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Investigation of Vector Antennas and their Applications
Gaurav Gupta
Date: 2008-08-22
Degree: PhD - Electrical Engineering
Advisory Committee
Dr. Brian L. Hughes - Committee Co-ChairDr. Gianluca Lazzi - Committee Chair
Dr. J. Keith Townsend - Committee Member
Dr. Robert T. Buche - Committee Member
Abstract
The use of multiple antennas at the transmitter and receiver can significantly improve the performance of wireless communication systems. In recent years, there has been a lot of interest in vector or multi-polarized antennas that can provide a compact alternative to spatial array antennas that are conventionally used in multiple antenna systems. In this dissertation, we investigate the performance benefits (like degrees of freedom, channel capacity, mean signal energy etc) that vector antennas can provide when used in two types of multiple antenna systems: multiple-input multiple-output (MIMO) communication systems and frequency selective surfaces (FSSs). We first characterize the degrees of freedom of fields produced by a linear array of tri-polarized antennas in a MIMO communication system. Using signal space concepts, we derive a sufficient condition for three-fold increase in the degrees of freedom for a system that uses tri-polarized antennas as compared to a system that uses uni-polarized antennas. We show that this condition holds true for three different types of antenna elements: infinitesimally small antennas, finite length dipoles and long wire traveling wave antennas. Since the degrees of freedom determines the slope of channel capacity vs. signal-to-noise ratio (SNR) curve at high SNRs, this result indicates the possibility of increasing channel capacity using tri-polarized antennas. We then consider practical aspects of vector antenna design. Using simulations, we design two types of vector antennas, one with half-wavelength dipole elements and second with meanderline traveling wave antenna elements. These vector antennas were then fabricated and tested to verify that they provide low return loss and inter-element mutual coupling. We calculated the channel capacity of the system obtained by arranging these antennas in a linear array configuration. As predicted by the degrees of freedom result, we found that the capacity increased three-fold in both cases as compared to an analogous system with uni-polarized antennas. Finally, we consider the use of vector antennas as unit elements of FSSs. FSSs act as filters to the incident electromagnetic waves based on their frequency and polarization. The reflected signal acts as a signature and can be used to detect the presence of FSS. We design and integrate two prototypes of vector antennas on fabric. Using a monostatic transceiver, we show that FSSs that use vector antennas provide a higher reflected mean-signal-energy (MSE) compared to FSSs that use uni-polarized antennas. We also found that the use of vector antennas makes the FSS more robust to misalignment with the transceiver.
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