### Browsing by Author "Besieris, Ioannis M."

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- Acoustic scattering analysis for remote sensing of manganese nodulesMa, Yushieh (Virginia Polytechnic Institute and State University, 1982)
Show more The theory of the scattering of plane waves in a fluid medium by an isotropic elastic sphere representing a manganese nodule is developed. Scattering cross sections were computed using the theory and the results are presented graphically. The scattering cross section and the reflectivity factor govern the characteristic acoustic signature of the Pacific where manganese nodules are present. Preliminary experimental data for the compressional and shear wave speeds in nodule material is given. This data was used in the scattering computations. Limiting cases of Rayleigh scattering and scattering from fixed rigid and fluid spheres are also shown for comparison. It is shown that the rigidity of the nodules dominates the high frequency response. The problem of the multiple scattering of acoustic waves by randomly distributed nodules on the flat ocean bottom is investigated analytically. The statistical description of nodule deposits is given. The concept of the configurational average is introduced in order to obtain the average scattered response. The size averaging is found to be able to smooth the acoustic response in the high frequency region. The plane wave analysis for the multiple scattering problem is justified by the narrow beam investigation. It shows that the beam effect on the average backscattered field can be neglected in the remote sensing. For a planar distribution of nodules, the average scattered field excited by a normally incident plane wave is verified to be plane waves characterized by coherent reflection and transmission coefficients. The multiple scattering effect is found to be a higher order correction to the average scattered field. For a sparse distribution of nodules, the average scattered field can be well evaluated using the single scattering theory in which the scattering process is also shown to be energy conserved. For a dense distribution of nodules, the radial distribution function is used in the Foldy-Lax hierarchy. The result shows that the pair correlation affects the phase of the second order correction term in the expression for the average scattered field when the higher order statistics are truncated using the quasi crystalline approximation.Show more - Acoustic X-wave reflection and transmission at a planar interface: Spectral analysisShaarawi, Amr M.; Besieris, Ioannis M.; Attiya, Ahmed M.; El-Diwany, Essam (Acoustical Society of America, 2000-01-01)
Show more The spectral structure of a three-dimensional X-wave pulse incident on a planar surface of discontinuity is examined. Introducing a novel superposition of azimuthally dependent pulsed plane waves, it is shown for oblique incidence that the reflected pulse has a localized wave structure. On the other hand, the transmitted field maintains its localization up to a certain distance from the interface, beyond which it starts disintegrating. An estimate of the localization range of the transmitted pulse is established; also, the parameters affecting the localization range are identified. The reflected and transmitted fields are deduced for X-waves incident from either a slower medium or a faster one. For the former case the evanescent fields in the second medium are calculated and their explicit time dependence is deduced for a normally incident X-wave. Furthermore, at near-critical incidence the transmitted pulse exhibits significant pulse compression and focusing.Show more - Adaptive Self-Tuning Neuro Wavelet Network ControllersLekutai, Gaviphat (Virginia Tech, 1997-03-31)
Show more Single layer feed forward neural networks with hidden nodes of adaptive wavelet functions (wavenets) have been successfully demonstrated to have potential in many applications. Yet applications in the process control area have not been investigated. In this paper an application to a self-tuning design method for an unknown nonlinear system is presented. Different types of frame wavelet functions are integrated for their simplicity, availability, and capability of constructing adaptive controllers. Infinite impulse response (IIR) recurrent structures are combined in cascade to the network to provide a double local structure resulting in improved speed of learning. In particular, neuro-based controllers assume a certain model structure to approximate the system dynamics of the "unknown" plant and generate the control signal. The capability of neuro-controllers to self-tuning of an unknown nonlinear plants is then illustrated through design examples. Simulation results demonstrate that the self-tuning design methods are directly applicable for a large class of nonlinear control systems.Show more - Analysis and Characterization of Fiber Nonlinearities with Deterministic and Stochastic Signal SourcesLee, Jong-Hyung (Virginia Tech, 2000-02-10)
Show more In this dissertation, various analytical models to characterize fiber nonlinearities have been applied, and the ranges of validity of the models are determined by comparing with numerical results. First, the perturbation approach is used to solve the nonlinear Schrödinger equation, and its range of validity is determined by comparing to the split-step Fourier method. In addition, it is shown mathematically that the perturbation approach is equivalent to the Volterra series approach. Secondly, root-mean-square (RMS) widths both in the time domain and in the frequency domain are modeled. It is shown that there exists an optimal input pulse width to minimize output pulse width based on the derived RMS models, and the functional form of the minimum output pulse width is derived. The response of a fiber to a sinusoidally modulated input which models an alternating bit sequence is studied to see its utility in measuring system performance in the presence of the fiber nonlinearities. In a single channel system, the sinusoidal response shows a strong correlation with eye-opening penalty in the normal dispersion region over a wide range of parameters, but over a more limited range in the anomalous dispersion region. The cross-phase modulation (CPM) penalty in a multi-channel system is also studied using the sinusoidally modulated input signal. The derived expression shows good agreement with numerical results in conventional fiber systems over a wide range of channel spacing, ∆*f*, and in dispersion-shifted fiber systems when ∆*f*> 100GHz. It is also shown that the effect of fiber nonlinearities may be characterized with stochastic input signals using noise-loading analysis. In a dense wavelength division multiplexed (DWDM) system where channels are spaced very closely, the broadened spectrum due to various nonlinear effects like SPM (self-phase modulation), CPM, and FWM (four-wave mixing) is in practice indistinguishable. In such a system, the noise-loading analysis could be useful in assessing the effects of broadened spectrum due to fiber nonlinearities on system performance. Finally, it is shown numerically how fiber nonlinearities can be utilized to improve system performance of a spectrum-sliced WDM system. The major limiting factors of utilizing fiber nonlinearities are also discussed.Show more - Analysis and design of broadband single-mode multi-clad fibersLu, Liang-Ju (Virginia Polytechnic Institute and State University, 1989)
Show more ln the last several years, considerable attention has been paid to the study of dispersion-flattened single-mode fibers which offer a high transmission capacity with low losses through a wide range of wavelengths. However, the existing designs are sensitive to bending and manufacturing tolerances, and are not truly single-mode at most wavelengths of interest. To remedy these problems a new series of broadband dispersion-flattened truly single-mode fiber designs are proposed. These fibers have both dispersion-shifted and dispersion-flattened features with low splice and bend losses. Results demonstrating a total dispersion of ±0.97 ps/km-nm over the entire spectral range between 1.31 μm to 1.66 μm are presented. Such dispersion-flattening is achieved while simultaneously maintaining a mode-field radius of 3 μm to 5 μm in the dispersion-flattened wavelength range. The most significant achievement is that the proposed muIti-clad fiber design is strictly single-mode and splice and bend losses are smaller than those of double-clad, triple-clad, and quadruple-clad fibers with the same value of dispersion. Ultralow dispersion fibers, whose chromatic dispersion and the first and second-order derivatives of the chromatic dispersion are zero at 1.5 μm or 1.55 μm, are described. This effectively increases the laser emission tolerance. Ultralow dispersion fibers open the way to wavelength multiplexing with currently available inexpensive multifrequency lasers, either in local or long distance networks. These fibers also have low splice and bend losses compared to double-clad, triple-clad, and quadruple-cIad fibers. An inverse waveguide synthesis program, which can trace multiple objective functions and optimize multiple parameters simultaneously, is developed. An objective function is applied, for the first time, to optimize the dispersion-flattened single-mode fiber index profile with respect to: (1) minimum dispersion, (2) the wavelengths of zero-dispersion, (3) maximum width of dispersion-flattened window, (4) maximum layer index difference less than 0.8%, and (5) layer thickness larger than 3.5 μm. The accuracy of chromatic dispersion calculations in dispersion-flattened fibers is evaluated. lt has been shown that the accuracy of approximate methods is influenced not only by the index differences, but also by their derivatives with respect to wavelength. The matrix method and direct numerical integration of the wave equation are used to compute the mode propagation constants, cutoff frequencies, field distributions, mode-field radius, and splice loss, and carry out production tolerance analysis for multi-clad step-index fibers and graded-index fibers, respectively. Detailed analysis and optimized fiber data are presented.Show more - Analysis and simulation of the Kerr effect in long haul in-line fiber amplifier transmission systemsMa, Xiaobing (Virginia Tech, 1994-12-05)
Show more The next generation of transoceanic submarine cable systems will use in-line fiber amplifiers to replace electrical regenerators. This new approach requires a better understanding of the fiber waveguide, especially the nonlinear characteristics. It has been demonstrated, both by numerical simulation and experiments, that the Kerr effect has the most significant degradation effects on these systems with a single or a few channels. In this dissertation, the numerical simulation is the principal approach but this is supplemented with some analytical studies. There are two phenomena that are directly associated with the Kerr effect: spectrum broadening and four wave mixing (FWM). The broadened signal spectrum enhances the dispersion effect and consequently increases the inter symbol interference. This distortion is significant when the dispersion is relatively large. U sing erbium doped fiber amplifiers, amplified spontaneous emission (ASE) noise is added to the signal The FWM effect between the noise and signal causes a loss in the signal power. In the close vicinity of the zero dispersion wavelength, the FWM effect is maximized due to the loose phase match condition. In order to reduce these degradation effects, dispersion compensation has recently been proposed. Using this approach, it is possible to optimize the system configuration to achieve the best performance. In this dissertation, the dispersion compensation distance, dispersion coefficient, channel bandwidth, bit rate and the degree of imperfect compensation are all subject to the optimization. The evaluation is obtained by the numerical simulation using the mean squared error (MSE), which can be derived as the difference between the wavefronts received by a back-to-back receiver and a receiver at the end of the channel. Although the MSE can't be related to the bit error directly, this research provides the insight into how dispersion and noise behave in the presence of the Kerr effect and points the direction for future experimental research.?Show more - Analysis of Aperture Radiation Using Computer Visualization and Image-Processing TechniquesMonkevich, James Matthew (Virginia Tech, 1998-05-04)
Show more In order to accurately describe the behavior of an antenna, one needs to understand the radiation mechanisms that govern its operation. One way to gain such an insight is to view the fields and currents present on a radiating structure. Unfortunately, in close proximity to an antenna empirical techniques fail because the measurement probe alters the operation of the radiating structure. Computational methods offer a solution to this problem. By simulating the operation of an antenna, one can obtain electromagnetic field data near (or even internal to) a radiating structure. However, these computationally intense techniques often generate extremely large data sets that cannot be adequately interpreted using traditional graphical approaches. A visualization capability is developed that allows an analysis of the above-mentioned data sets. With this technique, the data is viewed from a unique, global perspective. This format is well suited for analytical investigations as well as debugging during modeling and simulation. An illustrative example is provided in the context of a rectangular microstrip patch antenna. A comparison is performed between the visualized data and the theory of operation for the microstrip patch in order to demonstrate that radiation mechanisms can be obtained visually. An additional analysis tool is developed using Gabor filters and image-processing techniques. This tool allows one to detect and filter electromagnetic waves propagating with different velocities (both speed and direction). By doing so, each mode of an antenna can be analyzed independently. The fields of a multi-moded, open-ended rectangular waveguide are analyzed in order to demonstrate the effectiveness of these techniques.Show more - Analysis of three-dimensional field distributions for focussed unapodized/apodized annular beamsBhabu, Shaleen J. (Virginia Tech, 1990-08-14)
Show more The study of focal shift in focused beams using unapodized apertures has been well documented. However, not much work has been done on apodized apertures. In this thesis we use a Fourier-Optic approach to analyze the field distribution of a focused beam around the region of geometrical focus. The analytical formulation developed is general in nature as it is valid for any arbitrary aperture functions. This is then applied to some specific cases. Two cases of interest that are considered are the unapodized and the Gaussian apodized annular apertures. In order to study the intensity distributions around the geometrical focus, simulation results are presented using closed form analytical expressions and approximate integral forms. Specific emphasis is placed on the focal shift in the two apertures and on the effect of changing various parameters. A prognosis for future work using a-Modulation on Gaussian apodized annular apertures is also presented.Show more - Analysis, Design and Performance Evaluation of Optical Fiber Spectrum-Sliced WDM SystemsArya, Vivek (Virginia Tech, 1997-06-03)
Show more This dissertation investigates the design and performance issues of a recently demonstrated technique, termed as spectrum-slicing, for implementing wavelength-division-multiplexing (WDM) in optical fiber systems. Conventional WDM systems employ laser diodes operating at discrete wavelengths as carriers for the different data channels that are to be multiplexed. Spectrum-slicing provides an attractive low-cost alternative to the use of multiple coherent lasers for such WDM applications by utilizing spectral slices of a broadband noise source for the different data channels. The principal broadband noise source considered is the amplified spontaneous emission (ASE) noise from an optical amplifier. Each slice of the spectrum is actually a burst of noise that is modulated individually for a high capacity WDM system. The stochastic nature of the broadband source gives rise to excess intensity noise which results in a power penalty at the receiver. One way to minimize this penalty, as proposed and analyzed for the first time in this work, is to use an optical preamplifier receiver. It is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which optimizes the detection sensitivity (minimizes the average number of photons/bit) for a given error probability. Moreover the evaluated detection sensitivity represents an order of magnitude ( > 10 dB) improvement over conventional PIN receiver-based detection techniques for such spectrum-sliced communication systems. The optimum is a consequence of signal energy fluctuations dominating at low values of the signal time bandwidth product (m), and the preamplifier ASE noise dominating at high values of m. Operation at the optimum bandwidth renders the channel error probability to be a strong function of the optical bandwidth, thus providing motivation for the use of forward error correction coding (FEC). System capacity (for BER = ) is shown to be 23 Gb/s without coding, and 75 Gb/s with a (255,239) Reed Solomon code. The effect of non-rectangular spectra on receiver sensitivity is investigated for both OOK and FSK transmission, assuming the system (de)multiplexer filters to be N'th order Butterworth bandpass. Although narrower filters are recommended for improving power budget, it is shown that system penalty due to filter shape may be kept < 1 dB by employing filters with N > 2. Moreover spectrum-sliced FSK systems using optical preamplifier receivers are shown, for the first time, to perform better in a peak optical power limited environment. Performance-optimized spectrum-sliced WDM systems have potential use in both local loop and long-distance fiber communication systems which require low-cost WDM equipment for high data rate applications.Show more - Aperture synthesis of time-limited X waves and analysis of their propagation characteristicsChatzipetros, Argyrios A.; Shaarawi, Amr M.; Besieris, Ioannis M.; Abdel-Rahman, Mohammad J. (Acoustical Society of America, 1998-05-01)
Show more The feasibility of exciting a localized X-wave pulse from a finite aperture is addressed. Also, the possibility of using a finite-time excitation of a dynamic aperture to generate a finite-energy approximation to an X-wave pulse is explored. The analysis is carried out by using a Gaussian time window to time limit the infinite X-wave initial excitation. Huygens' construction is used to calculate the amplitude of the radiated wave field away from the finite-time source. The decay rate of the peak of the X wave is compared to that of a quasi-monochromatic signal. It is shown that the finite-time X-wave propagates to much farther distances without significant decay. Furthermore, the decay pattern of the radiated X-wave pulse is derived for a source consisting of an array of concentric annular sections. The decay behavior of the radiated pulse is similar to that of an X-wave launched from a finite-time aperture. This confirms the fact that time windowing the infinite energy X-wave excitation is a viable scheme for constructing finite apertures. A discussion of the diffraction limit of the X-wave pulse is also provided.Show more - Appling Machine and Statistical Learning Techniques to Intelligent Transport Systems: Bottleneck Identification and Prediction, Dynamic Travel Time Prediction, Driver Run-Stop Behavior Modeling, and Autonomous Vehicle Control at IntersectionsElhenawy, Mohammed Mamdouh Zakaria (Virginia Tech, 2015-06-30)
Show more In this dissertation, new algorithms that address three traffic problems of major importance are developed. First automatic identification and prediction algorithms are developed to identify and predict the occurrence of traffic congestion. The identification algorithms concoct a model to identify speed thresholds by exploiting historical spatiotemporal speed matrices. We employ the speed model to define a cutoff speed separating free-flow from congested traffic. We further enhance our algorithm by utilizing weather and visibility data. To our knowledge, we are the first to include weather and visibility variables in formulating an automatic congestion identification model. We also approach the congestion prediction problem by adopting an algorithm which employs Adaptive Boosting machine learning classifiers again something novel that has not been done previously. The algorithm is promising where it resulted in a true positive rate slightly higher than 0.99 and false positive rate less than 0.001. We next address the issue of travel time modeling. We propose algorithms to model travel time using various machine learning and statistical learning techniques. We obtain travel time models by employing the historical spatiotemporal speed matrices in conjunction with our algorithms. The algorithms yield pertinent information regarding travel time reliability and prediction of travel times. Our proposed algorithms give better predictions compared to the state of practice algorithms. Finally we consider driver safety at signalized intersections and uncontrolled intersections in a connected vehicles environment. For signalized intersections, we exploit datasets collected from four controlled experiments to model the stop-run behavior of the driver at the onset of the yellow indicator for various roadway surface conditions and multiple vehicle types. We further propose a new variable (predictor) related to driver aggressiveness which we estimate by monitoring how drivers respond to yellow indications. The performance of the stop-run models shows improvements after adding the new aggressiveness predictor. The proposed models are practical and easy to implement in advanced driver assistance systems. For uncontrolled intersections, we present a game theory based algorithm that models the intersection as a chicken game to solve the conflicts between vehicles crossing the intersection. The simulation results show a 49% saving in travel time on average relative to a stop control when the vehicles obey the Nash equilibrium of the game.Show more - Beam Propagation in Focusing Media with Random Axis Misalignments: Second-Order And Higher-Order MomentsBesieris, Ioannis M. (AIP Publishing, 1978-12-01)
Show more A novel statistical technique which allows the asymptotic evaluation of second_ and higher_order averaged observables related to the stochastic complex parabolic equation is applied to the problem of beam propagation in a focusing medium characterized by random_axis misalignments. Analytical and numerical results concerning on_ and off_axis statistics (e.g., the variance of intensity fluctuations, modal power transfer, the probability distribution density of the log_irradiance, etc.) are presented, and comparisons are made with previously reported findings.Show more - Bidirectional and unidirectional spectral representations for the scalar wave equationKoutoumbas, Anastasios M. (Virginia Tech, 1990-06-13)
Show more The Cauchy problem associated with the scalar wave equation in free space is used as a vehicle for a critical examination and assessment of the bidirectional and unidirectional spectral representations. These two novel methods for synthesizing wave signals are distinct from the superposition principle underlying the conventional Fourier method and they can effectively be used to derive a large class of localized solutions to the scalar wave equation. The bidirectional spectral representation is presented as an extension of Brittingham's ansatz and Ziolkowski's Focus Wave Mode spectral representations. On the other hand, the unidirectional spectral representation is motivated through a group-theoretic similarity reduction of the scalar wave equation.Show more - A Bidirectional Traveling Plane-Wave Representation Of Exact-Solutions Of The Scalar Wave-EquationBesieris, Ioannis M.; Shaarawi, Amr M.; Ziolkowski, R. W. (AIP Publishing, 1989-06-01)
Show more A new decomposition of exact solutions to the scalar wave equation into bidirectional, forward and backward, traveling plane wave solutions is described. The resulting representation is a natural basis for synthesizing pulse solutions that can be tailored to give directed energy transfer in space. The development of known free_space solutions, such as the focus wave modes, the electromagnetic directed energy pulse trains, the spinor splash pulses, and the Bessel beams, in terms of this decomposition will be given. The efficacy of this representation in geometries with boundaries, such as a propagation in a circular waveguide, will also be demonstrated.Show more - Calibration of time domain network analyzersSu, Wansheng (Virginia Tech, 1992)
Show more A calibration technique for time domain reflectometry and transmission (TDR and TDT) measurement system as applied to network analysis is presented. The calibration corrects for the errors caused by the response of the measurement system. A complete physically-based model has been established for the system. A set of calculable standards has been developed to satisfy the time domain requirements for calibration. The calibration technique was applied to determining the model parameters of a commercial TDR and TDT system. The errors of modeling and de-embedding are analyzed. The calibration enhanced the system bandwidth from 8 GHz to about 20 GHz. Experimental verification is given to demonstrate the validity and accuracy of the calibration technique.Show more - Capacitor-Probe Calibration and Optimization for NDE Application to Portland Cement ConcreteAlzaabi, Aref Alderbas (Virginia Tech, 2000-08-28)
Show more Three main objectives have been set for this research. The first is to develop an accurate method for measuring the dielectric constant of PCC using a capacitor probe (C-Probe) that has been recently developed at Virginia Tech and validate it for field application to detect internal PCC flaws such as delamination. The C-Probe consists of two flexible conducting plates, connected to a Network Analyzer, with a specific separation between them. The second is to optimize the C-Probe design configuration for different PCC slab thicknesses. The third objective is to develop a predictive model that correlates the bulk dielectric constant of PCC with its critical parameters (cement, aggregate, and air content). Five calibration methods have been developed and evaluated for the C-Probe to measure the dielectric properties of PCC. This evaluation has demonstrated that open, short, Teflon material (OSM) calibration method is the most appropriate one for the C-Probe. The selected calibration method was used to validate the C-Probe fixture for field application by measuring 1.5 x 1.5 m PCC slabs prepared with different mix properties, thicknesses, and induced deterioration. The C-Probe has been proved to detect induced voids in the PCC slabs. In addition, the effect of steel reinforcement on measurements can be mastered by controlling the penetration of electromagnetic (EM) field in the PCC slabs. The effective penetration depth of the EM field for different C-Probe design configuration was optimized by computer simulation. The results have been used to develop a predictive model that correlates the effective penetration depth with the plates' size, separation between them, and the dielectric constant of the PCC under test. Thus, an optimum design for different desired penetration depth was achieved. Two experimental designs were developed to identify the critical parameters that affect the bulk dielectric constant of PCC. A computer simulation was used to identify the significance of each parameter. A predictive model has been developed to correlate the PCC bulk dielectric constant to the critical parameters. The estimated dielectric constant of PCC using the predictive model was correlated to that obtained by other theoretical mixture models; the predictive model has found to correlate well with Looyenga theoretical mixture model.Show more - Characterization and modeling of magnetic materials and structuresAl-Mazroo, Abdulhameed Yousef (Virginia Polytechnic Institute and State University, 1988)
Show more This dissertation presents methods for wideband characterization and modeling of magnetic materials and structures over a wide frequency range (dc to a few GHz). A method for modeling the thick film inductor structures at high frequencies is presented in this dissertation. The thick film inductor under test is printed and located in shunt connection at the end of a reference transmission line. Time Domain Reflectometry (TDR) technique is used to measure the response waveform from the inductor under test. The response from a short circuit at the location of the inductor is acquired as the reference waveform. The two acquired waveforms are then transformed into the frequency domain using the Fast Fourier Transform algorithm (FFT). The reflection coefficient is then computed as the ratio between the Fourier Transforms of the response and reference waveforms. From the information contained, the complex impedance of the structure under study can be calculated. This information is used for modeling that structure by fitting the data to the network model using the computer network analysis program. Experimental and simulated response waveforms are compared and brought to a close match by changing the model components values. A cavity-like sample holder filled with ferrite material ls proposed in this dissertation to measure the complex permeability of the magnetic material filling this cavity. The cavity walls are deposited on a coaxially shaped sample using thick film techniques. The reflection coefficient from the cavity under study is measured by adapting the cavity to the end of a transmission line. The full field analysis of this proposed configuration is used to determine a relationship between the complex permeability of the ferrite material and the measured reflection coefficient. The method of moments ls used to achieve this task. Computer simulation experiments are performed to test the sensitivity of the technique and to predict the performance over the desired frequency range. Actual experimentation as well as verifications of these measurements are conducted to verify the merit of the proposed technique.Show more - Characterization of a nonrigid sphere using the backscattered fields of acoustic X wavesMoawad, Maged F.; Shaarawi, Amr M.; Besieris, Ioannis M. (Acoustical Society of America, 2004-06-01)
Show more The scattering of acoustic ultra-wideband X-wave pulses by a nonrigid sphere is simulated for purposes of material identification and characterization. Using the backscattered spectrum of the X-wave pulses, a procedure is described for estimating the radius, speed of sound, and density of-the sphere. The effectiveness of the suggested technique is verified in the case that the peak of the X-wave is incident on the centers of the sphere, as well as for the off-center incidence case.Show more - Characterization of Ultra Wideband Communication ChannelsMuqaibel, Ali Hussein (Virginia Tech, 2003-03-05)
Show more Ultra-wideband (UWB) communication has been the subject of extensive research in recent years due to its unique capabilities and potential applications, particularly in short-range multiple access wireless communications. However, many important aspects of UWB-based communication systems have not yet been thoroughly investigated. The propagation of UWB signals in indoor environments is the single most important issue with significant impacts on the future direction, scope, and generally the extent of the success of UWB technology. The objective of this dissertation is to obtain a more thorough and comprehensive understanding of the potentials of UWB technology by characterizing the UWB communication channels. Channel characterization refers to extracting the channel parameters from measured data. The extracted parameters are used to quantify the effect of the channel on communication UWB systems using this channel as signal transmission medium. Data are measured in different ways using a variety of time-domain and frequency-domain techniques. The experimental setups used in channel characterization effort also include pulse generators and antennas as integral parts of the channel, since the pulse shape and antenna characteristics have significant impact on channel parameters. At a fundamental level, the propagation of UWB signals, as any electromagnetic wave, is governed, among other things, by the properties of materials in the propagation medium. One of the objectives of this research is to examine propagation through walls made of typical building materials and thereby acquire ultra-wideband characterization of these materials. The loss and the dielectric constant of each material are measured over a frequency range of 1 to 15 GHz. Ten commonly used building materials are chosen for this investigation. These include, dry wall, wallboard, structure wood, glass sheet, bricks, concrete blocks, reinforced concrete (as pillar), cloth office partition, wooden door, and styrofoam slab. The work on ultra-wideband characterization of building materials resulted in an additional interesting contribution. A new formulation for evaluating the complex dielectric constant of low-loss materials, which involves solving real equations and thus requiring only one-dimensional root searching techniques, was found. The results derived from the exact complex equation and from the new formulation are in excellent agreement. Following the characterization of building materials, an indoor UWB measurement campaign is undertaken. Typical indoor scenarios, including line-of-sight (LOS), non-line-of-sight (NLOS), room-to-room, within-the-room, and hallways, are considered. Results for indoor propagation measurements are presented for local power delay profiles (local-PDP) and small-scale averaged power delay profiles (SSA-PDP). Site-specific trends and general observations are discussed. The results for pathloss exponent and time dispersion parameters are presented. The analyses results indicate the immunity of UWB signals to multipath fading. The results also clearly show that UWB signals, unlike narrowband signals, do not suffer from small scale fading, unless the receiver is too close to walls. Multipath components are further studies by employing a deconvolution technique. The application of deconvolution results in resolving multipath components with waveforms different from those of the sounding pulse. Resolving more components can improve the design of the rake receiver. The final part of this research elaborates on the nature of multiple access interference and illustrates the application of multi-user detection to improve the performance of impulse radio systems. Measured dispersion parameters and their effects on the multiple access parameters are discussed.Show more - A Class of Robust and Efficient Iterative Methods for Wave Scattering ProblemsAdams, Robert John (Virginia Tech, 1998-12-17)
Show more Significant effort has recently been directed towards the development of numerically efficient iterative techniques for the solution of boundary integral equation formulations of time harmonic scattering problems. The primary result of this effort has been the development of several advanced numerical techniques which enable the dense matrix-vector products associated with the iterative solution of boundary integral equations to be rapidly computed. However, an important aspect of this problem which has yet to be adequately addressed is the development of rapidly convergent iterative techniques to complement the relatively more mature numerical algorithms which expedite the matrix-vector product operation. To this end, a class of efficient iterative methods for boundary integral equation formulations of two-dimensional scattering problems is presented. This development is based on an attempt to approximately factor (i.e., renormalize) the boundary integral formulation of an arbitrary scattering problem into a product of one-way wave operators and a corresponding coupling operator which accounts for the interactions between oppositely propagating waves on the surface of the scatterer. The original boundary integral formulation of the scattering problem defines the coupling between individual equivalent sources on the surface of the scatterer. The renormalized version of this equation defines the coupling between the forward and backward propagating fields obtained by re-summing the individual equivalent sources present in the original boundary integral formulation of the scattering problem. An important feature of this class of rapidly convergent iterative techniques is that they are based on an attempt to incorporate the important physical aspects of the scattering problem into the iterative procedure. This leads to rapidly convergent iterative series for a number of two-dimensional scattering problems. The iterative series obtained using this renormalization procedure are much more rapidly convergent than the series obtained using Krylov subspace techniques. In fact, for several of the geometries considered the number of iterations required to achieve a specified residual error is independent of the size of the scatterer. This desirable property of the iterative methods presented here is not shared by other iterative schemes for wave scattering problems. Moreover, because the approach used to develop these iterative series depends only on the assumption that the total field can be approximately represented by a summation of independent and oppositely directed waves (and not on the presence of special geometries, etc.), the proposed iterative methods are very general and are thus applicable to a large number of complex scattering problems.Show more