Multi-element Antennas for Mobile Communication Systems: Design, Evaluation and User Interactions

Capacity improvement using Multiple-Input Multiple-Output (MIMO) technology necessitates multiple antennas at both transmitter and receiver ends. Antenna design challenges are related especially to mobile terminal, in which antennas need to be small, located relatively close to each other and interact with a user. Due to these limitations, mobile terminal antennas are potentially inefficient and do not provide a good MIMO performance. Traditional antenna design and characterization methods are not directly applicable to multiple antennas, and therefore research in these fields is needed. This thesis contributes two topics. First, characterization methods of multiple antennas in compact mobile terminals are studied. In the studied methods, the effects of propagation environment and user on the multiple antenna performance are taken into account. The time-varying propagation environment and user’s hand grip emulating the true scenario of the users in real-life environments are analyzed. In particular; different terminal orientations, hand grips and locations are investigated either with one or two hands holding the terminal. Second, the thesis has scientific contribution on introducing novel multi-antenna structures with elements ranging from two to eight, and frequencies ranging from 900 MHz UHF to 3500 MHz LTE bands. Specifically, this thesis advances the design, evaluation and user interaction of multiple antennas in mobile terminals operating at the 3500 MHz. The proposed structures are shown to provide good diversity and MIMO performances. Selection of suitable antenna type and their locations on the terminal chassis are shown to be important. Based on author’s experience, the proposed designs are particularly suitable for realizing a multi-element antenna structure that is tolerant to the user. Applicability of multiple antennas in compact mobile terminal for radio direction finding (RDF) is studied. Detailed analysis has also been carried out on the antenna topology on a mobile terminal and consideration of user interaction to ascertain the best use of antennas for such an application. It is found that ambiguity characterization method could be useful evaluation tool for designing a multi-antenna structure for RDF application.