Pattern Reconfigurable MIMO Antennas for Multiband LTE Operation

Sammanfattning: Nowadays, multiple antennas are becoming widely used in small mobile terminals as they can significantly improve wireless communication performance in terms of link reliability and spectral efficiency. Also, pattern reconfiguration is another new trend for antenna design as it enables the antennas to adapt to different propagation and user scenarios. However, due to the limited space in the terminals, it is difficult to implement both techniques and obtain good antenna performance, especially for frequency bands below 1 GHz. This is because the mobile chassis is often shared by different antennas as the main radiator, regardless of the antenna structure, which leads to high mutual coupling and correlation. In this thesis, a dual-band (824-894 MHz and 1850-1990 MHz) MIMO antenna system with pattern reconfiguration at the low band was designed based on the theory of characteristic modes (TCM). The work began with the design of a single reconfigurable antenna that can be switched between an inverted-F antenna (IFA) mode and a bezel mode, with low envelope correlation of around 0.2 between the states. This was followed by the implementation of a second antenna (i.e., a single-side T-strip) for MIMO operation. The inclusion of the second antenna required the antenna system to be re-optimized, and several performance trade-offs were observed and investigated, including the tradeoff in the mutual coupling between the MIMO antennas in different states as well as the tradeoff between the mutual coupling and the inter-state correlation. The final design of the pattern reconfigurable MIMO antennas yields an inter-state correlation of around 0.3, and an intra-state correlation of below 0.1 and 0.2, respectively. All the studies were carried out in CST Microwave Studio and Matlab.