AbstractsEngineering

The rapid growth of cellular technology over the past decade transformed our lives, enabling billions of people to enjoy interactive multimedia content and ubiquitous connectivity through a device that can fit into the palm of a hand. In part the explosive growth of the smartphone market is enabled by innovative antenna system technologies, such as multiple-input multiple-output (MIMO) systems, facilitating high data rates and reliable connections. Even though future deployment of Long Term Evolution Advanced (LTE-A) is expected to provide seamless internet connectivity at even higher speeds over a wide range of devices with different form factors, fundamental terminal antenna limitations can severely impact the actual performance of the terminal. One of the key challenges in terminal antenna design are user-induced losses. It has been shown that electromagnetic absorption in body tissues as well as antenna impedance mismatch due to user proximity significantly degrade terminal antenna performance. Moreover, user interactions are non-static, which further complicates terminal design by leading to the requirement of evaluating a wide range of hand grips and usage scenarios. This doctoral thesis explores these challenges and offers useful insight on effective user interaction mitigation. In particular, state-of-the-art multiple antenna designs have been investigated in an attempt to formulate guidelines on efficient terminal antenna design in the presence of a user (Paper I). Moreover, the major part of the thesis considers the method of adaptive impedance matching (AIM) for performance enhancements of MIMO terminals. Both ideal and very practical and realistic AIM systems have been studied in order to extend the knowledge in the area by determining achievable performance gains and providing insights on AIM gain mechanisms for different terminal antenna designs, propagation environments and user scenarios. In Paper I, five different MIMO terminal antenna designs were evaluated in 11 representative user scenarios. Two of the prototypes were optimized with the Theory of Characteristic Modes (TCM), whereas the remaining three were based on more conventional antenna types. Multiplexing efficiency (ME) was used as the MIMO system performance metric, assuming an ideal uniform 3D propagation environment. The paper focuses on performance at frequency bands below 1 GHz due to the more stringent size limitations. Paper II presents a simulation model of the complete physical channel link based on ideal lossless AIM and evaluates the potential of AIM to mitigate user effects for three terminal antennas in four user scenarios. The prototypes studied have different performances in terms of bandwidth and isolation. MIMO capacity was used as the main performance metric. In order to gain insight on the impact of terminal bandwidth, as well as system bandwidth on AIM performance, capacity calculations were performed both for the center frequency and over the full LTE Band 13. In Paper III, a practical AIM…