Design and analysis of reconfigurable MIMO antennas for next generation wireless communication systems

摘要

In the United States, the forthcoming Long Term Evolution (LTE) and cognitive radio (CR) systems are going to operate in the spectrum vacated with the digital television transition (LTE: 698 MHz ??? 806 MHz; CR: 512 MHz ??? 698 MHz ???TV white space frequencies???). Both of the systems are going to include multiple-input multiple-output (MIMO) technology as a standard feature due to the exemplary performance of MIMO links in rich-scattering environments over the traditional single-input single-output (SISO) configuration. To achieve the capacity gain from MIMO, it is essential to keep the mutual coupling between the antenna elements low so multiple signals can be resolved effectively. Given the lower operating frequency of the new systems as compared to the WiFi and the existing cellular standards, the antennas in portable devices must be electrically small and are likely to be inefficient. It is also impractical to separate antennas to get low mutual coupling. As a result, the coverage of the systems would not only be limited, but the capacity of the systems would also suffer. A remedy to the problem is to push the cellular concept further down to the femtocell level. The femtocell base station can be considered as a small-sized, low-power access node serving customer premises such as home or office areas. As the operating bands of the new systems are adjacent to each other, it could be economical to build a single hybrid LTE/CR low-power access node that supports both systems. Therefore, there is a need for a new MIMO antenna solution that covers the LTE and CR bands. The design must also be compact and robust in the face of unknown packaging and deployment positions.The first part of this dissertation describes the boundary perturbation method for inducing degenerated modes in dielectric resonator for MIMO operation. Based on the proposed method, a 700 MHz dual-mode MIMO dielectric resonator antenna for LTE femtocell base station is designed and tested. The maximum dimension of the proposed antenna is only 0.2??0 while the worst case measured mutual coupling between the radiating ports is less than ???30 dB. It is estimated that the proposed antenna is able to offer an average of 14% improvement of the channel capacity as compared to a pair of orthogonal dipole antennas. Furthermore, a frequency reconfigurable version of the proposed MIMO antenna for the hybrid LTE/CR low-power access node is described. Depending on the mobile user???s need and the propagation environment, the proposed antenna can have two modes of operation: MIMO mode for capacity enhancement and frequency sweep mode for channel optimization. The proposed antenna can hence allow for a more flexible and efficient use of the radio spectrum. Finally, this work concludes with a comprehensive study of nonlinear effects in frequency reconfigurable MIMO antennas with integrated solid state switching and tuning devices. The effect of mutual coupling and device placement on the antenna nonlinearity as well as potential remedies for the undesirable nonlinear effects are discussed thoroughly.