Asymptotic properties of wireless multi-hop networks

In this dissertation, we consider wireless multi-hop networks, where the nodes are randomly placed. We are particularly interested in their asymptotic properties when the number of nodes tends to infinity. We use percolation theory as our main tool of analysis. As a first model, we assume that nodes have a fixed connectivity range, and can establish wireless links to all nodes within this range, but no other (Boolean model). We compute for one-dimensional networks the probability that two nodes are connected, given the distance between them. We show that this probability tends exponentially to zero when the distance increases, proving that pure multi-hopping does not work in large networks. In two dimensions however, an unbounded cluster of connected nodes forms if the node density is above a critical threshold (super-critical phase). This is known as the percolation phenomenon. This cluster contains a positive fraction of the nodes that depends on the node density, and remains constant as the network size increases. Furthermore, the fraction of connected nodes tends rapidly to one when the node density is above the threshold. We compare this partial connectivity to full connectivity, and show that the requirement for full connectivity leads to vanishing throughput when the network size increases. In contrast, partial connectivity is perfectly scalable, at the cost of a tiny fraction of the nodes being disconnected. We consider two other connectivity models. The first one is a signal-to-interference- plus-noise-ratio based connectivity graph (STIRG). In this model, we assume deterministic attenuation of the signals as a function of distance. We prove that percolation occurs in this model in a similar way as in the previous model, and study in detail the domain of parameters where it occurs. We show in particular that the assumptions on the attenuation function dramatically impact the results: the commonly used power-law attenuation leads to particular symmetry properties. However, physics imposes that the received signal cannot be stronger than the emitted signal, implying a bounded attenuation function. We observe that percolation is harder to achieve in most cases with such an attenuation function. The second model is an information theoretic view on connectivity, where two arbitrary nodes are considered connected if it is possible to transmit data from one to the other at a given rate. We show that in this model the same partial connectivity can be achieved in a scalable way as in the Boolean model. This result is however a pure connectivity result in the sense that there is no competition and interferences between data flows. We also look at the other extreme, the Gupta and Kumar scenario, where all nodes want to transmit data simultaneously. We show first that under point-to-point communication and bounded attenuation function the total transport capacity of a fixed area network is bounded from above by a constant, whatever the number of nodes may be. However, if the network area increases linearly with the number of nodes (constant density), or if we assume power-law attenuation function, a throughput per node of order 1/√n can be achieved. This latter result improves the existing results about random networks by a factor (log n)1/2. In the last part of this dissertation, we address two problems related to latency. The first one is an intruder detection scenario, where a static sensor network has to detect an intruder that moves with constant speed along a straight line. We compute an upper bound to the time needed to detect the intruder, under the assumption that detection by disconnected sensors does not count. In the second scenario, sensors switch off their radio device for random periods, in order to save energy. This affects the delivery of alert messages, since they may have to wait for relays to turn on their radio to move further. We show that asymptotically, alert messages propagate with constant, deterministic speed in such networks

Volumetric Phased Arrays for Satellite Communications

The high amount of scientific and communications data produced by low earth orbiting satellites necessitates economical methods of communication with these satellites. A volumetric phased array for demonstrating horizon-to-horizon electronic tracking of the NASA satellite EO-1 was developed and demonstrated. As a part of this research, methods of optimizing the elemental antenna as well as the antenna on-board the satellite were investigated. Using these optimized antennas removes the variations in received signal strength that are due to the angularly dependent propagation loss exhibited by the communications link. An exhaustive study using genetic algorithms characterized two antenna architectures, and included optimizations for radiation pattern, bandwidth, impedance, and polarization. Eleven antennas were constructed and their measured characteristics were compared to those of the simulated antennas. Additional studies were conducted regarding the optimization of aperiodic arrays. A pattern-space representation of volumetric arrays was developed and used with a novel tracking algorithm for these arrays. This algorithm allows high-resolution direction finding using a small number of antennas while mitigating aliasing ambiguities. Finally, a method of efficiently applying multiple beam synthesis using the Fast Fourier Transform to aperiodic arrays was developed. This algorithm enables the operation of phased arrays combining the benefits of aperiodic element position with the efficiency of FFT multiple beam synthesis. Results of this research are presented along with the characteristics of the volumetric array used to track EO-1. Experimental data and the interpretations of that data are presented, and possible areas of future research are discussed.Ph.D.Committee Chair: Steffes, Paul; Committee Member: Durgin, Gregory; Committee Member: Peterson, Andrew; Committee Member: Roper, Robert; Committee Member: Williams, Dougla

Toward Sustainable Transparent and Flexible Electronics with Amorphous Zinc Tin Oxide

The present thesis addresses a sustainable approach to mechanically flexible and transparent electronic devices based on the amorphous oxide semiconductor zinc tin oxide (ZTO) as abundant and low-cost alternative to already industrially established materials such as amorphous indium gallium zinc oxide. ZTO thin films are deposited by radio frequency long-throw magnetron sputtering at room temperature to generally enable the implementation of common photolithography processes and further facilitate patterning of digital circuit elements on thermally unstable organic substrates. Starting with the most basic device building blocks of integrated circuitry, various types of field-effect transistors are fabricated by implementation of amorphous ZTO as active channel material. Metal-semiconductor field-effect transistors and pn heterodiode based junctions field-effect transistors as well as conventional metal-insulatorsemiconductor field-effect transistors are then compared regarding their electrical performance and long-term stability over a couple of months. A decisive step toward the successful interconnection of fundamental digital circuit elements, such as previously demonstrated simple inverters, is to ensure sufficient output level compatibility between the signals of associated logic components. Accordingly, the Schottky diode field-effect transistor logic approach is adapted for amorphous ZTO based devices in order to facilitate cascading of multiple inverters consisting of unipolar devices. Field-effect transistor properties as well as the circuit design have been continuously improved to enhance the overall performance in terms of functionality and low-voltage operation. Corresponding logic inverters are finally integrated in ring oscillator circuits to gain insights into the dynamic properties of digital circuit building blocks based on amorphous ZTO. Ultimately, ZTO has been fabricated on mechanically flexible polyimide substrates to determine the elastic and electrical properties of amorphous ZTO thin films in dependence on external tensile and compressive stress induced by mechanical bending. Further, associated flexible metal-semiconductor field-effect transistor are investigated regarding their performance stability under tensile strain.Die vorliegende Arbeit umfasst die Herstellung und Charakterisierung aktiver elektrischer Bauelemente und integrierter Schaltkreise auf Basis des amorphen Oxidhalbleiters Zink-Zinnoxid (ZTO). Als vielversprechende nachhaltige und kostengünstigere Alternative zu dem bereits industriell etablierten Halbleiter Indium-Gallium-Zinkoxid wird insbesondere die Eignung von ZTO in optisch transparenter sowie mechanisch flexibler Elektronik untersucht. Um entsprechend Kompatibilität mit thermisch instabilen organischen Substraten sowie herkömmlichen Fotolithografieverfahren zu gewährleisten, beschränkt sich die Züchtung von ZTO-Dünnfilmen mittels Hochfrequenz-Magnetron-Distanzkathodenzerstäubung ausschließlich auf Herstellungsprozesse bei Raumtemperatur. Zunächst wird auf die Umsetzung verschiedener Feldeffekttransistor-Typen auf Basis amorphen ZTOs eingegangen, welche elektrisch charakterisiert und schließlich vor dem Hintergrund der Anwendung in integrierten Schaltkreisen vergleichend gegenübergestellt werden. Neben konventionellen Metall-Isolator-Halbleiterstrukturen wird vor allem näher auf Metall-Halbleiter-Feldeffekttransistoren sowie Sperrschicht-Feldeffekttransistoren auf der Grundlage von pn-Heteroübergängen eingegangen, da diese hauptsächlich in Bereichen hoher geforderter Schaltfrequenzen zum Einsatz kommen. Da integrierte Schaltkreise auf Basis unipolarer Feldeffekttransistoren eines Ladungsträgertyps inkonsistente Signaleingangs- sowie -ausgangspegel aufweisen, wird die Schottky- Dioden-Transistorlogik adaptiert, um entsprechend die Verknüpfung mehrerer Logikgatter auf Basis amorphen ZTOs zu gewährleisten. Durch geeignete Signalrückkopplung werden komplexere Schaltungen wie Ringoszillatoren realisiert, welche anhand von Laufzeitanalysen Aufschluss über die Schaltgeschwindigkeit ZTO basierter Feldeffekttransistoren geben. Abschließend werden amorphe ZTO-Dünnfilme auf flexiblen Polyimid-Substraten hergestellt und bezüglich der elastischen sowie elektrischen Eigenschaften in Abhängigkeit von exzessivem mechanischen Stress untersucht. Darüber hinaus werden flexible Metall-Halbleiter-Feldeffekttransistoren hinsichtlich ihrer Funktionalität und Stabilität gegenüber durch Biegeprozesse induzierte Verspannungen elektrisch charakterisiert

Automated CAD Model Generation for Structural Optimisation

Computer-aided design (CAD) models play a crucial role in the design, manufacturing and maintenance of products. Therefore, the mesh-based finite element descriptions common in structural optimisation must be first translated into CAD models. Currently, this translation either can be performed semi-manually or fails to reserve the structural optimality found by the structural optimisation due to the intrinsic difference in geometric representation between finite element mesh and CAD model. This thesis propose a fully automated and topologically accurate approach to synthesise structurally sound parametric CAD models from topology-optimised finite element models to fill the long-existing gap between structural optimisation and CAD systems. This approach successfully preserves the optimal structural performance during the mesh-CAD conversion. The solution provided in this thesis is to first convert the topology-optimised structure into a spatial frame structure and then to regenerate it in a CAD system using standard constructive solid geometry (CSG) operations. The obtained parametric CAD models are compact, that is, have as few as possible geometric parameters, which makes them ideal for editing and further processing within a CAD system. The critical task of converting the topology-optimised structure into an optimal spatial frame structure is accomplished in several steps. The first step is to generate a one-voxel-wide voxel chain model from the topology-optimised voxel model using a topology-preserving skeletonisation algorithm from digital topology. The undirected graph defined by the voxel chain model yields a spatial frame structure after processing it with the proposed graph algorithms. Subsequently, the cross-sections and layout of the frame members are optimised to recover its optimality, which may have been compromised during the conversion process. At last, the obtained frame structure is generated in a CAD system by repeatedly combining primitive solids, like cylinders and spheres, using boolean operations. The resulting solid model is a boundary representation (B-Rep) consisting of trimmed non-uniform rational B-spline (NURBS) curves and surfaces. The numerical studies in this thesis clarify that the converted spatial frame structures are with equivalent structural performance. Moreover, CAD models generated from the spatial frame structures have significantly fewer geometric degree of freedom compared to the topology-optimised structures. Though the numerical studies use topology-optimised structures as input and compact CSG models as output, this thesis also provides the way to extend the proposed generation process to taking other optimised meshes and producing outputs of various geometric representations. This offers a wide range of possible applications and brings new thoughts to industrial design and manufacturing.Chinese Scholarship Counci

Merging neutron star and black hole binaries: Inference of their parameters and simulations of their formation and fate

In recent years, the growing numbers of black hole and neutron star merger candidates observed by the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo gravitational-wave observatories are rapidly expanding the frontiers of astrophysics. The observations enable (i) direct measurements of properties of these compact objects with information extraction from the gravitational-wave data, and seek the understanding of (ii) mechanisms by which the close compact object binaries come into existence and (iii) the astrophysical processes that take place after they merge. This thesis presents work on all these three fronts (i) We present measurements of properties of the binary neutron star and black hole observations from the LIGO-Virgo observatories\u27 second observing run, using Bayesian parameter estimation on the gravitational-wave data. During this observing run, LIGO-Virgo for the first time reported observations of gravitational waves from a binary neutron star inspiral, GW170817. This same source was also observed across the full electromagnetic spectrum. We combine gravitational-wave observations with a physical constraint on the component stars\u27 equation of state and information from electromagnetic observations, to measure tidal deformabilities and radii of the neutron stars in the source binary. (ii) We explore the ``common envelope\u27\u27 phase in the lives of binary stars in our universe. Common envelope is proposed to be the most probable mechanism of assembly of close compact object binaries. We present three-dimensional hydrodynamic simulations to model these episodes and discuss our understanding of the effect of this phase on the observable properties---such as masses and spins---of LIGO-Virgo\u27s stellar mass black hole populations. (iii) We discuss the aftermath of compact object mergers where at least one of the components is a neutron star. We use three-dimensional General Relativistic Magnetohydrodynamic simulations to model one of the typical outcomes---black hole surrounded by matter in the form of an accretion disk---for a variety of merger scenarios. We present connections of the binary parameters to properties of the disks, and the nucleosythetic yields they produce. Using the simulation results, we predict properties of kilonova emissions from future neutron star mergers

The Active Tail in the MMS Era: An Ion Perspective

The Earth\u27s magnetic field has a complex and dynamic relationship with the greater solar system. The solar wind and interplanetary magnetic field extend the influence of the Sun\u27s atmosphere to the orbit of Earth and well beyond, carrying charged particles in a constant stream of varying density and velocity. These solar influences carry energy which interacts with every object they encounter, including the Earth and its magnetic field. The primary mechanism for the energetic interaction and exchange of energy between the Earth\u27s magnetic field and the solar wind is called Magnetic Reconnection, a process by which two opposing magnetic fields may cancel each other in a limited region and allow the plasma restrained by each to cross the boundary between magnetic fields and interact. The effects of this interactions are as varied as they are wonderful, including the aurora, intercontinental radio communications, and threats to orbiting satellites. As such, understanding magnetic reconnection and its effects is an important task for space science research. This work is devoted to characterizing and identifying magnetic reconnection region in one part of the Earth\u27s magnetosphere, the magnetotail, as well as the conditions in the magnetotail necessary for reconnection to begin. This is done through the analysis of data from the Magnetospheric Multi-Scale Mission, a fleet of four identical orbiting observatories designed specifically to study reconnection. Methods to identify reconnection derived from historical assumptions as well relatively new techniques, so-called Scalar Parameters, are employed and compared. Finally, a combination of these methods is brought to bear in an attempt to understand why magnetic reconnection in the magnetotail occurs more often in some locations than others

Repulsion of determinantal point processes and stationary Poisson tessellations in high dimensions

In this dissertation, new results on stochastic geometric models in high dimensional space are presented. We first concentrate on a particular class of repulsive point processes called determinantal point processes (DPPs). We establish a coupling of a DPP and its reduced Palm version showing the repulsive effect of a point of the point process. This is used for discussing the degree of repulsiveness in DPPs, including Ginibre point processes and other specific parametric models for DPPs. We then study this repulsion for stationary DPPs in high dimensional Euclidean space. It is shown that for many families of DPPs, a typical point has no repulsive effect with high probability for large space dimension n. It is also proved that for some DPPs there exists an R* such that the repulsive effect occurs at a distance of [square root] nR* with high probability for large n. This R* is interpreted as the asymptotic reach of repulsion of the DPP. Examples of DPPs exhibiting this behavior are presented and an application to high dimensional Boolean models is given. The second half of this dissertation examines zero cells of stationary Poisson tessellations. First, a stationary stochastic geometric model is proposed for analyzing one-bit data compression. The data is assumed to be an unconstrained stationary set, and each data point is compressed using one bit with respect to each hyperplane in a stationary and isotropic Poisson hyperplane tessellation. Size metrics of the zero cell of the tessellation are studied to determine how the intensity of hyperplanes must scale with dimension to ensure sufficient separation of different data by the hyperplanes or sufficient proximity of the data compressed together. The results have direct implications in compressive sensing and source coding. We then study the concentration of the norm of a random vector Y uniformly sampled in the centered zero cell of a stationary random tessellation in high dimensions. It is shown that for a stationary and isotropic Poisson-Voronoi tessellation, [mathematical equation] approaches one as the dimension approaches infinity. For a stationary and isotropic Poisson hyperplane tessellation, we prove that [mathematical equation] will be within a fixed range (R [subscript ℓ], R [subscript u]) with probability approaching one as dimension n tends to infinity.Mathematic

Ultrasonic phased array testing in the power generation industry : novel wedge development for the inspection of steam turbine blades roots

The thesis presented herein comprises of the work undertaken to research novel methods of Phased array ultrasonic inspection of complex steam turbine blade roots as found in the power generation industry. The research was conducted as part of the Engineering Doctorate scheme, administered by the Research Centre for Non-Destructive Evaluation (RCNDE), in conjunction with RWE npower and the University of Warwick. Steam turbine blades, and in particularly last stage blades of low pressure steam turbines, are amongst the most highly stressed components on a power generating plant. Two of the most common blade root fixing types include ‘curved axial entry fir tree roots’ (CAEFTR), and axial pinned roots, both of which are prone to cracking due to the high stresses to which they are subjected under operating conditions. Failure of the blade root fixings of such components, leading to the release of the blades, has historically led to the catastrophic failure and destruction of the whole turbine; the cost of collateral damage to plant components and the loss in generating income are seconded only by the risk these failures pose to life. Due to the high price of failure, NDT plays a critical part in the support and management of engineering maintenance, offering insight into the condition and integrity of turbine components through regular planned inspection regimes. It will be shown in this thesis how the invention of a novel continuous wedge, used to refract ultrasound into the critical regions of the blade roots, has significantly improved the ability to detect defects. Combined with the development of bespoke scanning frames these wedges facilitate the efficient and accurate acquisition of scanned data to assess the integrity of the component. By combining the latest reverse engineering, modelling and simulation tools with novel application of rapid prototyping, the author has been able to demonstrate significant reduction in design cycles whilst improving accuracy, sensitivity and repeatability of the applied inspections. Furthermore, application of this design philosophy has led to the development of inspection techniques which have facilitated the inspection of remote regions of the blade roots where manual access is limited or impossible. The developments and techniques invented during this research have been successfully deployed across numerous RWE npower and customer projects, leading to estimated savings in excess of £1m

HAPTIC AND VISUAL SIMULATION OF BONE DISSECTION

Marco AgusIn bone dissection virtual simulation, force restitution represents the key to realistically mimicking a patient– specific operating environment. The force is rendered using haptic devices controlled by parametrized mathematical models that represent the bone–burr contact. This dissertation presents and discusses a haptic simulation of a bone cutting burr, that it is being developed as a component of a training system for temporal bone surgery. A physically based model was used to describe the burr– bone interaction, including haptic forces evaluation, bone erosion process and resulting debris. The model was experimentally validated and calibrated by employing a custom experimental set–up consisting of a force–controlled robot arm holding a high–speed rotating tool and a contact force measuring apparatus. Psychophysical testing was also carried out to assess individual reaction to the haptic environment. The results suggest that the simulator is capable of rendering the basic material differences required for bone burring tasks. The current implementation, directly operating on a voxel discretization of patientspecific 3D CT and MR imaging data, is efficient enough to provide real–time haptic and visual feedback on a low–end multi–processing PC platform.