Walid M. Dyab

Electromagnetic Macro Modeling of Propagation in Mobile Wireless Communication: Theory and Experiment

The objective of this paper is to illustrate that an electromagnetic macro modeling can properly predict the path loss exponent in a mobile cellular wireless communication [1]. Specifically, we illustrate that the path loss exponent in a cellular wireless communication is three preceded by a slow fading region and followed by the fringe region where the path loss exponent is four. Theoretically this will be illustrated through the analysis of radiation from a vertical electric dipole situated over a horizontal imperfect ground plane as first considered by Sommerfeld in 1909 [2,3]. To start with, the exact analysis of radiation from the dipole is made using the Sommerfeld formulation. The semi-infinite integrals encountered in this formulation are evaluated using a modified saddle point method for field points moderate to far distances away from the source point to predict the appropriate path loss exponents. The reflection coefficient method can also be derived by applying a saddle point method to the semi-infinite integrals and it is shown not to provide the correct path loss exponent. The various approximations used to evaluate the Sommerfeld integrals are described for different regions [3]. It is also important to note that Sommerfelds original 1909 paper had no error in sign [1]. However, Sommerfeld overlooked the properties associated with the pole. Both accurate numerical analyses along with experimental data are provided to illustrate the above statements. Both Okumuras experimental data [4,5] and experimental data taken from different base stations in urban environments [6-8] at two different frequencies will validate the theory. Experimental data reveal that a macro modeling of the environment using an appropriate electromagnetic analysis can accurately predict the path loss exponent for the propagation of radio waves in a cellular wireless communication scenario.

A Physics-Based Green's Function for Analysis of Vertical Electric Dipole Radiation Over an Imperfect Ground Plane

Sommerfeld integrals appear in the solution of radiation and scattering problems involving antennas in planar multi-layered media. In the conventional approach it is quite difficult to numerically integrate the tails related to Sommerfeld integrals as they are not only oscillatory but also slowly decaying. Numerous research efforts have been developed to accelerate the accurate computation of such integrals, for example, by changing the integration path in the complex plane, or by using extrapolation methods. In this paper, the physical origin of the problem of the Sommerfeld integral tails is studied. Based on the physical description of the problem, a new Greens function for the radiation of a vertical electric dipole over an imperfect ground plane is derived. The new Greens function involves what is called in this paper Schelkunoff integrals. The new formulation is compared to the conventional Sommerfeld formulation, mainly with respect to the speed of convergence when the fields are calculated near the ground plane. The characteristics of the new formulation show that if Schelkunoff integrals are used in the appropriate region, the problem of Sommerfeld integral tails, which plagued the electromagnetic community for decades, can be totally abolished.

A Critical Look at the Principles of Electromagnetic Time Reversal and its Consequences

A plethora of papers can be found in the scientific literature talking about time reversal. The term time reversal exists in the publications in a variety of different fields, such as ultrasonics, acoustics, wireless communications, electromagnetics, antennas and propagation, optics, physics, and philosophy. The term time reversal by itself sounds very interesting, and can provide many interpretations that are fascinating. However, due to a loose use of this term, it may in many cases lead to fallacious interpretations and conclusions if it is not interpreted in strict scientific terms. The main goal of this paper is to explain the appropriate definition of the term time reversal in electrical engineering in general, and in electromagnetics for wireless communications, to be specific. However, the motivation of the work presented in this paper is not to raise a controversy about time reversal and its related research. Rather, the aim of this work is to highlight the basic electrical engineering fundamentals that are necessary to study time reversal and to correctly define it, and to then accordingly interpret the results. Hence, the true and fallacious benefits of electromagnetic time reversal will be exposed. To serve this purpose, the paper is divided into two parts. First, a detailed literature review is presented. The true history of the term time reversal in electrical engineering is traced. In this part, most of the reported definitions of time reversal and its claimed capabilities are summarized. Some problems with the application of time reversal in electromagnetics are discussed, both theoretically and practically. All of the time-reversal papers talk about reciprocal networks, so the fact of the non-reciprocity of a single-antenna system in the time domain (the way we generally interpret it) is presented, to prove theoretically that there is a problem with how we interpret time reversal in electromagnetics. Finally, practical examples are shown, where time reversal is used in a wireless system and in an acoustic de-reverberation problem. The exact capabilities of time reversal in electrical engineering are exposed in those experiments.

Further Validation of an Electromagnetic Macro Model for Analysis of Propagation Path Loss in Cellular Networks Using Measured Driving-Test Data

Received signal level measurements are frequently used to check the performance and the quality of service (QOS) inside the coverage area in cellular networks. These expensive, time-consuming measurements are carried out using actual driving tests to assess the coverage area of a base station for a given cell, and to thus evaluate the quality of service. In a driving-test measurement system, a receiving antenna is placed on top of a vehicle. The vehicle is then driven along radial and circular lines around the base station, to measure the received power and thus assess the quality of service. These driving-test measurements are also used to tune the empirical models in the radio-planning tools that have to be employed for various types of environments. This model tuning is a lengthy procedure. In this paper, it is shown that an electromagnetic macro modeling of the environment can provide simulation results comparable to the data one would obtain in an actual driving-test measurement for a cellular environment. The input parameters for the electromagnetic macro model can be generated using only the physical parameters of the environment, such as the height of the transmitting and receiving antennas over the ground, their tilts towards the ground, and the electrical parameters of the ground. Such analysis can provide realistic plots for the received power as functions of the separation distance between the receiving and the transmitting base-station antennas. The novelty of the electromagnetic-analysis technique proposed in this paper lies in its ability to match the macro-model-based simulation results and the driving-test measurements without any statistical or empirical curve fitting or an ad hoc choice of a reference distance. In addition, a new concept, called the proper route, is introduced to enhance the analysis of the measured data. A Method-of-Moments-based integral-equation-solver code has been used to simulate the effects of the macro parameters of the environment on the propagation-path loss of the signals emanating from a base-station antenna. The perfect match between the simulation results and the driving-test data was illustrated by monitoring the signal levels from some cellular base stations in western India and Sri Lanka, and then comparing the observed results with the simulated results. The goal here is to illustrate that these numerical simulation tools can accurately predict the propagation path loss in a cellular environment without tweaking some non-physical models based on statistical modeling or heuristic assumptions.

Application of the Schelkunoff Formulation to the Sommerfeld Problem of a Vertical Electric Dipole Radiating Over an Imperfect Ground

The objective of this presentation is to illustrate the accuracy of the Schelkunoff formulation over the Sommerfeld solution for a vertical electric dipole radiating over an imperfect ground. In an earlier paper, the alternate form of the Sommerfeld Greens function developed by Schelkunoff was presented (Schelkunoff, 1943 and Dyab, 2013). Here we demonstrate the application of this new methodology for two classes of problems. First, the problem of predicting the propagation path loss in a wireless communication environment is illustrated. The second application problem described in this paper deals with the verification of experimental data related to propagation over an Aluminum sheet at THz frequencies. It is seen that the main contribution of the reflected field is due to a specular image point as expected for a metal and the presence of surface waves in the total reflected field is absent, even though the permittivity of the metal is negative at these frequencies. Both theoretical predictions and experimental data demonstrate that there is little contribution to the reflected field due to a surface wave. Also, a clear definition is made to characterize surface waves as there is confusion as to what a surface wave really is.

On the relation between Surface Plasmons and Sommerfeld's Surface Electromagnetic Waves

The term “Surface Plasmons, SP” was first coined in the middle of the twentieth century to study the interaction of plasma oscillations with the electrons on the surface of metal foils. Surface Plasmons have a wide variety of applications such as in Terahertz spectroscopy. In the literature, Surface Plasmons are frequently related to Surface Electromagnetic Waves, SEW, which were first studied by Zenneck and independently by Sommerfeld in the early 1900s. However, Zenneck and Sommerfeld surface waves are rarely examined critically in the current literature on SP. Looking for a solid understanding for the relation between SP and SEW, it was necessary to study Sommerfelds work thoroughly. The revisiting of Sommerfelds work on Surface waves led to some important conclusions which are communicated in this paper.

Green’s Function Using Schelkunoff Integrals for Horizontal Electric Dipoles Over an Imperfect Ground Plane

Recently, Schelkunoff integrals have been used to formulate a Greens function for analysis of radiation from a vertical electric dipole over an imperfect ground plane. Schelkunoff integrals were proved to be more suitable for numerical computation for large radial distances than the Sommerfeld integrals which are used conventionally to deal with antennas over an imperfect ground. This is because Schelkunoff integrals have no convergence problem on the tail of the contour of integration, especially when the fields are calculated near the boundary separating the media and for large source-receiver separations. In this paper, the Schelkunoff integrals are utilized to derive a Greens function for the case of a horizontal electric dipole radiating over an imperfect ground plane (a two-media problem where the lower medium is lossy). A detailed comparison between the presented expressions and the conventional ones based on Sommerfeld integrals is illustrated both numerically and analytically.

Further validation of an electromagnetic macro model for analysis of propagation path loss in cellular networks using measured drive test data

Received signal level measurements are frequently used to check the performance and the Quality of Service (QOS) coverage area in cellular networks. These expensive time consuming measurements are carried out using an actual drive tests to assess the coverage area of a base station for a given cell and thus evaluate the QOS. In this paper, the novelty of the proposed electromagnetic analysis technique lies in its ability to match the macro model based simulation and measurement results without any statistical or empirical curve fitting or an adhoc choice of a reference distance. Furthermore, a new concept called proper route has been introduced to enhance the quality of measured data. The input parameters for the electromagnetic macro model can be generated using only the physical parameters of the environment like the height of the transmitting and receiving antennas over the ground, their tilts toward the ground, and the electrical parameters of the ground. A method of moments-based integral equation solver code called AWAS has been used to simulate the effects of the macro parameters of the environment. Measurements were carried out for cellular networks in western India and Srilanka.

Examining the theoretical basis for the analysis of surface plasmons in the microwave and terahertz regimes

Surface electromagnetic modes guided by thin metal layers are usually called surface plasmons SP. SP were originally discovered by Ritchie in the fifties of the twentieth century in the optical regime. Then, surface plasmons found many applications in different areas of science including terahertz and microwave devices. Departing from optics to radio frequencies, an urgent need for a solid theoretical basis to analyze SP in those regimes has appeared. The only candidate was Sommerfelds analysis for radiation of electromagnetic waves over imperfect ground planes, published in 1909. This kind of analysis has been used recently by many researches working on SP applications at radio frequencies. The main goal of this paper is to give a theoretical analysis which proves the unsuitability of Sommerfelds analysis to the problem of SP.

Antenna reciprocity and the theory of electromagnetic time reversal

In this paper, the theory of electromagnetic time reversal mirrors is examined in the light of antenna reciprocity relations in time domain. The single antenna relation is used to disprove the fallacious view of electromagnetic time reversal as a back propagation mechanism of waves. Then, the two-antenna reciprocity relation is used to explain the observed behavior of wireless channels under time reversal procedures. The time domain impulse responses of 175% bandwidth wireless channels are measured to test the spatial and temporal focusing of TR.