Information Theoretic Modeling of Antennas

Abstract

Linear time-invariant (LTI) electrically small antennas (ESAs) have inherent limitations in their capabilities. Two significant limitations apply to LTI antennas: a lower bound on the antenna Q-factor, which causes a narrow bandwidth limitation; and impedance matching constraints, which restrict the ability to effectively radiate power. To surpass these limitations, recent research has analyzed the use of active antennas with nonlinear and time-varying elements. Previous work has demonstrated that a class of linear time-varying (LTV) antennas employing parametric amplification by varying the antenna’s reactance over time results in the capability for significant bandwidth and gain improvements past traditional limitations. However, due to the disruption of linearity or time-invariance in novel antenna designs, typical communications metrics in the context of these systems are not well-founded, and the integration of mathematical communications theory with the added physical capabilities of new active antenna designs is not formally addressed in either antenna design or communications theory. This thesis focuses on capturing information-theoretic metrics of ESAs, principally the maximum channel capacity and achievable data rate. The simulated responses of an electrically small LTI antenna are compared with the simulated responses of two different LTV parametric amplification antenna designs, and the maximum theoretical capacity and achievable data rates are analyzed. Data rates using realizable modulation and coding schemes are simulated using conventional quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM). The analysis demonstrates that an LTV antenna operating in an internally-noise-limited bandlimited channel can achieve a theoretical performance increase of 2-4x the LTI capacity. In a case where the communication signal is limited to the antenna’s 3 dB bandwidth, the capacity gains can improve as much as 12x the LTI capacity due to the significant bandwidth broadening capabilities, and the data rate improvement is achievable using standard communications methods. Considering the significantly increased data rates, these novel antenna designs show promise in dramatically improving the efficiency, reliability, and connectivity of modern communications.