When Does Relay Transmission Give a More Secure Connection in Wireless Ad Hoc Networks?

Relay transmission can enhance coverage and throughput, while it can be vulnerable to eavesdropping attacks due to the additional transmission of the source message at the relay. Thus, whether or not one should use relay transmission for secure communication is an interesting and important problem. In this paper, we consider the transmission of a confidential message from a source to a destination in a decentralized wireless network in the presence of randomly distributed eavesdroppers. The source-destination pair can be potentially assisted by randomly distributed relays. For an arbitrary relay, we derive exact expressions of secure connection probability for both colluding and non-colluding eavesdroppers. We further obtain lower bound expressions on the secure connection probability, which are accurate when the eavesdropper density is small. By utilizing these lower bound expressions, we propose a relay selection strategy to improve the secure connection probability. By analytically comparing the secure connection probability for direct transmission and relay transmission, we address the important problem of whether or not to relay and discuss the condition for relay transmission in terms of the relay density and source-destination distance. These analytical results are accurate in the small eavesdropper density regime.Comment: Accepted for publication in IEEE Transactions On Information Forensics and Securit

Towards Scalable Nanomanufacturing: Modeling The Interaction Of Charged Droplets From Electrospray Using Gpu

Electrospray is an atomization method subject to intense study recently due to its monodispersity and the wide size range of droplets it can produce, from nanometers to hundreds of micrometers. This thesis focuses on the numerical and theoretical modeling of the interaction of charged droplets from the single and multiplexed electrospray. We studied two typical scenarios: large area film depositions using multiplexed electrospray and fine pattern printings assisted by linear electrostatic quadrupole focusing. Due to the high computation power requirement in the unsteady n-body problem, graphical processing unit (GPU) which delivers 10 Tera flops in computation power is used to dramatically speed up the numerical simulation both efficiently and with low cost. For large area film deposition, both the spray profile and deposition number density are studied for different arrangements of electrospray and electrodes. Multiplexed electrospray with hexagonal nozzle configuration can not give us uniform deposition though it has the highest packing density. Uniform film deposition with variation \u3c 5% in thickness was observed with the linear nozzle configuration combined with relative motion between ES source and deposition substrate. For fine pattern printing, linear quadrupole is used to focus the droplets in the radial direction while maintaining a constant driving field at the axial direction. Simulation shows that the linear quadrupole can focus the droplets to a resolution of a few nanometers quickly when the interdroplet separation is larger than a certain value. Resolution began to deteriorate drastically when the inter-droplet separation is smaller than that value. This study will shed light on using electrospray as a scalable nanomanufacturing approach

Electro- and photocatalytic carbon dioxide reduction and thermal isomerization of highly strained tricyclocompounds

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic and photocatalytic carbon dioxide (CO2) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms whereby the cis conformer operates via cooperative bimetallic CO2 activation and conversion and the trans conformer reduces CO2 through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)3Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s−1) relative to the trans conformer (22.9 s−1) with both outperforming Re(bpy)(CO)3Cl (11.1 s−1). Photocatalytic reactivity shows the plausible pathway for dinuclear system is one site acting as an efficient covalently-linked photosensitizer to the second site performing CO2 reduction. The excited-state kinetics and emission spectra reveal that the anthracene backbone plays a more significant role beyond a simple structural unit. The isomerizations of 3 4-diazatricyclo[4.1.0.02 7]hept-3-ene and 3 4 diazatricyclo[4.1.0.02 7]heptane to their corresponding products were studied by ab initio calculations at multiconfiguration self-consistent field level to study the stabilization effects from π bond electrons and lone pair electrons. The isomerization of 3 4-diazatricyclo[4.1.0.02 7]hept-3-ene occurs through four unique pathways. The 12.2 kcal mol-1 disparity in the disrotatory barriers is explained through π electron delocalization in the transition state. The 3 4-diazatricyclo[4.1.0.02 7]heptane structure has eight separate reaction channels for isomerization. Resonance stabilization from lone pair electron for two of the forbidden pathways results in a relative energy lowering. The isomerization of benzvalene to benzene has barriers of 20.6 kcal mol-1 for the disrotatory channel and 26.8 kcal mol-1 for the conrotatory channel. The isomerization of benzvalene back to benzvalene occurs by the transition state disTSback with barrier of 29.8 kcal mol-1. For the isomerization of benzvalyne the barriers are 22.9 kcal mol-1 for disrotatory channel and 21.7 kcal mol-1 for conrotatory channel. Intrinsic reaction coordinate shows the 122.2 kcal mol-1 relative energy could be released only by 1.4 kcal mol-1 due to the absence of normally intermediate with trans double bond

Mapping the Relationships among the Cognitive Complexity of Independent Writing Tasks, L2 Writing Quality, and Complexity, Accuracy and Fluency of L2 Writing

Drawing upon the writing literature and the task-based language teaching literature, the study examined two cognitive complexity dimensions of L2 writing tasks: rhetorical task varying in reasoning demand and topic familiarity varying in the amount of direct knowledge of topics. Four rhetorical tasks were studied: narrative, expository, expo-argumentative, and argumentative tasks. Three topic familiarity tasks were investigated: personal-familiar, impersonal-familiar, and impersonal-less familiar tasks. Specifically, the study looked into the effects of these two cognitive complexity dimensions on L2 writing quality scores, their effects on complexity, accuracy, and fluency (CAF) of L2 production, and the predictive power of the CAF features on L2 writing scores for each task. Three hundred and seventy five Chinese university EFL students participated in the study, and each student wrote on one of the six writing tasks used to study the cognitive complexity dimensions. The essays were rated by trained raters using a holistic scale. Thirteen CAF measures were used, and the measures were all automated through computer tools. One-way ANOVA tests revealed that neither rhetorical task nor topic familiarity had an effect on the L2 writing scores. One-way MANOVA tests showed that neither rhetorical task nor topic familiarity had an effect on accuracy and fluency of the L2 writing, but that the argumentative essays were significantly more complex in global syntactic complexity features than the essays on the other rhetorical tasks, and the essays on the less familiar topic were significantly less complex in lexical features than the essays on the more familiar topics. All-possible subsets regression analyses revealed that the CAF features explained approximately half of the variance in the writing scores across the tasks and that writing fluency was the most important CAF predictor for five tasks. Lexical sophistication was however the most important CAF predictor for the argumentative task. The regression analyses further showed that the best regression models for the narrative task were distinct from the ones for the expository and argumentative types of tasks, and the best models for the personal-familiar task were distinct from the ones for the impersonal tasks

Response of Electrified Micro-Jets to Electrohydrodynamic Perturbations

The breakup of liquid jets is ubiquitous with rich underpinning physics and widespread applications. The natural breakup of liquid jets originates from small ambient perturbations, which can grow exponentially until the amplitude as large as the jet radius is reached. For unelectrified inviscid jets, surface energy analysis shows that only the axisymmetric perturbation is possibly unstable, and this mode is referred as varicose instability. For electrified jets, the presence of surface charge enables additional unstable modes, among which the most common one is the whipping (or kink) instability that bends and stretches the charged jet that is responsible for the phenomena of electrospinning. A closer examination of the two instabilities suggests that due to mass conservation, the uneven jet stretching from whipping may translate into radial perturbations and trigger varicose instabilities. Although the varicose and whipping instabilities of electrified micro-jets have both been extensively studied separately, there is little attention paid to the combined effect of these two, which may lead to new jet breakup phenomena. This dissertation investigates the dynamic response of electrified jets under transverse electrohydrodynamic (EHD) perturbations which were introduced by exciters driven by alternating voltage of sweeping frequency. Three different jetting mechanisms are used to generate jets with various ranges of jet diameters: ~150 micrometer inertial jets from liquid pressurized through a small orifice, ~50 micrometer flow focused jets, and ~20 micrometer electrified Taylor-cone jets. The transverse perturbations enable systematic triggering of varicose and whipping instabilities, and consequently a wide range of remarkable phenomena emerge. For inertial jets with zero or low charge levels, only varicose instability is observable due to suppressed whipping instability. At modest charge levels, inertia jets can respond to the fundamental perturbation frequency as well as the second harmonic of the perturbation frequency. Highly charged jets such as fine jets generated from Taylor cones exhibit distinct behavior for different perturbation wavenumber x. Typical behavior include: whipping jets with superimposed varicose instability at small x, jet bifurcation from crossover of whipping and varicose instabilities at x~0.5, Coulombic fission owing to the surge of surface charge density as the slender liquid segments recover spherical shapes at x~0.7, and simple varicose mode near wave numbers of unity. The phenomena observed in this work may be explained by a linear model and rationalized by the phase diagram in the space of wave number and dimensionless charge levels. The experimental apparatus used in this dissertation is simple, non-intrusive, and scalable to a linear array of jets. The rich phenomena combined with the versatile apparatus may spawn new research directions such as regulated electrospinning, generating strictly monodisperse micro/nano droplets, and manufacturing of non-spherical particles from drying droplets that undergo controlled Coulombic fissions

Market competition, efficiency and profitability: an empirical study on the Chinese banking industry 1997-2006

Since the economic reform was initiated in 1978, the Chinese banking sector has undergone significant changes, particularly during the period under our investigation. This is primarily induced by the WTO entry in 2001, which brought in to full openness the financial market in China. The ultimate objective of the recent banking reform is to promote competition and efficiency as a way of improving the overall competitiveness and banking performance, in order to cope with challenges from foreign competitors. With the purpose of examining whether the recent banking reform is effective in achieving the targets as well as suggesting future policy directions, this study investigates market competition, cost efficiency and profitability in the Chinese banking industry over those critical years (1997-2006) before and after the WTO entry. We first employ both structural (the SCP) and non-structural (the Panzar-Rosse) approach to evaluate market competition. Then we estimate cost efficiency for Chinese banks under the Stochastic Frontier Approach (SFA). Finally, we assess the relationship between profitability and market structure under the structure-performance hypothesis and the efficient-structure hypothesis. Our findings show that Chinese banking market become less concentrated and more competitive since the WTO entry. Chinese banks improve their cost efficiencies, with state-owned banks are the least efficient while joint equity banks are the most efficient. The explanation for the relationship between profitability and market structure is quite mixed. The acceptance of which hypothesis depends on which dependent variable is used