Guest Researcher, Dr.
Design and Evaluation of Compact Multi-antennas for Efficient MIMO Communications
Date: Tuesday, November 29, 2011
Location: Lecture hall E:1406 of the E Building, Ole Römers väg 3, Lund
Main Supervisor: Assoc. Prof. Buon Kiong (Vincent) Lau
Faculty opponent: Prof. Daniel D. Stancil, North Carolina State University, USA
Prof. Gert F. Pedersen, Aalborg University, Denmark
Dr. Mattias Frenne, Ericsson AB, Sweden
Assoc. Prof. Marco D. Migliore, University of Cassino, Italy
Prof. Mats Gustafsson, Lund University, Sweden
After completing the course work component of my Master of Science degree program, which concerns digital communication systems and technologies, a thesis work on radio channel modeling brought me into the world of MIMO. To my understanding, MIMO has been one of the hottest research topics in radio communications over the past decade. When I finished my master’s thesis project, I felt that I had learned something about MIMO and wondered if there were still exciting opportunities left in this research field. As it turned out, despite extensive research in MIMO technology, surprisingly little effort has been directed towards assessing and mitigating the impacts of different antenna impairments on the performance of MIMO systems in their realistic usage conditions. In this context, the overarching question that this thesis tries to answer is: how should we design and evaluate compact multi-antennas that can deliver efficient MIMO communications?
The thesis is a compilation of an introduction to the research field and a summary of my contributions, together with five research papers that present the main results achieved during my graduate study. It extends the understanding of MIMO systems from an antenna and propagation perspective, and offers valuable insights on the design of compact yet efficient multi-antennas.
The included papers are:
 R. Tian and B. K. Lau, “Experimental verification of degrees of freedom for co-located antennas in wireless channels,” submitted to IEEE Transactions on Antennas and Propagation, Jun. 2011 (revised Oct. 2011).
 R. Tian, V. Plicanic, B. K. Lau, and Z. Ying, “A compact six-port dielectric resonator antenna array: MIMO channel measurements and performance analysis,” IEEE Transactions on Antennas and Propagation, vol. 58, no. 4, pp. 1369 – 1379, Apr. 2010.
 R. Tian and B. K. Lau, “Uncoupled antenna matching for performance optimization in compact MIMO systems using unbalanced load impedance,” in Proc. IEEE Vehicular Technology Conference (VTC Spring 2008), pp. 299 – 303, Singapore, May 11 – 14, 2008.
 R. Tian, B. K. Lau, and Z. Ying, “Multiplexing efficiency of MIMO antennas,” IEEE Antennas and Wireless Propagation Letters, vol. 10, pp. 183 – 186, 2011.
 R. Tian, B. Wu, B. K. Lau, and J. Medbo, “On MIMO performance enhancement with multi-sector cooperation in a measured urban environment,” submitted to Electronics Letters, 2011.
The use of multi-antenna systems with multiple-input multiple-output (MIMO) technology will play a key role in providing high spectrum efficiency for next generation mobile communication systems. This thesis offers valuable insights on the design of compact multi-antennas for efficient MIMO communications. In the course of the thesis work, several novel six-port antenna designs have been proposed to simultaneously exploit all six possible degrees-of-freedom (DOFs) by means of various antenna diversity mechanisms (Paper I & II). Moreover, the thesis also examines the potential of using uncoupled matching networks to adaptively optimize compact multi-antenna systems to their dynamic usage environments (Paper III). Furthermore, a simple and intuitive metric is proposed for evaluating the performance of MIMO antennas when operating in the spatial multiplexing mode (Paper IV). Last but not least, cooperation among multi-antenna systems at all three sectors of a given cellular base station is shown to deliver significant benefit at sector edges (Paper V). The thesis with the five included research papers extend the understanding of MIMO systems from an antenna and propagation perspective. It provides important guidelines in designing compact and efficient MIMO antennas in their usage environments.
In Paper I, a fundamental question on the number of effective DOFs in a wireless channel is explored using two co-located six-port antenna arrays. The antenna elements of both arrays closely reproduce the desired characteristics of fundamental electric and magnetic dipoles, which can efficiently extract angle and polarization diversities from wireless channels. In particular, one of the two array designs is by far the most electrically compact six-port antenna structure in the literature. Analysis of measured channel eigenvalues in a rich multi-path scattering environment shows that six eigenchannels are successfully attained for the purpose of spatial multiplexing.
To study the potential of implementing different diversity mechanisms on a practical multi-port antenna, Paper II builds on an existing dielectric resonator antenna (DRA) to provide a compact six-port DRA array that jointly utilizes space, polarization and angle diversities. In order to fully substantiate the practicality of the DRA array for indoor MIMO applications, the compact DRA array together with two reference but much larger arrays were evaluated in an office scenario. The use of the compact DRA array at the receiver is shown to achieve comparable performance to that of the reference monopole array due to the DRA array’s rich diversity characteristics.
In Paper III, the study of uncoupled matching networks to counteract mutual coupling effects in multi-antenna systems is extended by allowing for unbalanced matching impedances. Numerical studies suggest that unbalanced matching is especially effective for array topologies whose effective apertures can vary significantly with respect to the propagation channel. Moreover, it is also demonstrated that unbalanced matching is capable of adapting the radiation patterns of the array elements to the dynamic propagation environment.
Paper IV introduces multiplexing efficiency as a performance metric which defines the loss of efficiency in decibel when using a multi-antenna prototype under test to achieve the same multiplexing performance as that of an ideal array in the same propagation environment. Its unique features are both its simplicity and the valuable insights it offers with respect to the performance impacts of different antenna impairments in multi-antenna systems.
In Paper V, intrasite cooperation among three 120◦-sector, each with a cross-polarized antenna pair, is investigated in a measured urban macrocellular environment. The single-user capacity improvement is found to exceed 40% at the sector edges, where improvements are most needed. In addition, a simple simulation model is developed to analyze the respective impact of antennas and specific propagation mechanisms on the measured cooperative gain.