Electromagnetic Waves And Signal Processing PDF Download

Abstract: This dissertation focuses on the application and development of time reversal (TR) based signal processing techniques for ultrawideband (UWB) electromagnetic (EM) waves in random media. TR techniques exploit the TR invariance of the wave equation to take advantage of the retransmitted (time-reversed) fields for improved imaging and focusing. This is because retransmitted signals propagate backwards through the same medium and undergo similar reflection, refraction and multiple scattering that they underwent during the forward propagation, resulting in focusing around the initial source locations. If multiple scattering occurs in the intervening media, refocusing resolution can overcome the classical diffraction limit characterizing superresolution, a somewhat counterintuitive result. With the success of initial TR experiments in acoustics, there has been a strong interest in the application of TR methods using radio frequency EM waves. It is also our motivation here to develop TR techniques for UWB electromagnetic waves. We start by investigating superresolution effects of TRed UWB EM waves under continuous random background media and examine their dependency on medium statistics. In contrast to the acoustic case, polarimetric TR exploiting EM polarization is also analyzed to observe depolarization effects on the refocusing of TRed EM waves. We continue with the application of TR in dispersive media (such as soil or biological tissues) where TR invariance is broken and yields performance degradation. We introduce a physical compensation technique based upon both space and frequency dependent inverse filters to improve the performance of TR techniques for homogeneous and random dispersive media. We also develop a physical full time-domain selective focusing method on the desired scatterers in the presence of others. Both narrowband and UWB TR-based imaging techniques for detection and localization of distinct scatterers in inhomogeneous background media are investigated under several perturbations. Specifically, we investigate the effects due to clutter, noise, dispersion and losses. Moving TRAs and restrictions on array elements on these TR-based imaging methods are also considered. Finally, we introduce a new TR-based imaging functional based on the simultaneous utilization of spatial and UWB frequency data to obtain a novel method for UWB imaging of embedded scatterers in homogeneous and random media.