Design of Frequency Selective Surface (FSS) based Fields Decorrelating Radome for MIMO Antennas

Abstract

Use of Multiple-Input Multiple-Output (MIMO) systems is the basic underlying technology employed in 4G wireless systems to achieve high data rates. The same will also be used in the upcoming 5G systems. The design of MIMO antennas is challenging due to size limitation and the required radiation characteristics. A major issue in the MIMO systems is the correlation induced among the propagation channels due to the radiation patterns of it’s antenna elements, quantified by the correlation coefficient. To be able to take advantage of MIMO systems, the antenna design should result in a low cor relation coefficient. Since multiple antennas are placed within a small form factor device, this necessitates careful antenna design with low correlated radiation patterns. Many works have appeared in literature which focuses on increasing the port isolation between the antenna elements of MIMO an tenna to reduce the correlation coefficient. However, recent studies have shown that isolation improvement may not guarantee a reduction in the correlation coefficient. Therefore, a separate design effort was required to reduce the correlation coefficient by deliberately decorrelating the individual radiation patterns of each antenna element. This thesis presents a new method to decorrelate the fields of MIMO elements and lower the correlation coefficient. The method utilizes a phase-gradient FSS radome / superstrate above the antenna elements in a low profile Fabry-Perot cavity configuration to achieve this task. A general design methodology for the FSS radome is presented and a system is designed for a two element patch based MIMO antenna op erating at 5.25 GHz. The method is validated through full wave simulations of the antenna model and measurements of a fabricated prototype. The results show a reduction of more than 95% in the correlation coefficient at the design frequency, thus projecting better MIMO performance. The proposed method may be employed for MIMO antennas in WLAN access points, or in any application where the FSS pattern can be printed on the radome of the antenna or on the dielectric enclosure of the system. In such applications, the proposed method will have the advantage of ensuring reduction in the correlation coefficient without occupation of additional space.