Donald G. Dudley

Wireless propagation in tunnels

In this article, we give a presentation of wireless electromagnetic propagation in tunnels. We discuss in depth the characteristics of multimodal propagation. We show analytical models whereby we are able to study electromagnetic fields in tunnels. To complement the analysis, we give an account of experiments performed in the Channel Tunnel between France and England, and the Massif Central in south-central France. We conclude with a discussion of the results, followed by some comments about bandwidth and the possibility of using multiple-input multiple-output (MIMO) systems to improve communication-channel capacity.

Interpretation of MIMO Channel Characteristics in Rectangular Tunnels From Modal Theory

We develop a modal approach for analyzing multiple-input-multiple-output (MIMO) wireless channel propagation in a tunnel with lossy walls. We use parametric methods to study the effects of the number of modes and of the separation among antennas. We evaluate the performance of the MIMO channel in terms of capacity as a function of range and tunnel size.

Propagation of ultra-wide-band electromagnetic pulses through dispersive media

We develop an efficient method for the analysis of ultra-wide-band (UWB) electromagnetic pulses (e.g., double-exponential pulse) propagating through a waveguide or cold plasma (i.e., the ionosphere). First we show that the inverse Fourier-transform representations for the electric and magnetic fields satisfy second order, nonhomogeneous, ordinary, differential equations. These differential equations are solved analytically, thereby yielding closed-form expressions involving incomplete Lipschitz-Hankel integrals (ILHIs). The ILHIs are computed using efficient convergent and asymptotic series expansions. We demonstrate the usefulness of the ILHI expressions by comparing them with the fast Fourier-transform technique (FFT). Because of the long tails associated with UWB pulses, a large number of sample points are required in the FFT, to avoid aliasing errors. In contrast, the ILHI expressions provide accurate and efficient numerical results, regardless of the number of points computed. An asymptotic series representation for the ILHIs is also employed, to obtain a relatively simple, late-time approximation for the transient fields. This approximate late-time expression is shown to accurately model the waveform over a large portion of its time history. >

Wireless propagation in circular tunnels

We study models for propagation in circular tunnels. The excitation is either a circular magnetic or circular electric current loop. We produce expressions for the fields in terms of a Fourier transform over the axial variable. We then produce the modes in the lossy structure by contour integration techniques. We include numerical results for the field intensity both as a function of axial distance and as a function of radial distance for several frequencies, radii, and constitutive parameters of interest. We comment in particular concerning the specific form of the field both near to and far from the antenna source.

Linear source in a circular tunnel

We study models for propagation in circular tunnels. The excitation is a fundamental electric current source. We produce expressions for the fields in terms of a Fourier transform over the axial variable. We perform an asymptotic analysis of the complex wavenumbers for use as initial estimates in a complex root finder. We then produce the modes in the lossy structure by contour integration techniques. We include numerical results for the field intensity as a function of axial distance for several frequencies and constitutive parameters of interest.We comment in particular concerning the modal content and the specific form of the field both near to and far from the antenna source.

Progress in identification of electromagnetic systems

We review progress in identification of electromagnetic systems. We introduce the basic ideas with a brief discussion of forward and inverse problems. We next give some history, emphasizing the basic issues involved. We follow with some results and an assessment of present capabilities and limitations. We conclude with some recommendations for future work in electromagnetic identification

Linear inverse problems in wave motion: nonsymmetric first-kind integral equations

We present a general framework to study the solution of first-kind integral equations. The integral operator is assumed to be compact and nonself-adjoint and the integral equation can possess a nonempty null space. An approach is presented for adding contributions from the null space to the minimum-energy solution of the integral equation through the introduction of weighted Hilbert spaces. Stability, accuracy, and nonuniqueness of the solution are discussed through the use of model resolution, data fit, and model covariance operators. The application of this study is to inverse problems that exhibit nonuniqueness.

System identification for wireless propagation channels in tunnels

Wireless communication channels in large tunnels at microwave frequencies involve a large number of guided wave modes. It has been reported that spatial records of field strength versus axial distance in such tunnels can be divided into two zones. Close to the transmitting source, the field consists of many modes interacting in such a way as to produce rapid decay and strong local variations. For greater distances, the field appears dominated by only a few principal modes, or a single mode, with less rapid decay and smoother variations. We apply methods of system identification to estimate parameters in a model that includes these characteristics in the two regions. We validate our model with theoretical data from a circular tunnel with lossy walls.

Parametric identification of transient electromagnetic systems

Abstract Data into and out of a transient eletromagnetic system are considered in the framework of modern system identification. System solutions that take the form of a complex exponential series are discussed. Since the identification of the parameters in the series is non-linear, emphasis shifts to the identification of the parameters in the difference equation whose solution is the exponential series. The subject is cast in the formalism of system identification with generalizations to more complex systems. Two examples are given, one involving an actual electromagnetic experiment.

Ultra-wideband electromagnetic pulse propagation in a homogeneous, cold plasma

In this paper, we investigate the propagation of an ultra-wideband electromagnetic pulse in a homogeneous, cold plasma which is used to represent a simplified model of the atmosphere. The standard procedure for the computation of the corresponding transient field involves the application of a fast Fourier transform (FFT) to a well-known, analytical, frequency-domain solution. However, because of the long tails in both the time and frequency domains, a large number of sample points are required to compute the transient response using this FFT approach. In this paper, we introduce a new asymptotic extraction technique which dramatically reduces the number of sample points required by the FFT. First, we review the recently derived closed-form expression for a double-exponential pulse propagating in a homogeneous, collisionless, cold plasma. Since the high-frequency behavior does not depend on the electron collision frequency, an analytical frequency-domain expression, which is similar in form to the one encountered for the collisionless, cold plasma and encompasses this high-frequency behavior, can be subtracted from the exact expression for the plasma with a nonzero collision frequency. The extracted term is evaluated analytically. The remaining expression, which can be transformed to the time domain with a FFT, requires only a modest number of sample points. This dramatically improves the numerical efficiency of the approach. We find that the extracted analytical term provides a very good approximation for the early-time behavior of the transient pulse.