Sharif Digital Repository

In this research, diffraction limit is introduced as the major challenge in wave focusing. Common wave focusing techniques have been reviewed and classified into two categories: far-field and near-field techniques. Through examples that illustrate its major inherent and practical challenges, super-oscillation has been presented as a specific idea of focusing waves using remote sources. By introducing subwavelength inhomogeneities in the region of interest, we have demonstrated sub-diffraction wave focusing in a near-field approach. Using the multipole method, the effect of injecting sub-wavelength cylindrical particles on focusing electromagnetic waves has been studied. Since when there are... 

Electromagnetic wave scattering from rough surfaces has been the subject of intensive investigations over the past several decades for its applications in important remote sensing problems. The problem of scattering from random rough surfaces hasn't full analytical closed form solution. In millimeter-wave band some surfaces like asphalt, concrete, which has roughness in millimeter range, must be considered as rough surfaces. A major part of the literatures are specified to numerical and statistical approach. In this thesis the electromagnetic waves scattering from rough surfaces in millimeter-wave band is studied. The main goal of this thesis is to find the available ways for applying the... 

In recent years, there has been a great studying on site-specific indoor wave propagation mechanism. Conventional ray tracing method is the most common way to predict the profile of the signal in these environments. This method provides us with a deep understanding of signal propagation mechanism including reflection, refraction and diffraction. Knowing these characteristics, we can design a wireless system with optimal performance. Modified Wavefrond Decomposition Method is a ray tracing speed up technique. In this method, a bundle of rays will be traced instead of tracing single rays. If a ray bundle illuminates the receiver location, a ray that exactly crosses the receiver point would be... 

Always, there has been a great interest toward electromagnetic wave propagation and new ways for simulation of the problem. Level set methods provide a powerful mathematical framework for propagating surfaces (in 3-D) or curves (in 2-D). The contours of the moving objects are viewed as the zero level sets of a function and propagated in a Eulerian point of view by means of a Hamilton–Jacobi partial differential equation expressing the conservation of the level sets under a given velocity field. Level set methods have been applied in a wide range of domains, such as physics, chemistry, fluid dynamics, combustion, image processing… In this thesis, we introduce a new numerical procedure for... 

In this dissertation, hybrid meshless method for electromagnetic wave scattering has been presented. The idea of the dissertation is the use of two robust meshless shape functions i.e. MLS and RPIM simultaneously to solve differential and integral equations from 3D scattering equations respectively. Finite element method (FEM) and method of moments (MoM) are used extremely for computing scattering problems. These methods are powerful and their results are accurate enough. One of their disadvantages is the need of mesh generation. Mesh generation in FEM is a time consuming process. Furthermore, in electromagnetic problems that involve geometrical, the use of an underlying mesh creates... 

In this dissertation, site-specific ultrawideband propagation modeling has been presented based on ray tracing. The empirical, statistical and site-specific methods are compared and the ray tracing method is selected as an efficient and accurate site-specific approach for indoor propagation modeling. The challenges of the ray tracing method in propagation modeling of ultrawideband electromagnetic waves are outlined and some solutions are presented.In order to increase the efficiency of finding the propagation paths, the modified wavefront decomposition method is presented. This method intelligently reduces the number of emitted rays and finds the exact propagation paths from the transmitter... 

There is an ever rising interest for propagation simulation and communication channel modeling in indoor environments. Thus the demand for the development of proper software to perform these computations has been the motivation for a lot of researches on proper algorithms for these calculations. The most popular one is the ray tracing which is faster than other algorithms and needs only simple calculations. Still, the large number of these calculations needed to achieve proper accuracy does not allow ray tracing to achieve the desired speed in indoor environments. Numbers of methods have been proposed to increase its speed. One of which is the modified wavefront decomposition method which is... 

In site-specific ultrawideband propagation modeling, electromagnetic field calculations need to be performed by ray tracing at numerous frequencies of the band in order to properly include the frequency variations of the propagation mechanisms and antenna patterns. The number of frequencies at which calculations must be done, must be followed by Nyquist rate. In the previous study, it is showed that, this number can be reduced a lot by using interpolation. In this study the object is optimization of post-processing by using optimized interpolator functions. So, this study develops a systematic procedure for determining the number of required frequency samples of the interpolator... 

Nowadays, wireless communication plays a key role in everyday lives and has many different applications. Today’s communication systems require high bandwidths in order to transfer data with higher bitrates. Among these, modeling a wireless channel is one of the most important challenges in wireless communications. Modeling an indoor Ultra-Wide-Band (UWB) wireless channel has been involved in a great number of researches. In order to model a UWB channel, we need to correctly investigate electromagnetic waves inside the buildings. Amongst different methods that are used for this aim, ray tracing method is one of the most accurate and fast ones. In this thesis, time-domain reflection and... 

The purpose of this study is to estimate the temperature distribution of the internal body organs using electromagnetic waves. When the external electromagnetic field propagates in biological tissues under a process like microwave hyperthermia, the temperature of the tissue at the field concentration area increases. Due to temperature and frequency dependence of the tissue dielectric, the tissue permittivity changes; thus, the electric field scattered by the tissue changes. Therefore, by measuring the field scattered from the object, dielectric changes of the tissue structure can be calculated. Since the biological tissues’ dielectric coefficient at frequencies lower than 2GHz is linear,... 

In this thesis, a microwave imaging system has been designed and fabricated for medical applications. Compared to the conventional imaging methods such as X-ray and Magnetic Resonance Imaging (MRI), this technique is safer and more cost efficient, specifically for patients who need more frequent screenings over a short period of time, such as patients diagnosed with cancer. In electromagnetics, microwave imaging is referred to locating the heterogeneities or finding complex relative permittivity distribution in the imaging domain. In this project, in order to find the complex dielectric constant distribution of the imaged object, an array of 8 monopole antennas have been positioned on the... 

In recent years, the application of machine learning algorithms for predicting physical phenomena and achieving a balance between problem-solving speed and accuracy has gained prominence. The propagation of radio waves in indoor environments, due to various phenom- ena such as multipath signals, requires a significant amount of time for simulation. This thesis explores the phenomenon of indoor wave propagation using machine learning algorithms. By utilizing physical features, the error in the predicted received power map is reduced. These features include the presence of a line-of-sight ray, the number of obstacles between the transmitter and receiver, the distance between the transmitter... 

This thesis explores the feasibility of implementing terahertz wireless communication in indoor environments (as suggested in 6G) by modeling the propagation of terahertz band waves. The roughness of indoor surfaces at the terahertz frequencies is investigated through the detailed examination of the electromagnetic fields scattered from these surfaces using the method of moments (MoM). To account for the random roughness of these surfaces, the Monte Carlo method is utilized, generating surfaces with Gaussian statistical distribution, and obtaining the scattered electromagnetic fields using MoM. The final results represent an average of all simulated scenarios. In addition, the study... 

Ray-tracing method is extensively used for wave propagation analysis of indoor and outdoor environment for narrowband systems. The main goal of this research is to extend ray-tracing method for site-specific modeling of wideband electromagnetic propagation. In this thesis, Time-domain reflection and transmission coefficients have been computed for dielectric half-space and dielectric slab with frequency dependent permittivity. First, analytical form of Fresnel reflection coefficient for vertical and parallel polarizations in time-domain have been computed for dispersive half-spaces with lossy, extended Debye, two-pole Debye and multi-pole Debye models. By using these time-domain... 

In this thesis the scattering analysis of periodic rough surfaces is studied using the Spectral Finite Difference Time Domain (SFDTD) method. This method takes advantages of both wideband frequency response and stability criterion for all incident angles compared to other methods in solving the periodic problems. In the SFDTD method a wave with Constant Transverse Wavenumber (CTW) is used as the incident wave. As a result, the problem of time delay in periodic boundary is solved and Periodic Boundary Condition (PBC) can be implemented in time domain directly. Periodic rough surfaces are important part of electromagnetic propagation surfaces. Therefore analysis of scattering from these... 

The goal of the research presented in this thesis is assessment of millimeter-wave (mm-wave) reflectometry as a potential method for non-invasive detection of skin cancers. The reasons for choosing mm-waves are lower penetration, higher resolution and technology maturity. The main work of this research is numerical simulation of reflection from skin and other media as well as reflection measurement of some skin-equivalent phantoms at mm-wave frequencies. By exposing the skin to mm-waves and studying the characteristics of the reflected wave malignant lesions are differentiated from benign and normal skin tissues. Since Oxygen molecules have strong attenuation peaks around 60 GHz band, this... 

In this dissertation, a proper closed-form representation has been presented for the site specific indoor radio-wave propagation. Characteristic Green’s function (CGF) technique is used to obtain the integral representation of the Green’s function of a rectangular 2D room with dielectric slab walls. Based on numerical simulations, this method provide quite accurate results for wave propagation in the interior region (away from the walls) but the error increases for source or field points near the walls. The presented integral form of Green’s function has slow convergence and computed using Real images, complex images and rational function fitting methods. An error of less than 20 percent is... 

Microwave imaging has been introduced as a substitute to previous diagnostic medical devices due to its sufficiency, portability and nondestructive radiations. Studies over the feasibility of embedding Microwave imaging in medical application has been done by VNAs which are considered as large and expensive devices; therefore, these studies are in fact not addressing the goal we are thriving for regarding low cost and portability characteristics of sought for devices. This document introduces a novel medical imaging device which can be sufficiently designed in low cost and indeed portable. It consists of 8 monopole antennas located in a circular configuration with radius 15.5 centimeters... 

Recently, there has been a great interest toward site–specific modeling of outdoor radio wave propagation. Wave propagation in very large environments compared to a wavelength builds up an important category of electromagnetic wave propagation problems which occurs in
versatile criteria from optical devices to long wireless microwave links. Well-known methods such as different ray tracing schemes used in these problems suffer from long execution time and great amount of needed memory. A new method has been proposed in this thesis to overcome such difficulties for high frequency wave propagation in large scale problems. The proposed method is based on tracing 3D vectorial Gaussian beam.... 

Analysis of periodic structures is an important topic in the electromagnetic wave theory. Also, scattering from rough surfaces is of importance due to its theoretical and practical aspects. Integral equation approaches in combination with the Method of Moments (MoM) are among the most frequently used techniques for solving scattering problems related to rough surfaces due to their flexibility in modeling different geometries. Here, the electromagnetic wave scattering from periodic rough surfaces is carried out using MoM and a closed form Green’s function that accelerates the computation. Different methods, previously proposed to accelerate the convergence of periodic Green’s function are...