Compact MIMO Antenna Design Using Characteristic Mode Theory

179 Pages·2017·10.67 MB·English

by  Binbin Yang

ABSTRACT
YANG, BINBIN. A Modal Approach to Compact MIMO Antenna Design. (Under the
direction of Dr. Jacob J. Adams.)
MIMO(Multiple-InputMultiple-Output)technologyoffersnewpossibilitiesforwireless
communication through transmission over multiple spatial channels, and enables linear
increases in spectral efficiency as the number of the transmitting and receiving antennas
increases. However, the physical implementation of such systems in compact devices
encounters many physical constraints mainly from the design of multi-antennas. First, an
antenna’s bandwidth decreases dramatically as its electrical size reduces, a fact known
as antenna Q limit; secondly, multiple antennas closely spaced tend to couple with each
other, undermining MIMO performance. Though different MIMO antenna designs have
been proposed in the literature, there is still a lack of a systematic design methodology
and knowledge of performance limits.
In this dissertation, we employ characteristic mode theory (CMT) as a powerful
tool for MIMO antenna analysis and design. CMT allows us to examine each physical
mode of the antenna aperture, and to access its many physical parameters without
even exciting the antenna. For the first time, we propose efficient circuit models for
MIMO antennas of arbitrary geometry using this modal decomposition technique. Those
circuit models demonstrate the powerful physical insight of CMT for MIMO antenna
modeling, and simplify MIMO antenna design problem to just the design of specific
antenna structural modes and a modal feed network, making possible the separate design
of antenna aperture and feeds. We therefore develop a feed-independent shape synthesis
technique for optimization of broadband multi-mode apertures. Combining the shape
synthesis and circuit modeling techniques for MIMO antennas, we propose a shape-first
feed-next design methodology for MIMO antennas, and designed and fabricated two
planar MIMO antennas, each occupying an aperture much smaller than the regular size
of λ/2×λ/2.
Facilitated by the newly developed source formulation for antenna stored energy and
recently reported work on antenna Q factor minimization, we extend the minimum Q
limit to antennas of arbitrary geometry, and show that given an antenna aperture, any
antenna design based on its substructure will result into minimum Q factors larger than
or equal to that of the complete structure. This limit is much tighter than Chu’s limit
based on spherical modes, and applies to antennas of arbitrary geometry.
Finally, considering the almost inevitable presence of mutual coupling effects within
compact multiport antennas, we develop new decoupling networks (DN) and decoupling
network synthesis techniques. An information-theoretic metric, information mismatch
loss (Γ ), is defined for DN characterization. Based on this metric, the optimization of
info
decoupling networks for broadband system performance is conducted, which demonstrates
the limitation of the single-frequency decoupling techniques and room for improvement.
© Copyright 2017 by Binbin Yang
All Rights Reserved
A Modal Approach to Compact MIMO Antenna Design
by
Binbin Yang
A dissertation submitted to the Graduate Faculty of
North Carolina State University
in partial fulfillment of the
requirements for the Degree of
Doctor of Philosophy
Electrical Engineering
Raleigh, North Carolina
2017
APPROVED BY:
Dr. Brian A. Floyd Dr. Brian L. Hughes
Dr. Michael B. Steer Dr. Ruian Ke
Dr. Jacob J. Adams
Chair of Advisory Committee
DEDICATION
To my father, Ming-Yu Yang (杨明玉), whose endless love, encouragement and
extraordinary confidence in me makes me who I am today, yet to whom I owe the most in
my life.
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BIOGRAPHY
Binbin Yang was born in Nanyang, Henan Province, China in 1988. He received his
Bachelor’s degree in Electrical Engineering from Hunan University, Changsha, China, in
2010, and his Master’s degree in Electrical Engineering from the University of Chinese
Academy of Sciences, Beijing, China, in 2013. From 2013 to 2017, he pursued his doctoral
degree at North Carolina State University, Raleigh, NC, USA, and worked as a research
assistant at Antennas and Electromagnetics Lab under Dr. Jacob J. Adams.
His research interests include MIMO antennas, characteristic mode theory, RF and
microwave systems, wireless communication and computational electromagnetics.
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ACKNOWLEDGEMENTS
First and foremost, I would like to thank my mentor and advisor in my doctoral education,
Dr. Jacob J. Adams. I am really grateful to him for offering me the opportunity and
assistantship to pursue my Ph.D. degree here at North Carolina State University. He is
not only an experienced researcher, but also a great mentor offering valuable suggestions
and guidance. I benefit a lot from his insight, perseverance and professional attitude, and
really appreciate his trust and support. He has allowed great freedom and flexibility in
managing my time and choosing interested research topics. I feel very lucky to have him
as my advisor while I am learning to be an independent researcher.
I sincerely express my gratitude to my other committee members, Dr. Brian A.
Floyd, Dr. Brian L. Hughes, Dr. Michael B. Steer and Dr. Ruian Ke, for their professional
suggestionsandhelpfuldiscussions,whichsignificantlyimprovedthequalityofmyresearch
work. I am also grateful to all the professors who has taught me at NC State, for imparting
their knowledge that prepares me for future careers.
I would like to thank all the collaborators in EARS project: Dr. Jacob J. Adams, Dr.
Brian A. Floyd, Dr. Brian L. Hughes, Shaohan Wu, Wuyuan Li, Charley Wilson and
Dr. Lopamudra Kundu. Their many valuable suggestions and inspirational discussions
throughout the past four years really help shape my research work.
I also want to thank Futurewei Technologies at Bridgewater, New Jersy, and my
mentors Sean Ma and Leonard Piazzi for offering me the internship opportunity and
sponsoring part of my research work on DN synthesis during the summer of 2016.
To all my colleagues at Antennas and Electromagnetics Lab (AEL) at NCSU, Meng
Wang, Kurt Schab, Shruti Srivastava, Clifford Muchler, Danyang Huang, Vivek Bharambe
and Munirah Boufarsan, thank you all for the discussions in office, feedback in our group
meetings and lending a hand on lab fabrication and measurement.
I would like to say thank you to many friends in China, Ying-Yong Zhang, Dr. Feng-
Man Liu, Dr. Zhi-Hua Li, Dr. Da-Quan Yu and Dr. Hai-Fei Xiang. They have supported
me in one way or another during my Master education. I also owe special thanks to Dr.
Daniel Guidotti, whose friendship and supportive advices have been with me throughout
my Master and Ph.D. career.
I owe eternally to my beloved father, Ming-Yu and my elder sister, Yan. My debt to
them is immeasurable.
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At last, I would like to thank my wife, Xiao, and our precious two kids she has been
raising most of the time all by herself. My life in the past four years has been so joyfully
fulfilled because of their love, encouragement, support and sometimes distraction.
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TABLE OF CONTENTS
LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 MIMO System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Challenges in Compact MIMO System . . . . . . . . . . . . . . . . . . . 3
1.2.1 Antenna Physical Limits . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.2 Mutual Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 MIMO Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Decoupling Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Overview of the Dissertation . . . . . . . . . . . . . . . . . . . . . . . . . 7
Chapter 2 Characteristic Mode Theory . . . . . . . . . . . . . . . . . . . . 9
2.1 Numerical Modeling of Radiating Objects . . . . . . . . . . . . . . . . . 10
2.1.1 PEC Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.2 Dielectric Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.3 Method of Moment . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Characteristic Mode Theory . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Modal Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2.3 Eigenvalues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.4 Characteristic Modal Q Factor . . . . . . . . . . . . . . . . . . . . 17
Chapter 3 Broadband Circuit Models for MIMO Antennas . . . . . . . . 19
3.1 Circuit Model for Arbitrary Wire MIMO Antennas . . . . . . . . . . . . 20
3.1.1 Formulation of Multi-port Admittances Using CMT . . . . . . . . 20
3.1.2 Proposed Multi-port Circuit Model . . . . . . . . . . . . . . . . . 22
3.1.3 Demonstration Examples . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Circuit Model for Probe-fed Planar MIMO Antennas . . . . . . . . . . . 32
3.2.1 Formulation of Multi-port Impedances Using CMT . . . . . . . . 33
3.2.2 Proposed Multi-port Circuit Model . . . . . . . . . . . . . . . . . 36
3.2.3 Calculation of the Circuit Elements . . . . . . . . . . . . . . . . . 38
3.2.4 Demonstration Examples . . . . . . . . . . . . . . . . . . . . . . . 42
3.3 Application to Feed Specification . . . . . . . . . . . . . . . . . . . . . . 48
3.3.1 Visualizing Input Parameters Using Heat Maps . . . . . . . . . . 48
3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Chapter 4 Feed-independent Shape Synthesis of MIMO Antennas . . . 52
4.1 Challenges in MIMO Antenna Shape Synthesis . . . . . . . . . . . . . . . 53
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4.2 A Simple Feed Network to Extract Characteristic Modes of Symmetric
Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.3 Framework of Antenna Shape Synthesis . . . . . . . . . . . . . . . . . . . 60
4.3.1 Binary Genetic Algorithm . . . . . . . . . . . . . . . . . . . . . . 60
4.3.2 Optimization Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.4 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.1 Minimize the Sum of the Modal Qs . . . . . . . . . . . . . . . . . 63
4.4.2 Minimize the Sum of the Normalized Modal Qs and Occupied Area 65
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Chapter 5 Systematic Design of Planar MIMO Antennas . . . . . . . . . 68
5.1 Shape-first Feed-next Design Methodology . . . . . . . . . . . . . . . . . 68
5.2 Design Example 1: Two-Port Planar MIMO Antenna on Air Substrate . 69
5.2.1 Shape Synthesis of Self-Resonant MIMO Antennas . . . . . . . . 69
5.2.2 Feed Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.2.3 Fabrication and Measurement . . . . . . . . . . . . . . . . . . . . 73
5.3 Modeling of Antennas on Dielectric Substrates . . . . . . . . . . . . . . . 76
5.3.1 Spectral Domain DGF of Microstrip Substrate . . . . . . . . . . . 76
5.3.2 Spatial Domain DGF Using DCIM . . . . . . . . . . . . . . . . . 78
5.3.3 Numerical Examples of Dyadic Green’s Functions . . . . . . . . . 80
5.3.4 Extension of Planar Antenna Circuit Model to Microstrip Antennas 83
5.4 Design Example 2: Two-Port Microstrip MIMO Antenna . . . . . . . . . 86
5.4.1 Shape Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.4.2 Feed Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5.4.3 Fabrication and Measurement . . . . . . . . . . . . . . . . . . . . 88
5.4.4 Antenna Pattern and Correlation . . . . . . . . . . . . . . . . . . 92
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Chapter 6 Physical Limits of Antennas of Arbitrary Geometry . . . . . 96
6.1 Problem of Q Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.2 Relation Between Impedance Matrices of A Structure and Its Substructures 98
6.3 Substructure Eigenvalue Bound . . . . . . . . . . . . . . . . . . . . . . . 100
6.4 Limit on Minimum Tuned Q Factor . . . . . . . . . . . . . . . . . . . . . 102
6.4.1 Numerical Examples . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.5 Limit on Untuned Modal Q Factors . . . . . . . . . . . . . . . . . . . . . 104
6.5.1 Numerical Examples . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.5.2 Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Chapter 7 Decoupling Networks for MIMO Antennas . . . . . . . . . . . 108
7.1 A Decoupling Network Based on Characteristic Port Modes . . . . . . . 109
7.1.1 Mathematics and Topology of the DN . . . . . . . . . . . . . . . 109
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