Study of reconfigurable antennas and their applications in MIMO systems

Reconfigurable antennas (RAs) are at the frontier of current antenna research and are excellent antenna candidates for advanced wireless communication applications. The basic structure of a RA is similar to that of a conventional one; however, by using electrical, mechanical or other means to change the fundamental operating mechanism of an individual radiator, the frequency and radiation characteristics of the RA can be varied significantly. This makes a single RA equal to multiple antennas with fixed functionalities. Thanks to the agility and diversity of RAs, they find wide application in Software Defined Radios, multiple-input-multiple-output (MIMO) systems, diversity communication systems, satellite communications, and military communication systems. One of the greatest design challenges for antenna reconfigurability is changing one characteristic of an antenna, such as the frequency response, without affecting others (polarization or radiation pattern), since the frequency response and the radiation characteristics of an antenna are always highly related. Furthermore, for RAs using electronic components to achieve reconfigurability, the design of an effective dc bias network that has a limited influence on the antenna performance is another important design issue for antenna engineers. In this dissertation, motivated by the increasing significance of RAs as well as the demand for new design methods for compact RAs with simple dc bias networks, a thorough investigation is presented on RAs in terms of frequency, polarization, pattern, as well as compound frequency and polarization RAs. A few novel RAs with dc bias networks are designed, fabricated and measured. And they have demonstrated many advantages over other corresponding reported designs. In addition, a literature review reveals that, although a considerable number of papers on RAs have been published, much fewer are reported on the study of the applications of RAs in wireless communication systems. In order to examine the practical application of RAs, the proposed polarization and pattern RAs are incorporated separately in 2 x 2 MIMO orthogonal frequency division multiplexing (OFDM) systems. Real-time channel measurements are conducted in both line-of-sight (LOS) and non-LOS (NLOS) scenarios. Compared to the systems with non-reconfigurable antennas, a significant enhancement of the system capacity is accomplished.