concepts of visual supercomputing, where complex distributed systems are used toward interactive visualisation of large datasets. Such complex systems inherently trigger management and optimisation problems; in recent years the concepts of autonomic computing have arisen to address those issues. Distributed visualisation systems are a very challenging area to apply autonomic computing ideas as such systems are both latency and compute sensitive, while most autonomic computing implementations usually concentrate on one or the other but not both concurrently. A major contribution of this thesis is to provide a case study demonstrating the application of autonomic computing concepts to a computation intensive, real-time distributed visualisation system. The first part of the thesis proposes the realisation of a layered multi-agent system to enable autonomic visualisation. The implementation of a generic multi-agent system providing reflective features is described. This architecture is then used to create a flexible distributed graphic pipeline, oriented toward real-time visualisation of volume datasets. Performance evaluation of the pipeline is presented. The second part of the thesis explores the reflective nature of the system and presents high level architectures based on software agents, or visualisation strategies, that take advantage of the flexibility of the system to provide generic features. Autonomic capabilities are presented, with fault recovery and automatic resource configuration. Performance evaluation, simulation and prediction of the system are presented, exploring different use cases and optimisation scenarios. A performance exploration tool, Delphe, is described, which uses real-time data of the system to let users explore its performance

Control predictivo basado en modelo para control lateral de vehículos

200 p.La mejora tecnológica ha supuesto el desarrollo de sistemas de seguridad avanzada en vehículos, entrelos que destacan los sistemas de guiado avanzados para vehículos autónomos y semiautónomos. Deforma general, este tipo de sistemas se basan en la resolución de diferentes subproblemas independientes:la definición de la trayectoria a seguir, el control para que el vehículo siga dicha trayectoria y la toma dedecisión para la asistencia al conductor. De estos tres subproblemas la presente tesis abarca el relacionadocon el control.Para dicho fin, se plantea el desarrollo de un control predictivo para control deseguimiento lateral de trayectoria de un vehículo, de forma que éste sea capaz de seguiruna trayectoria de referencia conocida de forma segura y confortable para todoslos pasajeros, cumpliendo las restricciones impuestas y garantizando así la permanenciadentro del carril y evitando giros bruscos de volante. Además, esta metodologíapropuesta, a diferencia de otros trabajos, puede ser aplicada para diferentes trazadosy en un amplio rango de velocidades.En primer lugar, se desarrolla un modelo del sistema para ser empleado adecuadamente como modelo depredicción en las diferentes estrategias de control. A continuación, se plantean tres propuestas de controlpredictivo: un control predictivo basado en modelo y con modelo de predicción variable, que sirve comobase para las siguientes estrategias de control; un control predictivo basado en modelo con estabilidadgarantizada y modelo de predicción variable; y un control predictivo basado en modelo robusto conestabilidad garantizada basado en tubos de trayectorias. Los tres controladores propuestos han sidovalidados experimentalmente

HIV and Early Life Stress on Neuroimaging and Risky Behavior

This study examined the interactive effects of early life stress (ELS) and HIV on brain morphometry, diffusion-basis-spectrum-imaging (DBSI), risky decision-making, and sex-risk behavior. 122 people with HIV (PWH) and 113 people without HIV (PWoH), free of major psychiatric illness and neurological confounds, were stratified into high (≥ 3 events) vs. low (\u3c 3 events) ELS [PWoH/low ELS (n = 57), PWoH/high ELS (n =56), PWH/low ELS (n = 43), PWH/high ELS (n = 79)] and underwent structural magnetic resonance imaging, DBSI, neuropsychological, and risky-behavior assessment; all PWH were virologically controlled. Compared to PWoH, PWH had smaller orbitofrontal cortex (OFC), parietal lobes, insula, caudate and anterior cingulate. No ELS effects were detected in volumetric measures. Significant interactions were found between HIV serostatus and ELS on the OFC and on cellularity of the inferior fronto-occipital fasciculus after multiple comparisons adjustment. Specifically, PWH/high ELS exhibited significantly smaller OFC and PWoH/high ELS show significantly larger OFC than the other groups. PWoH/high ELS exhibited higher DBSI cellularity (neuroinflammation proxy) of the inferior-occipital-fasciculus compared to PWoH/high ELS. Regardless of HIV status, executive function moderated the relationship between the OFC and sex-risk behavior such that individuals within the sample who performed above average on a measure of executive function and had a larger OFC reported fewer sex partners in past six months than individuals with smaller volumes. No interaction was found between HIV serostatus and ELS on risky behavior measures. Clustering analyses defined ELS subgroups in PWH that were determined by demographic characteristics, duration of infection, recent CD4+ T-cell count, nadir CD4+ T-cell count and high/low ELS.Even in PWH that are virologically controlled, without major current psychiatric comorbidities, there is evidence of a synergistic impact of ELS and HIV on OFC volumes. Higher volumes in the OFC were detrimental when associated with lower executive function scores or advantageous when associated with higher executive function. Findings suggest that ELS is associated with different brain signatures among PWoH and virally suppressed PWH. However, ELS was not directly associated with risky behaviors, and subgroups in PWH were characterized by demographic variables, past substance use and HIV clinical variables

Influencia de las propiedades foliares, la hidrodinámica y el cambio global en las interacciones entre angiospermas marinas y sus consumidores

Seagrass-grazer interactions play a fundamental role in ecological processes by regulating structure and functioning of plant communities. Within biotic connections, herbivory is a crucial process because it involves the matter and energy transference from primary producers to higher trophic levels, affecting the physical structure of the community and ecosystem productivity. Contrary to previous beliefs, new insights suggest that grazing may have an important impact on seagrass communities and that their grazers may be key to understand seagrass food webs. Thus, the contrasting response found in previous research may indicate that the susceptibility of seagrass species to be grazed may have a temporal and spatial component and even within a community, a preferential consumption of some species among others may exist depending on the palatability of their tissues. Therefore, the assessment of the strength of the seagrass-grazers links and those mechanisms that regulate this relationship, it is of crucial importance to increase our understanding on community and ecosystem structure and dynamics. In addition, it is well known that seagrasses are living under local and global threats, and consequently, there is a pressing need to include the global change factors (eutrophication, acidification and warming) into the current experimental approaches, in order to forecast the future responses of this fundamental community in the changing world. This PhD Thesis is structured in 6 chapters to answer particular questions about seagrass-grazer interactions. In the first two chapters, the importance of hydrodynamics in the seagrass-grazer interactions was studied, and it demonstrated how this abiotic factor acts as a top-down regulator in the community. Moreover, seagrass acclimation to hydrodynamics modulates intraspecific seagrass traits, and then seagrass palatability, influencing the selection of leaves by consumers (chapters 2 and 3). On the other hand, eutrophication and the main global change factors (i.e. warming and acidification) influenced positively Cymodocea nodosa palatability through the alteration of leaf traits and by promoting the growth of epiphytes (chapters 4 and 5). In the last chapter, we looked at the seagrass responses to biomass loss promoted by herbivory, noting how the colonization states influences on the compensation responses to herbivory in the fast-growing tropical species such as Halodule wrightii. The research presented in this PhD Thesis contributes to identify changes on seagrass palatability under different environmental conditions and global change factors. Particularly, this research displays how hydrodynamics modulate feeding behaviour of consumers and how indirect effects of eutrophication and global change enhance seagrass consumption by herbivores. In addition, this Thesis shows how seagrass colonization states affect to compensatory responses derived by herbivore damage. The importance of considering biotic processes in conservation and management plans is emphasized in order to enhance seagrass ecosystems resilience and to identify solutions to support the maintenance of key ecosystem services provided by seagrass meadows to our planet

When Cardiac Biophysics Meets Groupwise Statistics: Complementary Modelling Approaches for Patient-Specific Medicine

This habilitation manuscript contains research on biophysical and statistical modeling of the heart, as well as interactions between these two approaches

When Cardiac Biophysics Meets Groupwise Statistics: Complementary Modelling Approaches for Patient-Specific Medicine

This habilitation manuscript contains research on biophysical and statistical modeling of the heart, as well as interactions between these two approaches