Design of MIMO Antennas using Transformation Electromagnetics

Abstract

Multiple-input-multiple-output (MIMO) system is an important underlying technology for all of the current and future wireless communication standards. By employing multiple radiating elements at both the transmit and receive sides, these systems offer several benefits such as better link reliability (in spatial diversity) and enhanced channel capacity (in spatial multiplexing). The diversity or multiplexing gain offered by such systems depends upon the number of uncorrelated channels formed between the transmitter and receiver. Correlation coefficient is a measure of how much the communication channels formed in a MIMO systems are independent or isolated from each other. For an isotropic propagation environment, the correlation coefficient only depends on the radiation pattern of a MIMO antenna elements. In order to reap the benefits of a MIMO system, the antenna design should induce low correlation among channels. Since in a small form factor device multiple antenna elements are placed in closed proximity, this necessitates careful antenna design with low correlated radiation patterns. Recently, the concept of radiation patterns decorrelation was achieved using a phase-gradient partially reflective surface (PRS) placed above a patch based MIMO antenna elements in a Fabry-Perot cavity configuration. This technique results in considerable reduction in correlation coefficient but a drawback associated with this techniques is that there is an increase in mutual coupling between the antenna elements due to presence of a reflecting surface above them. This thesis presents a new method to design a field decorrelated MIMO antenna with high port isolation level, comprising of closely placed patch elements in a MIMO configuration, using a systematic methodology based on transformation electromagnetics. A general design methodology is presented for the design of all-dielectric device for MIMO antennas such that it is applicable for the design of antenna elements for different frequency bands. A two elements patch-based MIMO antenna system is designed at 3GHz in order to verify the effectiveness of the proposed methods with the help of simulations of the antenna system modelled in a finite element method based full wave EM software as well as measurements of the fabricated prototype. The proposed technique not only helps reduce the correlation coefficient value in the range of 62% to 99% in the operating bandwidth but also improve the port isolation between the antenna elements by better than 3.9 dB, thereby offering enhanced MIMO diversity and multiplexing performance. The large and small-sized base station sides including rooftop towers and indoor wireless access points are some of the systems where this technique can be employed in their MIMO antenna design.