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The Global ETD Search service is a free service for researchers
to find electronic theses and dissertations. This service is
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Networked Digital Library of Theses and Dissertations.
Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
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Showing 491 to 498 of 498 (0.037 seconds)Spelling suggestions: "subject:"space time"" "subject:"space lime""
| 491 |
Precoder Design Based on Mutual Information for Non-orthogonal Amplify and Forward Wireless Relay NetworksSyed, Tamseel Mahmood 09 June 2014 (has links)
No description available.
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| 492 |
An Experimental Investigation on Waves and Coherent Structures in a Three-Dimensional Open Cavity Flow / Étude Expérimentale des Ondes et Structures Cohérentes dans un Écoulement Tridimensionnel de Cavité Ouverte.Basley, Jérémy 19 October 2012 (has links)
Une écoulement de cavité ouverte tridimensionnel saturé non-linéairement est étudié par une approche spatio-temporelle utilisant des données expérimentales résolues à la fois en temps et en espace. Ces données ont été acquises dans deux plans longitudinaux, respectivement perpendiculaire et parallèle au fond de la cavité, dans le régime incompressible, en air ou en eau. À l'aide de multiples méthodes de décompositions globales en temps et en espace, les ondes et les structures cohérentes constituant la dynamique dans le régime permanent et pouvant être produites par des mécanismes d'instabilités différents sont identifiées et caractérisées.Tout d'abord, on approfondit la compréhension de l'effet des non-linéarités sur les oscillations auto-entretenues de la couche cisaillée impactante et leurs interactions avec l'écoulement intra-cavitaire. En particulier, l'analyse spectrale d'une portion de l'espace des paramètres permet de mettre en évidence un lien entre l'accrochage des modes d'oscillations auto-entretenues, la modulation d'amplitude au niveau du coin impactant et l'intermittence de ces modes. De plus, l'observation des basses fréquences intéragissant fortement avec les oscillations de la couche de mélange démontre l'existence d'une dynamique tridimensionnelle intrinsèque à l'intérieur de la cavité malgré les perturbations causées par la couche cisaillée instable.Les analyses de stabilité linéaire ont montré que des instabilités centrifuges peuvent résulter de la courbure induite par la recirculation. L'étude de la dynamique après saturation révèle de nombreuses structures cohérentes dont les propriétés sont quantifiées et classées en s'appuyant sur la forme des instabilités sous-jacentes: des ondes transverses progressives ou stationnaires. Enfin, certains comportements des structures saturées suggèrent que les mécanismes non-linéaires gouvernant le développement de l'écoulement une fois sorti du régime linéaire pourraient être étudiés dans le cadre des équations d'amplitude. / A space-time study of a three-dimensional nonlinearly saturated open cavity flow is undertaken using time-resolved space-extended experimental data, acquired in both cross-stream and spanwise planes, in incompressible air and water flows. Through use of multiple modal decompositions in time and space, the waves and coherent structures composing the dynamics in the permanent regime are identified and characterised with respect to the instabilities arising in the flow.Effects of nonlinearities are thoroughly investigated in the impinging shear layer, regarding the self-sustained oscillations and their interactions with the inner-flow. In particular, the analysis conducted throughout the parameter space enlightens a global connection between the selection of locked-on modes and the amplitude modulation at the impingement and the mode switching phenomenon. Furthermore, observations of low frequencies interacting drastically with the shear layer flapping motion underline the existence of intrinsic coherent three-dimensional dynamics inside the cavity in spite of the shear layer disturbances.Linear stability analyses have demonstrated that centrifugal instabilities are at play along the main recirculation. The present investigation of the dynamics after onset of the saturation reveals numerous space-time coherent structures, whose properties are quantified and classified with respect to the underlying instabilities: travelling or standing spanwise waves. Finally, some patterns exhibited by the saturated structures suggest that the nonlinear mechanisms governing the mutations of the flow after the linear regime could gain more insight in the frame of amplitude equations.
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| 493 |
Modelling of Mobile Fading Channels with Fading Mitigation Techniques.Shang, Lei, lei.shang@ieee.org January 2006 (has links)
This thesis aims to contribute to the developments of wireless communication systems. The work generally consists of three parts: the first part is a discussion on general digital communication systems, the second part focuses on wireless channel modelling and fading mitigation techniques, and in the third part we discuss the possible application of advanced digital signal processing, especially time-frequency representation and blind source separation, to wireless communication systems. The first part considers general digital communication systems which will be incorporated in later parts. Today's wireless communication system is a subbranch of a general digital communication system that employs various techniques of A/D (Analog to Digital) conversion, source coding, error correction, coding, modulation, and synchronization, signal detection in noise, channel estimation, and equalization. We study and develop the digital communication algorithms to enhance the performance of wireless communication systems. In the Second Part we focus on wireless channel modelling and fading mitigation techniques. A modified Jakes' method is developed for Rayleigh fading channels. We investigate the level-crossing rate (LCR), the average duration of fades (ADF), the probability density function (PDF), the cumulative distribution function (CDF) and the autocorrelation functions (ACF) of this model. The simulated results are verified against the analytical Clarke's channel model. We also construct frequency-selective geometrical-based hyperbolically distributed scatterers (GBHDS) for a macro-cell mobile environment with the proper statistical characteristics. The modified Clarke's model and the GBHDS model may be readily expanded to a MIMO channel model thus we study the MIMO fading channel, specifically we model the MIMO channel in the angular domain. A detailed analysis of Gauss-Markov approximation of the fading channel is also given. Two fading mitigation techniques are investigated: Orthogonal Frequency Division Multiplexing (OFDM) and spatial diversity. In the Third Part, we devote ourselves to the exciting fields of Time-Frequency Analysis and Blind Source Separation and investigate the application of these powerful Digital Signal Processing (DSP) tools to improve the performance of wireless communication systems.
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| 494 |
Weak nonergodicity in anomalous diffusion processesAlbers, Tony 23 November 2016 (has links)
Anomale Diffusion ist ein weitverbreiteter Transportmechanismus, welcher für gewöhnlich mit ensemble-basierten Methoden experimentell untersucht wird.
Motiviert durch den Fortschritt in der Einzelteilchenverfolgung, wo typischerweise Zeitmittelwerte bestimmt werden, entsteht die Frage nach der Ergodizität.
Stimmen ensemble-gemittelte Größen und zeitgemittelte Größen überein, und wenn nicht, wie unterscheiden sie sich?
In dieser Arbeit studieren wir verschiedene stochastische Modelle für anomale Diffusion bezüglich ihres ergodischen oder nicht-ergodischen Verhaltens hinsichtlich der mittleren quadratischen Verschiebung.
Wir beginnen unsere Untersuchung mit integrierter Brownscher Bewegung, welche von großer Bedeutung für alle Systeme mit Impulsdiffusion ist.
Für diesen Prozess stellen wir die ensemble-gemittelte quadratische Verschiebung und die zeitgemittelte quadratische Verschiebung gegenüber und charakterisieren insbesondere die Zufälligkeit letzterer.
Im zweiten Teil bilden wir integrierte Brownsche Bewegung auf andere Modelle ab, um einen tieferen Einblick in den Ursprung des nicht-ergodischen Verhaltens zu bekommen.
Dabei werden wir auf einen verallgemeinerten Lévy-Lauf geführt.
Dieser offenbart interessante Phänomene, welche in der Literatur noch nicht beobachtet worden sind.
Schließlich führen wir eine neue Größe für die Analyse anomaler Diffusionsprozesse ein, die Verteilung der verallgemeinerten Diffusivitäten, welche über die mittlere quadratische Verschiebung hinausgeht,
und analysieren mit dieser ein oft verwendetes Modell der anomalen Diffusion, den subdiffusiven zeitkontinuierlichen Zufallslauf. / Anomalous diffusion is a widespread transport mechanism, which is usually experimentally investigated by ensemble-based methods.
Motivated by the progress in single-particle tracking, where time averages are typically determined, the question of ergodicity arises.
Do ensemble-averaged quantities and time-averaged quantities coincide, and if not, in what way do they differ?
In this thesis, we study different stochastic models for anomalous diffusion with respect to their ergodic or nonergodic behavior concerning the mean-squared displacement.
We start our study with integrated Brownian motion, which is of high importance for all systems showing momentum diffusion.
For this process, we contrast the ensemble-averaged squared displacement with the time-averaged squared displacement and, in particular, characterize the randomness of the latter.
In the second part, we map integrated Brownian motion to other models in order to get a deeper insight into the origin of the nonergodic behavior.
In doing so, we are led to a generalized Lévy walk.
The latter reveals interesting phenomena, which have never been observed in the literature before.
Finally, we introduce a new tool for analyzing anomalous diffusion processes, the distribution of generalized diffusivities, which goes beyond the mean-squared displacement, and we analyze with this tool an often used model of anomalous diffusion, the subdiffusive continuous time random walk.
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| 495 |
Channel Modeling Applied to Robust Automatic Speech RecognitionSklar, Alexander Gabriel 01 January 2007 (has links)
In automatic speech recognition systems (ASRs), training is a critical phase to the system?s success. Communication media, either analog (such as analog landline phones) or digital (VoIP) distort the speaker?s speech signal often in very complex ways: linear distortion occurs in all channels, either in the magnitude or phase spectrum. Non-linear but time-invariant distortion will always appear in all real systems. In digital systems we also have network effects which will produce packet losses and delays and repeated packets. Finally, one cannot really assert what path a signal will take, and so having error or distortion in between is almost a certainty. The channel introduces an acoustical mismatch between the speaker's signal and the trained data in the ASR, which results in poor recognition performance. The approach so far, has been to try to undo the havoc produced by the channels, i.e. compensate for the channel's behavior. In this thesis, we try to characterize the effects of different transmission media and use that as an inexpensive and repeatable way to train ASR systems.
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| 496 |
Performance evaluation and protocol design of fixed-rate and rateless coded relaying networksNikjah, Reza 06 1900 (has links)
The importance of cooperative relaying communication in substituting for, or complementing,
multiantenna systems is described, and a brief literature review is presented.
Amplify-and-forward (AF) and decode-and-forward (DF) relaying are investigated and
compared for a dual-hop relay channel. The optimal strategy, source and relay optimal
power allocation, and maximum cooperative gain are determined for the relay channel. It
is shown that while DF relaying is preferable to AF relaying for strong source-relay links,
AF relaying leads to more gain for strong source-destination or relay-destination links.
Superimposed and selection AF relaying are investigated for multirelay, dual-hop relaying.
Selection AF relaying is shown to be globally strictly outage suboptimal. A necessary
condition for the selection AF outage optimality, and an upper bound on the probability of
this optimality are obtained. A near-optimal power allocation scheme is derived for superimposed
AF relaying.
The maximum instantaneous rates, outage probabilities, and average capacities of multirelay,
dual-hop relaying schemes are obtained for superimposed, selection, and orthogonal
DF relaying, each with parallel channel cooperation (PCC) or repetition-based cooperation
(RC). It is observed that the PCC over RC gain can be as much as 4 dB for the outage
probabilities and 8.5 dB for the average capacities. Increasing the number of relays deteriorates
the capacity performance of orthogonal relaying, but improves the performances of
the other schemes.
The application of rateless codes to DF relaying networks is studied by investigating
three single-relay protocols, one of which is new, and three novel, low complexity multirelay
protocols for dual-hop networks. The maximum rate and minimum energy per bit and
per symbol are derived for the single-relay protocols under a peak power and an average
power constraint. The long-term average rate and energy per bit, and relay-to-source usage
ratio (RSUR), a new performance measure, are evaluated for the single-relay and multirelay
protocols. The new single-relay protocol is the most energy efficient single-relay scheme
in most cases. All the multirelay protocols exhibit near-optimal rate performances, but are
vastly different in the RSUR.
Several future research directions for fixed-rate and rateless coded cooperative systems,
and frameworks for comparing these systems, are suggested. / Communications
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| 497 |
Performance evaluation and protocol design of fixed-rate and rateless coded relaying networksNikjah, Reza Unknown Date
No description available.
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| 498 |
GIS-based Episode Reconstruction Using GPS Data for Activity Analysis and Route Choice Modeling / GIS-based Episode Reconstruction Using GPS DataDalumpines, Ron 26 September 2014 (has links)
Most transportation problems arise from individual travel decisions. In response, transportation researchers had been studying individual travel behavior – a growing trend that requires activity data at individual level. Global positioning systems (GPS) and geographical information systems (GIS) have been used to capture and process individual activity data, from determining activity locations to mapping routes to these locations. Potential applications of GPS data seem limitless but our tools and methods to make these data usable lags behind. In response to this need, this dissertation presents a GIS-based toolkit to automatically extract activity episodes from GPS data and derive information related to these episodes from additional data (e.g., road network, land use).
The major emphasis of this dissertation is the development of a toolkit for extracting information associated with movements of individuals from GPS data. To be effective, the toolkit has been developed around three design principles: transferability, modularity, and scalability. Two substantive chapters focus on selected components of the toolkit (map-matching, mode detection); another for the entire toolkit. Final substantive chapter demonstrates the toolkit’s potential by comparing route choice models of work and shop trips using inputs generated by the toolkit.
There are several tools and methods that capitalize on GPS data, developed within different problem domains. This dissertation contributes to that repository of tools and methods by presenting a suite of tools that can extract all possible information that can be derived from GPS data. Unlike existing tools cited in the transportation literature, the toolkit has been designed to be complete (covers preprocessing up to extracting route attributes), and can work with GPS data alone or in combination with additional data. Moreover, this dissertation contributes to our understanding of route choice decisions for work and shop trips by looking into the combined effects of route attributes and individual characteristics. / Dissertation / Doctor of Philosophy (PhD)
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