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Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

This thesis explores the queueing performance of a wireless communication system that transmits packets over a correlated erasure channel using the IEEE 802.11 protocol suit. The channel states and the queue length together form a Markov chain. Exploiting this mathematical structure, the probability of the queue exceeding a certain threshold can be obtained. Most previous contributions in this area treat code-rate selection, channel erasure probability and network congestion separately. In this thesis, a simple integrated approach, which jointly considers these factors, is introduced. This approach becomes especially valuable for capturing the performance of delay-sensitive communication systems over time-varying channels. This thesis starts with a review of related work about correlated bit-erasure wireless channel models. A numerical study is then conducted to demonstrate the importance of optimizing overall system performance, and how this process impacts error-control coding at the physical layer. Following this exercise, a packet-erasure channel model with a Poisson arrival process is analyzed. The Baum-Welch algorithm is subsequently presented as a means to estimate the parameters of wireless communication systems. Furthermore, a matrix geometric method for obtaining the stationary distribution of the ensuing Markov chain is discussed. This offers a new perspective on wireless communication in the context of delay-sensitive applications. To complement the analysis platform put forth in this work, illustrative numerical results are contained in the last section of the thesis. From these results, design guidelines for improving the performance of delay-sensitive wireless communication systems are established. Although these results are obtained under simplifying assumptions, the overall methodology applies to more general situations, especially for wide-band delay-sensitive wireless communication applications.

Wireless communication systems continue to face the challenge of time varying quality of the underlying communication channel. When a slow fading channel goes into a deep fade, the corresponding communication system might face successive decoding failures at the destination, and for delay-sensitive communication systems, this amounts to delays that are not desired. In such situations, it becomes a priority to get out of the deep fades. Many techniques and approaches are already available in the literature to counteract fading effects. This work is motivated by recent advances in fast reconfigurable antennas, which provide new means to change the statistical profile of fading channels, and hence reduce the probability of prolonged fades. Fast reconfigurable antennas are poised to improve overall performance, especially for delay-sensitive traffic in slow-fading environments. This potential enhanced performance motivates this study of the queueing behavior of point-to-point communication systems with reconfigurable antennas. We focus on finite-state channels with memory, and we analyze the queueing behavior of the wireless communication system over erasure channels, for a traditional system versus a reconfigurable antenna implementation. We provide numerical results for situations where using reconfigurable antennas yield substantial performance gains in terms of throughput, delay and buffer overflow.

The simulation and design of advanced materials for fatigue resistance requires an understanding of the response of their hierarchical microstructure attributes to imposed load, temperature, and environment over time. For Ni-base superalloy components used in aircraft jet turbine engines, different competing mechanisms (ex. surface vs. subsurface, crystallographic vs. inclusion crack formation, transgranular vs. intergranular propagation) are present depending on applied load, temperature, and environment. Typically, the life-limiting features causing failure in Ni-base superalloy components are near surface inclusions. Compressive surface residual stresses are often introduced in Ni-base superalloy components to help retard fatigue crack initiation and early growth at near surface inclusions and shift the fatigue crack initiation sites from surface to sub-surface locations, thereby increasing fatigue life. To model the effects of residual stresses, inclusions, and microstructure heterogeneity on fatigue crack driving force and fatigue scatter, a computational crystal plasticity framework is presented that imposes quasi-thermal eigenstrain to induce near surface residual stresses in polycrystalline Ni-base superalloy IN100 smooth specimens with and without nonmetallic inclusions. In addition, the effect of near surface inclusions in notched Ni-base superalloy components on MSC growth and fatigue life scatter was investigated in this work. A fatigue indicator parameter (FIP)-based microstructurally small crack (MSC) growth model incorporating crack tip/grain boundary effects was introduced and fit to experiments (in both laboratory air and vacuum) for the case of 1D crack growth and then computationally applied to 3D crack growth starting (1) from a focused ion beam (FIB) notch in a smooth specimen, (2) from a debonded inclusion located at different depths within notched components containing different notch root radii, and (3) from inclusions located at different depths relative to the surface in smooth specimens containing simulated shot peened induced residual stresses. Computational predictions in MSC growth rate scatter and distribution of fatigue life were in general accordance with experiments. The general approach presented in this Dissertation can be used to advance integrated computational materials engineering (ICME) by predicting variation of fatigue resistance and minimum life as a function of heat treatment/microstructure and surface treatments for a given alloy system and providing support for design of materials for enhanced fatigue resistance. In addition, this framework can reduce the number of experiments required to support modification of material to enhance fatigue resistance, which can lead to accelerated insertion (from design conception to production parts) of new or improved materials for specific design applications. Elements of the framework being advanced in this research can be applied to any engineering alloy.

This study has created a GIS model and comprehensive analysis of environmentally sensitive areas in Delaware County, Indiana. Values were assigned to environmentally sensitive areas for four categories, including woodlands, wetlands, floodplains, and threatened and endangered species. There was an inverse relationship between the size of an area and the environmental sensitivity of the area. These areas occupy twenty-three percent of the total county area. The distribution of these areas is almost equal throughout the county; however, a large portion is located along the banks of the White River and other water bodies. Forty two soil types were identified within environmentally sensitive areas. Poorly drained soils are slightly more represented in the environmentally sensitive areas, and somewhat poorly drained soils are under-represented compared with soils in other drainage classes. / Department of Natural Resources and Environmental Management

Despite their acknowledged efficacy, currently available antidepressants still demonstrate undesirable side effects, shortfalls in effectiveness and a delayed onset of action. All these agents act via monoaminergic mechanisms, although recent studies have begun to note the potential role of the cholinergic system as well as the amino acid pathways in affective isorders. It has been suggested that glutamate NMDA receptor activation may be involved in hippocampal degeneration seen in patients with depression, as well as contributing as a molecular target for the antidepressant action of known antidepressant drugs. Glutamate either separately or via the release of nitric oxide, regulates the release of various transmitters in the brain critical for affective state, e.g. monoamines (noradrenaline, dopamine), indoleamines (5HT), y-aminobutyric acid (GABA) and acetylcholine. The aim of this study was to investigate N-methyl-D-aspartate (I\IMDA) and muscarinic M1 receptor characteristics and also GABA and acetylcholine levels in a genetic animal model of depression, the Flinders Sensitive Line (FSL) rat, with respect to its control, viz. Flinders Resistant Line (FRL) rat, thereby establishing a possible role for the amino acid and cholinergic pathways in the hippocampus and frontal cortex, two brain areas implicated in depression. In addition, anxietylike behaviours were assessed using the open field and social interaction tests. A sensitive liquid chromatography tandem mass spectrometer (LC/MS/MS) method was used in the quantification of acetylcholine as well as high performance liquid chromatography with electrochemical detection (HPLG-EGD) for the quantification of GABA in the above-mentioned brain areas of FSL and FRL rats. NMDA and muscarinic M1 receptor characteristics were expressed in terms of receptor denSity (Bmax) and affinity (Kd) values and were performed using [3H]-MK801 (27.5 Gi/mmol) and quinuclidinyl benzilate (52.0 Gilmmol) for NMDA and M1 receptors, respectively. In addition, to provide evidence for face validity, behavioural assessments were routinely performed using the open field test and social interaction test. Significantly elevated levels of acetylcholine were found in the frontal cortex but with significantly reduced levels in the hippocampus of FSL rats. Cortical and hippocampal muscarinic receptor binding characteristics remained unchanged, while no differences with regard to GABA levels and NMDA receptor binding characteristics were noted in these brain areas. In concordance with studies from the literature, aversive and locomotor behaviour as measured in the open field test, provided evidence of anxiogenic behaviour in the FSL rat, evinced by significantly less social interaction than their FRL counterparts. In addition, evidence for a lack in general activity of the FSL rat in the open field was also noted. Our data therefore suggest the presence of a cholinergic dysfunction in both the frontal cortex and hippocampus of the FSL rat, although this is not accompanied by simultaneous changes in muscarinic M1 receptor binding in key limbic brain regions. Although increased cholinergic drive is a recognised characteristic of FSL rats and is representative of the model's' construct validity, we suggest that the depressive phenotype of these animals is not related to altered cholinergic activity in a single brain region, but instead involves various limbic brain regions, possibly being more dependent on opposing cholinergic deficits in the cortex and hippocampus. / Thesis (M.Sc. (Pharmacology)--North-West University, Potchefstroom Campus, 2009.

Motivation: Ischemic heart disease is the eighth leading cause of death in an African population. Silent ischemia can be defined as an ischemic episode without associated pain. The clinical significance of silent ischemia is growing and can now be considered as a risk factor in the development of coronary disease. Hypertension and associated risk factors, hypercholesterolemia and diabetes are associated with silent ischemia. Other factors such as higher pulse pressure, double product, heart rate and higher carotid intima-media thickness are also associated with silent ischemia. Urbanisation is rising in South-Africa. This new lifestyle is associated with several risk factors including: poor diets, lower physical activity levels, hypertension and increased smoking and alcohol abuse. The prevalence of stroke is high among Africans, which can be due to a higher prevalence of hypertension, diabetes and obesity. Purpose: The purpose of this study was to determine the associations between silent ischemia and cardiovascular function in African men. The focus fell on hypertension and associated risk factors, higher total cholesterol levels, and increased pulse pressure, heart rate and sub-clinical atherosclerosis. vi Methodology: This study constituted a population study in the North-West province carried out on urbanized African male teachers aged between 20-60 years. The SABPA (Sympathetic activity and Ambulatory Blood Pressure in Africans) sub-study consisted of a total of 80 African male volunteers. The Cardiotens apparatus was placed on each participant on the first morning. This apparatus took ambulatory blood pressure measurements as well as Electrocardiogram measurements. Hereafter, participants continued with their normal work day until 1700. After an overnight stay at the Metabolic unit of the North-West University Potchefstroom campus, the apparatus was removed at 0600. During statistical analyses, the African males were divided into groups of participants with silent ischemia (SI) and those without silent ischemia (nSI), as determined by the ambulatory electrocardiogram. Statistical analyses were performed by means of the Statistica version 10 software program. Results: In comparison with the nSI men, the SI showed the following: above normal high sensitivity C-reactive protein and glucose, higher ambulatory blood pressure, heart rate, pulse pressure, resting ST-segment depression and carotid intima-media thickness. Multiple regression analyses indicated that ambulatory silent ischemia is associated with sub-clinical atherosclerosis, possibly increasing their stroke risk. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2012.

The main focus of this study was to explore how personal constructions, consisting of latent and conscious schema, and obtained from a culturally sensitive interpretation of the RCS, informed the transactional resilience of Black South African adolescents. This study was motivated in view of limited knowledge available about the (i) processes that are generic to the resilience-promoting transactions of Black South African adolescents and (ii) to potentiate insight into an adolescent’s construction of personal meaning of her conscious and unconscious experiences that may not always be easily recognised in her overt behaviour or by self-report measures often used in South African resilience research. Six Black South African adolescents aged 14 to 16 years volunteered to take part in this multiple case study. The participants were identified as resilient by an advisory panel consisting of learners and educators at an English-medium school in the Vaal Triangle area. In this essentially qualitative study, qualitative data obtained from an unstructured individual interview, unstructured observations, and a follow-up interview were integrated with the data obtained from a culturally sensitive, conceptual interpretation of these Black adolescents’ Rorschach protocols. The structural, quantitative data obtained from specific indicators in Exner’s Comprehensive System were interpreted in a culturally sensitive manner and integrated with the qualitative data obtained from the Rorschach protocols. A culturally sensitive, conceptual framework for interpreting Rorschach indicators associated with adolescents’ transactional resilience was provided. These findings indicated individual and ecological protective resources well known within South African resilience research. Findings that contributed new understanding of the transactional processes associated with Black South African adolescent resilience were also obtained. Four case-specific self-reflective strategies were identified, namely, emotional stoicism, frequent introspection, honouring the past, and adopting a new identity. These selfreflective strategies served as the participants’ unique ways of compensating for their adversity-informed schema as well as encouraging them to navigate towards the sustained support of specific significant others and resilience-promoting ecological resources. The participants indicated that attachment challenges brought about repressed feelings and specific security needs, which shaped their resilience-promoting navigation and enabled them to self-knit in a resilient way. / Thesis (Ph.D. (Education))--North-West University, Vaal Triangle Campus, 2011