Characterisation of Multiple Antennas and Channel for Small Mobile Terminals by Yue Gao A thesis submitted to the University of London for the degree of Doctor of Philosophy Department of Electronic Engineering Queen Mary, University of London United Kingdom June 2007 TO MY FAMILY Abstract MIMO (Multiple Input Multiple Output) technology has been regarded as a practical approach to increase the wireless channel capacity and reliability. In this technology, the performance of multiple antennas has a significant effect on the MIMO channel capacity. Currently, the MIMO channel capacity is mostly evaluated without accounting for the practical aspects of multiple antennas. Though multiple antennas have been considered in a number of studies, only dipole or monopole antennas were used to evaluate the channel capacity. Practically, multiple dipoles and monopoles are no longer used for small mobile terminals because of their high profile and poor isolation performance. Implementingmultipleantennasonasmallmobileterminalremainsamajorengineering challenge. Theobjectivesofthisthesisaretodesignpracticalantennaarraysforsmallmobiletermi- nalsrunningMIMOapplications,andtoobtainthecorrespondingchannelcharacteristics in a realistic environment. The research is conducted in the following areas. AnovelPIFA(PlanarInverted-FAntenna)designanditsarrayareproposedandstudied. The single PIFA with a small ground plane constitutes a stand-alone structure. The groundplane,assmallastheantenna,islocatedbetweenthePIFAandthePCB(Printed Circuit Board) so that the PCB is no longer acting as a ground plane for the PIFA. The two PIFAs mounted on the PCB do not share the same ground plane. Consequently, the isolation performance between the two modified PIFAs is significantly improved. The characteristics of both single-element and dual-element PIFA in the 5.2GHz and 2.5GHz frequency bands are evaluated in simulation and measurement. Following the study of multiple antennas, the concept of antenna diversity technology was introduced into the Galileo navigation system for the first time in the GAC (Galileo i Advanced Concept) project funded by GJU (Galileo Joint Undertaking). A statistical model was developed to analyse antenna diversity performance. The key parameters such as angle of arrival and cross-polar ratio of this model are specified for the Galileo system. The antenna diversity technique studied could be deployed for a small Galileo terminal. A realistic MIMO channel model is established based on a ray tracing simulator, i.e. Wireless InSite. Practical aspects such as the antenna configurations and orientations, radiation patterns, and specific environments are included in the model to evaluate the MIMO channel capacity. The capacity of a MIMO system employing four ideal dipole antennas in an indoor environment is investigated numerically by using the realistic MIMO channel model above. The results agree with those of the IEEE 802.11 MIMO model. Furthermore, the model is used to study the channel capacity of a dual-element modified PIFA array on a mobile terminal. It has been demonstrated that this PIFA array is suitable for practical MIMO applications. ii Acknowledgments Over the past three years of my PhD studies, I have been lucky to study and work with some of the most renowned researchers in the Department of Electronic Engineering at Queen Mary, University of London. When thinking back of my first day in London and the achievement of today’s work, I am beholden to a long list of research staffs, friends and my parents. First of all, I express my deep and sincere thanks to my supervisor, Professor Xiaodong Chen for his guidance, support and encouragement over the past three years. His office door has always been opened, both literally and metaphorically, and I could always talk to him no matter how busy he was. I can only feel flattered for the confidence he has always shown in me. Fortunately, I have benefited from his extraordinary motivation, great intuition and technical insight. I just hope my thinking and working attitudes have been shaped according to such outstanding qualities. I would also like to express my warm and sincere thanks to Professor Clive G. Parini for his valuable suggestions and encouragement during the course of my PhD study. A special acknowledgement goes to Dr Zhinong Ying and Mr John Dupuy for their guidance and assistance during my measurement works conducted in the Mobile Com- municationsABatSonyEricssonandAntennaMeasurementLaboratoryatQueenMary University of London. A special appreciation is given to my colleagues, Dr Choo Chiau, Dr Weekian Toh, Miss Maria-Anna Setta, Dr Jianxin Zhang and Dr Yasir Alfadhl for their valuable technical and scientific discussions and feasible advices. Last but certainly not least, I would like to thank my parents, my sister and Miss Ying Chen for their patience, understanding, support and love. iii Table of Contents Abstract i Acknowledgments iii Table of Contents iv List of Figures viii List of Tables xv List of Abbreviations xvii 1 Introduction 1 1.1 Background of MIMO systems . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1.1 Space time coding . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Spatial multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1.3 Hybrid scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Review of the State-of-Art . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.4 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 MIMO systems 16 2.1 SISO channels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 iv 2.1.1 AWGN channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.1.2 Fading channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2 Impulse response of a multipath fading channel . . . . . . . . . . . . . . . 19 2.3 Outage capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4 MIMO channel structure and capacity . . . . . . . . . . . . . . . . . . . . 21 2.4.1 Orthogonal parallel channels . . . . . . . . . . . . . . . . . . . . . 23 2.4.2 MIMO channel capacity . . . . . . . . . . . . . . . . . . . . . . . . 25 2.5 Physical understanding of MIMO channel . . . . . . . . . . . . . . . . . . 26 2.5.1 Antenna spacing effect . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.5.2 Environment effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3 Small and diversity antennas 32 3.1 Development of antennas on small mobile terminals . . . . . . . . . . . . 33 3.1.1 Monopole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.1.2 Helix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.1.3 Inverted-L Antenna (ILA) and Inverted-F Antenna (IFA) . . . . . 37 3.2 Requirements for multiple antennas on small mobile terminals . . . . . . . 41 3.2.1 Mutual coupling effects . . . . . . . . . . . . . . . . . . . . . . . . 42 3.2.2 Diversity technology . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.3 Antenna diversity techniques . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.3.1 Classification of antenna diversity . . . . . . . . . . . . . . . . . . 44 3.3.2 Combining methods . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.3.3 Diversity gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.3.4 Diversity performance analysis . . . . . . . . . . . . . . . . . . . . 52 3.3.5 Propagation factors . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.4 Galileo navigation applications of multiple antennas . . . . . . . . . . . . 58 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 v References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4 Multiple antenna design 67 4.1 Single modified PIFA at 5.2GHz . . . . . . . . . . . . . . . . . . . . . . . 68 4.1.1 Design of a single modified PIFA . . . . . . . . . . . . . . . . . . . 69 4.1.2 Characteristics of the single modified PIFA . . . . . . . . . . . . . 70 4.2 Dual-element modified PIFA array at 5.2GHz . . . . . . . . . . . . . . . . 75 4.3 Modified PIFA and its array at 2.5GHz . . . . . . . . . . . . . . . . . . . 80 4.3.1 Single modified PIFA at 2.5GHz . . . . . . . . . . . . . . . . . . . 80 4.3.2 Dual-element modified PIFA array at 2.5GHz . . . . . . . . . . . . 86 4.4 Dual-helical antenna array for Galileo/GPS terminals . . . . . . . . . . . 87 4.4.1 Design of helical antennas . . . . . . . . . . . . . . . . . . . . . . . 89 4.4.2 Performance of helical antennas . . . . . . . . . . . . . . . . . . . . 90 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5 Diversity performance and channel capacity of the dual-PIFA 96 5.1 Diversity performance of the dual-element PIFA array . . . . . . . . . . . 96 5.1.1 Correlation and MEG . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.1.2 Diversity gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 5.1.3 Measurement on diversity gain . . . . . . . . . . . . . . . . . . . . 98 5.2 Channel capacity of the dual-element PIFA array . . . . . . . . . . . . . . 101 5.2.1 Design of a RT MIMO channel model . . . . . . . . . . . . . . . . 101 5.2.2 Comparison with the IEEE MIMO channel model . . . . . . . . . 106 5.2.3 Investigation of practical antenna arrays in the RT MIMO channel model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 6 Diversity performance of the dual-helical antenna array 113 vi 6.1 Introduction to the WP2160 of the GAC project . . . . . . . . . . . . . . 113 6.2 Characterisation of propagation factors for Galileo/GPS system . . . . . . 114 6.2.1 Set-up of metrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 6.2.2 Full 3D ray tracing . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.3 AOA data collection and analysis . . . . . . . . . . . . . . . . . . . 119 6.3 Diversity performance of the dual-helical antenna array for Galileo/GPS systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 6.3.1 Correlation and MEG of the dual-helical antennas . . . . . . . . . 123 6.3.2 Diversity gain of the dual-helical antennas . . . . . . . . . . . . . . 124 6.3.3 The effect of XPR on the diversity performance . . . . . . . . . . . 125 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 7 Conclusions and future work 129 7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 7.2 Key contributions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 7.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Appendix A Author’s publications 135 Appendix B Solutions for the Examples in Chapter 2 138 B.1 Example 1: Antenna Spacing Effect . . . . . . . . . . . . . . . . . . . . . 138 B.2 Example 2: Environment Effect . . . . . . . . . . . . . . . . . . . . . . . . 139 Appendix C Introduction to the methods used in Wireless InSite 141 C.1 Shooting and bouncing ray method . . . . . . . . . . . . . . . . . . . . . . 141 C.2 Two different diffraction situations . . . . . . . . . . . . . . . . . . . . . . 142 C.3 Method of images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 vii List of Figures 1.1 A laptop used as a MIMO receiver terminal which contains 16 antenna elements [26]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 NXP’s lastest transceiver: UXA23466 [42]. . . . . . . . . . . . . . . . . . . 7 1.3 Wirelessrouters: (a)Buffalo(Model: WZR-G300N)[44],(b)Belkin(Model: N1 Wireless Router) [45] and (c) Linksys (Model: WRVS4400N) [46]. . . 8 2.1 CapacitygrowthofaSISOchannelbyincreasing(a)meanSNRforafixed bandwidth and (b) channel bandwidth for a fixed SNR. . . . . . . . . . . 18 2.2 The mobile radio channel as a function of time and space. . . . . . . . . . 19 2.3 CDF(CumulativeDistributionFunction)ofthecapacityfora4×4MIMO channelwithaSNRof10dB,theMIMOchannelisexplainedinnextsection. 21 2.4 Two antenna arrays in a scattering environment. Representation of an uplink scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.5 The process of orthogonal parallel channels for the MIMO system [10]. . . 24 2.6 Illustration of parallel subchannels for a 4×4 MIMO topology . . . . . . . 24 2.7 Two transmitters and two receivers in free space. . . . . . . . . . . . . . . 26 2.8 Capacity of a SISO system compared with those of the MIMO systems in the example when d=3m and d=0.5m, respectively. . . . . . . . . . . . . . 27 2.9 Two transmitters and two receivers in free space with one reflector. . . . . 28 2.10 Capacity of a SISO system compared with those of the MIMO system in the example with one reflector and no reflector, respectively. . . . . . . . . 29 viii