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The Global ETD Search service is a free service for researchers
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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 1 to 9 of 9 (0.077 seconds)| 1 |
Investigation of Methods for Arbitrarily Profiled Cylindrical Dielectric WaveguidesHong, Qing-long 07 July 2005 (has links)
Cylindrical dielectric waveguides such as the optical fiber and photonic crystal fiber are very important passive devices in optical communication systems. There are many kinds of commercial software and methods of simulation at present. In this thesis, we proposed the following four methods to analyze arbitrarily profiled cylindrical dielectric waveguides: The first two methods are modified from published work while the last two methods are entirely developed by ourselves.
1. Cylindrical ABCD matrix method: We take the four continuous electromagnetic field components as main variables and derive the exact four-by-four matrix (with Bessel functions) to relate the four field vector within each homogeneous layer. The electromagnetic field components of the inner and outer layer can propagate toward one of the selected interface of our choice by using the method of ABCD matrix. We can then solve for the £]-value of the waveguide mode with this nonlinear inhomogeneous matrix equation.
2. Runge-Kutta method: Runge-Kutta method is mostly used to solve the initial value problems of the differential equations. In this thesis, we introduce the Runge-Kutta method to solve the first-order four-by-four nonlinear differential equation of the electromagnetic field components and find the £]-value of the cylindrical dielectric waveguides in a similar way depicted in method one.
3. Coupled Ez and Hz method: It uses the axial electromagnetic filed components to solve cylindrical dielectric waveguides. The formulation is similar to cylindrical ABCD matrix method, but it requires less variables then cylindrical ABCD matrix method. The numerical solution obtained from this method is most stable, but it is more complicated to derive harder to write the program.
4. Simple basis expansion method: The simple trigonometric functions (sine or cosine) are chosen as the bases of the horizontal coupled magnetic field equation derived from the second-order differential equation of the transverse magnetic field components. We do not select the horizontal coupling electric field because the normal component of the electric field is discontinuous on the interface. But the normal and tangential components of the magnetic field are continuous across the interfaces. The modal solution problem is converted to a linear matrix eigenvalue-eigenvector equation which is solved by the standard linear algebra routines.
We will compare these four numerical methods with one another. The characteristics and advantage as well as the disadvantage of each method will be studied and compared in detail.
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An investigation into Functional Linear Regression ModelingEssomba, Rene Franck January 2015 (has links)
Functional data analysis, commonly known as FDA", refers to the analysis of information on curves of functions. Key aspects of FDA include the choice of smoothing techniques, data reduction, model evaluation, functional linear modeling and forecasting methods. FDA is applicable in numerous applications such as Bioscience, Geology, Psychology, Sports Science, Econometrics, Meteorology, etc. This dissertation main objective is to focus more specifically on Functional Linear Regression Modelling (FLRM), which is an extension of Multivariate Linear Regression Modeling. The problem of constructing a Functional Linear Regression modelling with functional predictors and functional response variable is considered in great details. Discretely observed data for each variable involved in the modelling are expressed as smooth functions using: Fourier Basis, B-Splines Basis and Gaussian Basis. The Functional Linear Regression Model is estimated by the Least Square method, Maximum Likelihood method and more thoroughly by Penalized Maximum Likelihood method. A central issue when modelling Functional Regression models is the choice of a suitable model criterion as well as the number of basis functions and an appropriate smoothing parameter. Four different types of model criteria are reviewed: the Generalized Cross-Validation, the Generalized Information Criterion, the modified Akaike Information Criterion and Generalized Bayesian Information Criterion. Each of these aforementioned methods are applied to a dataset and contrasted based on their respective results.
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Spectral Analysis Using Multitaper Whittle Methods with a Lasso PenaltyTang, Shuhan 25 September 2020 (has links)
No description available.
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Communication over Doubly Selective Channels: Efficient Equalization and Max-Diversity PrecodingHwang, Sung Jun 15 January 2010 (has links)
No description available.
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Bandwidth Efficiency and Power Efficiency Issues for Wireless TransmissionsChen, Ning 31 March 2006 (has links)
As wireless communication becomes an ever-more important and pervasive part of our everyday life, system capacity and quality of service issues are becoming more critical. In order to increase the system capacity and improve the quality of service, it is necessary that we pay closer attention to bandwidth and power efficiency issues.
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique for high speed data transmission and is generally regarded as bandwidth efficient. However, OFDM signals suffer from high peak-to-average power ratios (PARs) which lead to power inefficiency in the RF portion of the transmitter. Moreover, in OFDM, the well-known pilot tone assisted modulation (PTAM) technique utilizes a number of dedicated training pilots to acquire the channel state information (CSI), resulting in somewhat reduced bandwidth efficiency.
In this dissertation, we will address the above mentioned bandwidth and power efficiency issues in wireless transmissions. To avoid bandwidth efficiency loss due to dedicated training, we will first develop a superimposed training framework that can be used to track the frequency selective as well as the Doppler shift characteristics of a channel. Later on, we will propose a generalized superimposed training framework that allows improved channel estimates. To improve the power efficiency, we adopt the selected mapping (SLM) framework to reduce the PARs for both OFDM and forward link Code Division Multiple Access (CDMA). We first propose a dynamic SLM algorithm to greatly reduce the computational requirement of SLM without sacrificing its PAR reducing capability. We propose a number of blind SLM techniques for OFDM and for forward link CDMA; they require no side information and are easy to implement. Our proposed blind SLM technique for OFDM is a novel joint channel estimation and PAR reduction algorithm, for which bandwidth efficiency power efficiency - complexity - bit error rate tradeoffs are carefully considered.
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Dynamics of diatomic molecules in intense laser fields / Alignment, Ionization and Fragmentation of dimers / Die Dynamik zweiatomiger Moleküle in intensiven LaserfeldernUhlmann, Mathias 16 May 2006 (has links) (PDF)
A realistic description of ionization in intense laser fields is implemented into the Non-Adiabatic Quantum Molecular Dynamics (NA-QMD) formalism. First, the error of a finite basis expansion is considered and a new measure is proposed for time-dependent calculations. This is used to investigate systematically the influence of the used basis set in calculations on the hydrogen atom in intense laser fields. Second, absorbing boundary conditions in basis expansion are introduced via an imaginary potential into the effective one-particle Hamiltonian. It is shown that the used form of the absorber potential is valid in many-electron time-dependent density functional theory calculations, i.e. that only ionized states are affected by the absorbing potential. The absorber is then tested on reference calculations that exist for H and aligned H+2 in intense laser fields. Excellent agreement is found. Additionally, an approximative treatment of the missing electron-nuclear correlations is proposed. It is found in calculations on H+2 that a qualitative improvement of the description of nuclear dynamics results. The extension of the NA-QMD formalism is then used to investigate the alignment behavior of diatomic molecules. Recent experiments on H+2 and H2 are reviewed and explained. It is found that dynamic alignment, i.e. the laser induced rotation of the molecule, plays a central role. The alignment behavior of H+2 and H2 and its intensity dependence is investigated after that. A drastic difference between H+2 and H2 is found in NA-QMD as well as model calculations. Then, the focus is on an astonishing new effect that has been found in N2 calculations. This effect which is called "rotational destabilization" is studied on the model system H+2. Yet, it might be observable only in heavy dimers and might have already been found in an experiment on I2.
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Dynamics of diatomic molecules in intense laser fields: Alignment, Ionization and Fragmentation of dimers: Die Dynamik zweiatomiger Moleküle in intensiven LaserfeldernUhlmann, Mathias 16 June 2006 (has links)
A realistic description of ionization in intense laser fields is implemented into the Non-Adiabatic Quantum Molecular Dynamics (NA-QMD) formalism. First, the error of a finite basis expansion is considered and a new measure is proposed for time-dependent calculations. This is used to investigate systematically the influence of the used basis set in calculations on the hydrogen atom in intense laser fields. Second, absorbing boundary conditions in basis expansion are introduced via an imaginary potential into the effective one-particle Hamiltonian. It is shown that the used form of the absorber potential is valid in many-electron time-dependent density functional theory calculations, i.e. that only ionized states are affected by the absorbing potential. The absorber is then tested on reference calculations that exist for H and aligned H+2 in intense laser fields. Excellent agreement is found. Additionally, an approximative treatment of the missing electron-nuclear correlations is proposed. It is found in calculations on H+2 that a qualitative improvement of the description of nuclear dynamics results. The extension of the NA-QMD formalism is then used to investigate the alignment behavior of diatomic molecules. Recent experiments on H+2 and H2 are reviewed and explained. It is found that dynamic alignment, i.e. the laser induced rotation of the molecule, plays a central role. The alignment behavior of H+2 and H2 and its intensity dependence is investigated after that. A drastic difference between H+2 and H2 is found in NA-QMD as well as model calculations. Then, the focus is on an astonishing new effect that has been found in N2 calculations. This effect which is called "rotational destabilization" is studied on the model system H+2. Yet, it might be observable only in heavy dimers and might have already been found in an experiment on I2.
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Ab-initio molecular dynamics studies of laser- and collision-induced processes in multielectron diatomics, organic molecules and fullerenes / Ab-initio Molekulardynamik-Studien von laser- und stoßinduzierten Prozessen in Vielelektronen-Dimeren, organischen Molekülen und FullerenenHandt, Jan 22 December 2010 (has links) (PDF)
This work presents applications of an ab-initio molecular dynamics method, the so-called nonadiabatic quantum molecular dynamics (NA-QMD), for various molecular systems with many electronic and nuclear degrees of freedom. Thereby, the nuclei will be treated classically and the electrons with time-dependent density functional theory (TD-DFT) in basis expansion. Depending on the actual system and physical process,
well suited basis sets for the Kohn-Sham orbitals has to be chosen. For the ionization process a novel absorber acting in the energy space as well as additional basis functions will be used depending on the laser frequency.
In the first part of the applications, a large variety of different laser-induced molecular processes will be investigated. This concerns, the orientation dependence of the ionization of multielectronic diatomics (N2, O2), the isomerization of organic molecules (N2H2) and the giant excitation of the breathing mode in fullerenes (C60).
In the second part, fullerene-fullerene collisions are investigated, for the first time in the whole range of relevant impact velocities concerning the vibrational and electronic energy transfer (\"stopping~power\").
For low energetic (adiabatic) collisions, it is surprisingly found, that a two-dimensional, phenomenological collision model can reproduce (even quantitatively) the basic features of fusion and scattering observed in the fully microscopic calculations as well as in the experiment.
For high energetic (nonadiabatic) collisions, the electronic and vibrational excitation regimes are predicted, leading to multifragmentation up to complete atomization.
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Ab-initio molecular dynamics studies of laser- and collision-induced processes in multielectron diatomics, organic molecules and fullerenesHandt, Jan 18 October 2010 (has links)
This work presents applications of an ab-initio molecular dynamics method, the so-called nonadiabatic quantum molecular dynamics (NA-QMD), for various molecular systems with many electronic and nuclear degrees of freedom. Thereby, the nuclei will be treated classically and the electrons with time-dependent density functional theory (TD-DFT) in basis expansion. Depending on the actual system and physical process,
well suited basis sets for the Kohn-Sham orbitals has to be chosen. For the ionization process a novel absorber acting in the energy space as well as additional basis functions will be used depending on the laser frequency.
In the first part of the applications, a large variety of different laser-induced molecular processes will be investigated. This concerns, the orientation dependence of the ionization of multielectronic diatomics (N2, O2), the isomerization of organic molecules (N2H2) and the giant excitation of the breathing mode in fullerenes (C60).
In the second part, fullerene-fullerene collisions are investigated, for the first time in the whole range of relevant impact velocities concerning the vibrational and electronic energy transfer (\"stopping~power\").
For low energetic (adiabatic) collisions, it is surprisingly found, that a two-dimensional, phenomenological collision model can reproduce (even quantitatively) the basic features of fusion and scattering observed in the fully microscopic calculations as well as in the experiment.
For high energetic (nonadiabatic) collisions, the electronic and vibrational excitation regimes are predicted, leading to multifragmentation up to complete atomization.
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