Advanced diffraction and wave propagation models for characterization of wireless communication channels.

摘要

Accurate prediction of radio wave propagation in a wireless communication channel is essential in the development and design of an efficient, and reliable wireless system. Current channel models are heuristic in nature and have limited applicability while a physics based approach to channel characterization can give insight into the mechanisms of radio wave propagation and inherently provides highly accurate results for any communication scenarios.; With this as a motivation, this thesis focuses mainly on the development of physics-based models for application in both rural and urban areas. For rural environments two novel models are developed for a forested environments, for two different type of communication configurations. For point-to-point communications in a forest the operating frequency must be below VHF, due to high attenuation at higher frequencies. A forest can be modeled by a dielectric slab that contains many finite cylinders, which model the tree trunks. Interactions among the tree trunks are calculated in an exact manner by using a numerical technique, Method of Moments (MoM), while the canopy is accounted for in an analytic fashion using a perturbation technique. For the other configuration, air-to-ground communication, a model is developed based on a fractal tree model and the single scattering theory. To account for near-field scattering from constituents of the fractal tree (finite cylinders) an analytical uniform solution for the scattering problem is formulated based on the known exact surface currents on an infinite cylinder. This approximation is accurate for a finite cylinder whose ratio of length to radius is large. Monte Carlo simulation is carried out to generate the desired statistical behavior of the channel. The traditional model (three layer model) is modified for a more accurate estimation of mean-field by including the existing roughness between the air and the canopy. As an application of the forest models a signaling scheme is proposed for ultra-wide band communications and the performance of the signaling scheme is assessed when a receiver and transmitter are both embedded in a forest.; In this thesis for an urban area a rigorous analysis method using MoM is applied to channel estimation. The ground plane can be approximated by an impedance surface, and a Fast Multipole method (FMM) identity is developed for an known efficient Green's function. This Green's function is applied in conventional MoM code to investigate cosite interference for DSCDMA system.