two element printed mimo antennas for wireless...

TWO ELEMENT PRINTED MIMO ANTENNAS FOR

WIRELESS APPLICATIONS

WAN NOOR NAJWA BINTI WAN MARZUDI

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

TWO ELEMENT PRINTED MIMO ANTENNAS FOR WIRELESS

APPLICATIONS

WAN NOOR NAJWA BINTI WAN MARZUDI

A thesis submitted in fulfilment of the requirement for the award of the Degree of

Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

MARCH 2016

iv

To my beloved parents, Wan Marzudi Wan Omar and Azizah

Yunus and beloved siblings,

Wan Noor Marniza and Wan Mohd Azreen

for their motivation and inspiration.

To all those who believe in the richness of learning.

v

ACKNOWLEDGEMENT

First of all, Alhamdulillah. I am so grateful to Allah S.W.T for giving me the strength

and guidance throughout my studies. Again, thank you Allah for easing my path

towards understanding a part of Your vast knowledge.

I would like to express my deepest and sincere gratitude to my supervisor Dr.

Zuhairiah Zainal Abidin for her excellent guidance, supervision and support that

helped me a lot in completing this thesis.

Special thanks to Encik Mohd Rostam Anuar and Puan Miskiah Muhamad

Ihsan for their guidance and assistance during my measurement works conducted in

the EMCentre at Universiti Tun Hussein Onn Malaysia (UTHM). Thanks to all

Project Lab technicians who had contributed to the succesful completion of this

project.

I would like to express my sincere gratitude to Kementerian Pengajian

Malaysia (KPM) for funding this work through “Geran Insentif Penyelidik Siswazah

(GIPS)”.

Last but not least, I would like to give my big thanks to my parents, siblings

and friends for their support, love and patience. Without their encouragement,

motivation and understanding it would have been impossible for me to complete this

work.

vi

ABSTRACT

Multiple Input Multiple Output (MIMO) technology is a promising approach to fulfil

the demand of current wireless systems that requires higher data rate and larger

capacity in a compact size. The main challenge in designing a MIMO antenna in a

close packed space is to achieve high isolation between the antenna elements. This

research study concentrates on the design and analysis of several small sized two -

elements MIMO antennas that have a working frequency for the following

applications, such as Bluetooth (2.4 GHz), LTE2300 (2.3 – 2.4 GHz), LTE2600 (2.5-

2.69 GHz), Wi-Fi (2.4 – 2.485 GHz), WLAN (2.4/5.2/5.8 GHz) and WiMAX

(2.5/3.5/5.5 GHz). The proposed design were analysed with a different set of

parameters. The aforementioned antennas are design by means of different methods

that includes neutralization line technique, stub and defected ground plane to

suppress the mutual coupling. The MIMO parameters such as mutual coupling,

envelope correlation coefficient, total active reflection coefficient (TARC), capacity

loss and channel capacity were investigated and analysed. All the proposed MIMO

antenna design achieved satisfactory results with a reflection coefficient less than

-10 dB, mutual coupling less than -13 dB and envelope correlation coefficient less

than 0.5. Therefore, it could be concluded that all the proposed MIMO antenna

designs demonstrated a considerable good performance and met the required

prerequisite of a MIMO antenna.

vii

ABSTRAK

Teknologi berbilang masukan dan berbilang keluaran (MIMO) adalah pendekatan

yang menjanjikan kadar data yang lebih tinggi dan keupayaan yang lebih besar

dalam saiz yang kompak untuk memenuhi permintaan semasa sistem tanpa wayar.

Cabaran utama dalam merekabentuk antena berbilang masukan dan berbilang

keluaran di dalam satu ruang yang rapat adalah untuk mencapai pengasingan tinggi

diantara elemen antena. Kajian penyelidikan ini tertumpu kepada merekabentuk dan

menganalisis beberapa antenna dua elemen berbilang masukan dan berbilang

keluaran dengan saiz yang kecil yang mempunyai kekerapan kerja untuk aplikasi

Bluetooth (2.4 GHz), LTE2300 (2.3 - 2.4 GHz), LTE2600 (2.5-2.69 GHz), Wi-Fi

(2.4-2.485 GHz), WLAN (2.4 / 5.2 / 5.8 GHz) dan WiMAX (2.5 / 3.5 / 5.5 GHz.

Reka bentuk yang dicadangkan dianalisis dengan parameter yang berbeza. Beberapa

antenna berbilang masukan dan berbilang keluaran di rekabentuk dengan

menggunakan kaedah yang berbeza termasuk peneutralan teknik talian, punting dan

struktur gangguan satah pembumian untuk menindas gandingan bersama telah

dicadangkan. Parameter berbilang masukan dan berbilang keluaran adalah seperti

gandingan bersama, pekali korelasi sampul surat, jumlah pekali pantulan aktif

(TARC), kehilangan keupayaan dan kapasiti saluran telah disiasat dan dianalisis.

Semua reka bentuk antena berbilang masukan dan berbilang keluaran yang

dicadangkan mencapai prestasi yang baik dengan pekali pantulan kurang daripada -

10 dB, gandingan bersama kurang daripada -13 dB dan pekali korelasi sampul surat

kurang daripada 0.5. Oleh itu, semua reka bentuk antena MIMO dicadangkan

menunjukkan prestasi yang baik dan memenuhi kehendak antena MIMO.

viii

TABLE OF CONTENTS

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENTS vii

LIST OF PUBLICATIONS xii

LIST OF TABLES xiv

LIST OF FIGURES xv

LIST OF SYMBOLS AND ABBREVIATIONS xxi

LIST OF APPENDICES xxii

CHAPTER 1 INTRODUCTION 1

1.1 Project Background 1

1.2 Problem Statements 4

1.3 Aims and objectives 5

1.3.1 Aims 5

1.3.2 Objectives 5

1.4 Scope of Projects 6

1.5 Thesis Organizations 6

CHAPTER 2 THEORETICAL BACKGROUND OF MIMO AND

ITS APPLICATIONS 8

2.1 Introduction 8

2.2 MIMO Antenna System 8

2.3 MIMO Antenna Parameters 10

2.3.1 Total Active Reflection Coefficient (TARC) 10

2.3.2 Correlation Coefficient 11

ix

2.3.3 Capacity Loss 11

2.3.4 Channel Capacity 12

2.3.5 Mutual Coupling and Isolation 13

2.4 Techniques to Enhance Isolation and Its Applications 13

2.4.1 Neutralization Line (NL) 13

2.4.2 Stub on the Ground Plane 19

2.4.3 Defected Ground Plane 25

2.5 Channel capacity enhancement using MIMO

Technology 29

2.6 Summary of Previous Work 30

CHAPTER 3 ANTENNA DESIGN CONCEPT 33

3.1 Introduction 33

3.2 Flow Chart 33

3.3 Design Requirement 35

3.3.1 Frequency Band 35

3.3.2 Dielectric Substrate 35

3.4 Mathematical Method 36

3.5 Single Element Antenna 37

3.5.1 Wideband G-Shaped Slotted Printed Monopole

Antenna for WLAN/WiMAX Applications 37

3.6 MIMO/Diversity Antenna 38

3.6.1 Design 1: Two Elements Crescent Shaped

Antenna 38

3.6.2 Design 2: The Effect of Meander Line Slit on Lower

Band Printed MIMO Antenna 39

3.6.3 Design 3: Compact Orthogonal Wideband Printed

MIMO Antenna for Wi-Fi/WLAN/LTE

x

Applications 40

3.6.4 Design 4: Dual Band Printed G-Shaped Slotted

MIMO Antenna for WLAN/WiMAX Applications 41

3.7 Methods for Antenna Measurements 42

3.7.1 S-Parameters Measurement 42

3.7.2 Far-Field Measurement in Anechoic Chamber 43

3.7.3 Gain Determination Using Gain Transfer Method 46

3.7.3.1 Gain Transfer Method using VNA 46

3.7.3.2 Gain Transfer Method using Spectrum

Analyzer with Separate Signal Generator 48

3.8 Summary 50

CHAPTER 4 ANTENNA PROTOTYPING , RESULTS ANALYSIS 52

4.1 Introduction 52

4.2 Single Element Antenna 52

4.2.1 Wideband G-Shaped Slotted Printed Monopole


4.2.2 Design Summary 60

4.3 MIMO/Diversity Antenna 60

4.3.1 Design 1: Two Elements Crescent Shaped

Antenna 60

4.3.1.1 Design Summary 68

4.3.2 Design 2: The Effect of Meander Line Slit

on Lower Band

Printed MIMO Antenna 69


xi

4.3.3 Design 3: Compact Orthogonal Wideband Printed

MIMO Antenna for Wi-Fi/WLAN/LTE

Applications 76


4.3.4 Design 4: Dual Band Printed G-Shaped Slotted MIMO



4.4 Summary 94

CHAPTER 5 CONCLUSION AND RECOMMENDATION FOR FUTURE

WORK 95

5.1 Summary of Thesis 95

5.2 Achievements and Research Contributions 96

5.3 Recommendation of Future Work 98

REFERENCES 99

APPENDICES 106

xii

LIST OF PUBLICATIONS

1. Marzudi, W.N.W and Abidin Z.Z, “Dual Wideband G-Shaped Slotted

Printed Monopole Antenna for WLAN and WiMAX Application”, RF and

Microwave Conference (RFM), 2013 IEEE International: pp. 225-227, 2013.

2. W.N.N.W.Marzudi, Z.Z.Abidin, S.Z.Muji, Ma Yue, and R.A.A.Alhameed, “

Mutual Coupling Reduction of Two Elements Antenna for Wireless

Applications”, In The Second International Conference on Technological

Advance in Electrical, Electronics and Computer Engineering

(TAEECE2014), pp 131-136, The Society of Digital Information and

Wireless Communication, 2014.

3. W.N.N.W.Marzudi, Z.Z.Abidin, S.Z.Muji, Ma Yue, and R.A.A.Alhameed, “

Minimization of Mutual Coupling Using Neutralization Line Technique for

2.4 GHz Wireless Applications”, International Journal of Digital

Information and Wireless Communicatiob (IJDIWC), Vol 4(3), pp. 26-32,

2014.

4. W.N.N.W.Marzudi, Z.Z.Abidin, S.Z.M.Muji, M.Yue and R.A.A.Alhameed,

“ Wideband G-Shaped Slotted Printed Monopole Antenna for WLAN and

WiMAX Applications”, International Journal on Electrical and

Informatics,vol.6,no.3, September 2014.

5. Wan Noor Najwa Wan Marzudi, Zuhairiah Zainal Abidin, Mohd Zarar

Mohd Jenu, Ma Yue, “The Effect of Meander Line Slit on Lower Band

Printed MIMO Antenna”, IEEE Asia Pacific Conference on Applied

Electromagnetic (APACE), pp.135-138, 2014.

6. W.N.N.W.Marzudi, Z.Z.Abidin, Ma Yue, R.A.A.Alhameed, “ Two

Elements Crescent Shaped Printed Antenna for Wireless Applications”,

Advanced Computer and Communication Engineering Technology, Lecture

Notes In Electrical Engineering,Vol 315, pp.195-203, 2015.

xiii

7. N.N.Othman, W.N.N.W.Marzudi, N.F.M.Yusof, Z.Z.Abidin, S.Z.M.Muji

and Ma Yue, “MIMO Antenna Performance on Microstrip Antenna with

EBG Structure for WLAN Applications”, Applied Mechanics and Materials,

vol.773-774, 2015, pp.756-760.

8. W.N.N.W.Marzudi, Z.Z.Abidin, S.H.Dahlan, Ma Yue, Raed A.Abd-

Alhameed, M.B.Child, “ A Compact Orthogonal Wideband Printed MIMO

Antenna for WiFi/WLAN/LTE Applications”, Microwave and Optical

Technology Letter, Vol.6, no.7, July 2015.

xiv

LIST OF TABLE

2.1 Summary: Performances Comparison of MIMO Antenna 30

3.1 Working Frequencies band of proposed antenna 35

3.2 The Dimension for Single Element G-Shaped Slotted Antenna 38

3.3 Design parameter of the proposed antenna 41

4.1 Comparison of Antenna Performances with and without Neutralization

Line 68

5.1 Comparison of Proposed Antennas and Previous Works 97

xv

LIST OF FIGURES

1.1 Wireless Technology [2] 1

1.2 Bluetooth Technology [3] 2

1.3 Home WLAN [4] 3

1.4 WiMAX [5] 3

1.5 MIMO System 4

1.6 MIMO antenna design summary using different techniques to reduce

mutual coupling 7

2.1 Basic model of SISO, SIMO, MISO and MIMO [11] 9

2.2 MIMO System Concept 10

2.3 MIMO Capacity 12

2.4 The structure of the proposed MIMO antenna (unit: mm) [21]. 14

2.5 The simulated isolation with and without neutralizing line [21]. 14

2.6 Detailed dimensions of the proposed antenna [22]. 15

2.7 Simulated reflection coefficient and isolation of the proposed antenna.

(a) Without neutralization line; (b) With neutralization line [22]. 15

2.8 Geometry of the proposed dual-antenna with dimensions in millimeters (the

front side is in black, and the back side is in grey); (a) Perspective view. (b)

Front view. (c) Bottom view (metal on the front is omitted) [23]. 16

2.9 Simulated S-parameters with and without three neutralization lines.

(a) S11; (b) S21 [23]. 16

2.10 Isolation level with and without neutralization line [24]. 17

2.11 Simulated and measured envelope correlation coefficient (ECC) [24]. 17

2.12 Geometry of the proposed antenna [25]. 18


2.14 Simulated S11 and S21 for the proposed antenna (the proposed dual antenna)

the case with the F-like monopole (dual-ant 1), the dual-ant without NL

xvi

(dual-ant 2) and the dual-antenna without match branch (dual-ant 3) [27]. 19



2.17 Simulated S-Parameters for single element antenna with and without

Long ground stubs [29] 21

2.18 Geometry of the proposed antenna [30] 22

2.19 Simulated S-Parameters of proposed antenna with and without

Decoupling structure (a) S11; (b)S21 [30] 22

2.20 Geometry of the proposed antenna [33] 23

2.21 Simulated and measured S-Parameters. (a) S11 and (b) S21[33]. 23

2.22 Geometry of the proposed diversity antenna [34]. 24

2.23 Geometry of the proposed MIMO antenna array [35]. 24

2.24 S-parameter characteristic of the proposed antenna [35]. 24


2.26 Simulated S-parameter [37]. 26

2.27 Configuration of the proposed antenna [38]. 27

2.28 Simulated scattering parameters [38]. 27

2.29 Antenna configurations [39]. 28

2.30 Simulated S-parameters characteristics [39]. 28

2.31 Simulated S-parameters with and without ground slits [41]. 29

3.1 Flowchart of Antenna Production 34

3.2 Configuration of the proposed Single Element Antenna 37

3.3 Configuration of the proposed MIMO Antenna Design. (a) Front View;

(b) Back View 38

3.4 Configuration of the proposed antenna; (a) Reference Antenna ( without

L-stub, semi-circular slot and meander line); (b) Proposed antenna (with

L-stub, semi-circle slot and meander line); (c) Meander slit

configuration (in mm) 40

3.5 Configuration of the proposed antenna. (a) Front view; (b) back view;

(c) patch view and (d) slot views 41

3.6 Configuration of the proposed antenna. (a) Front view; (b) Back view 42

3.7 Antenna Measurement for S11 result 43

3.8 Antenna measurement for S21 result 43

3.9 Far-filed anechoic chamber measurement 44

xvii

3.10 Linear position in vertical and horizontal direction 45

3.11 Antenna under test terminated with 50Ω load 45

3.12 Example of measured radiation pattern result. (a) Linear position in vertical.

(b) Linear position in horizontal. 46

3.13 Photograph of normalized S21 47

3.14 Replaced Reference antenna with AUT for relative gain measurement 48

3.15 Horn antenna as transmitting and receiving antenna 49

3.16 Measurement setup to obtain power receives of reference antenna 49

3.17 Power received, PR2 of AUT measured 50

4.1 Simulated Return Loss. (a) with various dimensions of Ws; (b) with

various dimensions of Ls 53

4.2 Simulated Surface Current Distribution of the Proposed G-Shaped

Slotted Antenna at; (a) 2.4 GHz; (b) 2.5 GHz; (c) 3.5 GHz; (d) 5.2 GHz;

(e) 5.5 GHz; and (f) 5.8 GHz 54

4.3 Simulated return loss. (a) with various dimensions of Wd; (b) with

various dimensions of Ld 55

4.4 Comparison of Return Loss, S11 with and without defected ground

Plane 56

4.5 Practical prototype of the proposed antenna design 57

4.6 Simulated and measured return loss of the proposed antenna 57

4.7 Simulated and measured VSWR of the proposed antenna 57

4.8 Simulated radiation pattern of the proposed antenna at 2.4, 2.5, 3.5, 5.2,

5.5 and 5.8 GHz in the x-z Plane. "xxxx" Simulated cross-polarization,

"oooo" simulated co-polarization 58

4.9 Simulated radiation pattern of the proposed antenna at 2.4, 2.5, 3.5, 5.2,

5.5 and 5.8 GHz in the y-z Plane. "xxxx" Simulated cross-polarization,

"oooo" simulated co-polarization 59

4.10 Antenna gain 59

4.11 Simulated Transmission Coefficient, S21 of the various distance location

of neutralization line 61

4.12 The practical prototype of the proposed MIMO antenna 61

4.13 Comparative plot S-Parameters output for simulated and measured result

for the proposed antenna with neutralization line 62

xviii

4.14 Contour plot surface current distributions (a) with; (b) without neutralization

line at 2.4 GHz. Port 1 (left) is excited, and port 2 (right) is terminated in

50Ω. 63

4.15 Simulated capacity loss of the proposed antenna with and without

neutralization line 64

4.16 Simulated TARC of the proposed antenna with and without

neutralization line 64

4.17 Comparative plot envelope correlations output for simulated and

measured results for the proposed antenna with neutralization line 64

4.18 Comparison between simulated and measured capacity loss of the

proposed antenna 65

4.19 Comparison between simulated and measured TARC of the

proposed antenna 65

4.20 Simulated capacity of the proposed antenna with and without neutralization

line 65

4.21 Measured radiation pattern of the proposed antenna at 2.4,

GHz in the XZ-Plane and YZ plane. "xxxx" Simulated cross-

polarization "oooo" simulated co-polarization 66

4.22 Antenna gain without neutralization line at 2.4 GHz 67

4.23 Antenna gain with neutralization line at 2.4 GHz 67

4.24 The development of U-slit from zero to fifth iteration 69

4.25 Changes in S11 by increased number of iteration 69

4.26 Changes in S-Parameters with different number of meander slits etched

on the patch 70

4.27 Changes in S-Parameters with and without semi-circle slot 70

4.28 Practical prototype of the proposed antenna 71

4.29 Comparison of S-Parameters between reference and proposed antenna 71

4.30 Contour plot surface current distribution without (top) and with (bottom) L-

stub at (a) 2.5 GHz; (b) 3.5 GHz; and (c) 5.8 GHz. Port 1 (left) is excited and

port 2 (right) is terminated in 50Ω 72

4.31 Comparison of S-Parameters between simulated and measured results 72

4.32 Envelope correlation coefficient of the proposed antenna 73

4.33 Simulated capacity loss and TARC of the proposed antenna 73

xix

4.34 Simulated radiation patterns of the proposed antenna for two planes [(left) x-z

plane, (right) y-z plane] at (a) 3.5 GHz, (b) 5.2 GHz, (c) 5.5 GHz,and (d) 5.8

GHz). Port 1 is excited and port 2 is terminated in 50Ω”oooo” simulated co-

polarization, “xxxx” simulated cross-polarization. 74

4.35 Simulated antenna gain of the proposed antenna at (a) 3.5 GHz, (b) 5.2 GHz,

(c) 5.5 GHz and (d) 5.8 GHz. 75

4.36 Simulated capacity of the proposed antenna. 76

4.37 Simulated S-Parameters of the proposed antenna with and without T

Slot 77

4.38 Simulated S-Parameters of the proposed antenna with and without L-

Shaped Stub 78

4.39 Simulated S-Parameters of the proposed antenna with and without

Rectangular slot 78

4.40 The current distribution at 2.4 GHz. (a) without rectangular slot; (b)

with rectangular slot. 79

4.41 The simulated S-Parameters with various Ws3. (a) Return loss; (b)

Mutual coupling 79

4.42 Practical prototype of the proposed antenna 80

4.43 Simulated and measured S-Parameters. (a) S11,S22; (b) S12,S21 81

4.44 Measured envelope correlation coefficient of the proposed antenna 81

4.45 Simulated and measured capacity loss of the proposed antenna

design 82

4.46 Simulated and measured TARC of the proposed antenna

design 83

4.47 Simulated and measured VSWR of the proposed antenna design 83

4.48 Antenna gain 83

4.49 Simulated and measured normalized radiation patterns of the proposed

antenna for two planes [(left) x-z plane, (right) y-z plane] at (a) 2.4 GHz,

(b) 3.5 GHz, (c) 5.2 GHz,and (d) 5.5 GHz). Port 1 is excited and port 2

is terminated in 50Ω”oooo” measured co-polarization, “xxxx” measured

cross-polarization, “----” simulated co-polarization, “- - - -” simulated cross-

polarization 84

4.50 Simulated and measured normalized radiation patterns of the proposed

antenna for two planes [(left) x-z plane, (right) y-z plane] at (a) 2.4 GHz,

xx

(b) 3.5 GHz, (c) 5.2 GHz,and (d) 5.5 GHz). Port 2 is excited and port 1

is terminated in 50Ω”oooo” measured co-polarization, “xxxx” measured

cross-polarization, “----” simulated co-polarization, “- - - -” simulated cross-

polarization 85

4.51 Simulated capacity of the proposed antenna design. 86

4.52 Simulated S-Parameters with and without rectangular slots 87

4.53 Simulated S-Parameters with and without neutralization line 87

4.54 Simulated S-Parameters with and without rectangular slots 88

4.55 Simulated S-Parameters with and without stub 88

4.56 Prototype of the proposed antenna design 89

4.57 Simulated S-Parameters. (a) Reflection Coefficient, S11; (b) Transmission

Coefficient, S21 90

4.58 Measured envelope correlation coefficient 90

4.59 Comparison between simulated and measured result. (a) Capacity loss

(b) TARC 91

4.60 Simulated and measured radiation patterns of the proposed antenna for two

planes [(left) x-z plane and (right) y-z plane] at (a) 2.4 GHz, (b) 3.5 GHz, (c)

5.5 GHz and (d) 5.8 GHz . Port 1 is excited, and port 2 is terminated. “____”

simulated co-polarization, “_ _ _” simulated cross-polarization, „oooo‟

measured co-polarization and “xxxx” measured cross-polarization 92

4.61 Comparison between simulated and measured proposed antenna gain 93

4.62 Simulated capacity of the proposed antenna design. 93

xxi

LIST OF SYMBOLS AND ABBREVIATIONS

% Percentage

BER Bit Error Rate

CST Computer Simulation Technology

dB Decibel

DGS Defected Ground Structure

EBG Electromagnetic Band Gap

GHz Giga Hertz

GSM Global System for Mobile

IEEE Institute of Electrical and Electronics Engineers

LTE Long Term Evolution

MIMO Multiple Input Multiple Output

MISO Multiple Input Single Output

Rx Receiver

S11 Return Loss

S21 Mutual Coupling

SIMO Single Input Multiple Output

SISO Single Input Single Output

TARC Total Active Reflection Coefficient

Tx Transmitter

UMTS Universal Mobile Telecommunication System

Wi-Fi Wireless Fidelity

WiMAX Worldwide Interoperability for Microwave Access

WLAN Wireless Local Area Network

xxii

LIST OF APPENDICES

A Dual Wideband G-Shaped Slotted Printed Monopole Antenna for WLAN

and WiMAX Applications 107

B Mutual Coupling Reduction of Two Elements Antenna for Wireless

Applications 108

C Minimization of Mutual Coupling Using Neutralization Line Technique for

2.4 GHz Wireless Applications envelope correlation coefficient 109

D Wideband G-Shaped Slotted Printed Monopole Antenna for WLAN and

WiMAX Applications 110

E The Effect of Meander Line Slit on Lower Band Printed MIMO

Antenna 111

F MIMO Antenna Performance on Microstrip Antenna with EBG Structure

for WLAN Applications. 112

G Two Elements Crescent Shaped Printed Antenna for Wireless

Applications 113

H A Compact Orthogonal Wideband Printed MIMO Antenna

for WiFi/WLAN/LTE Applications 114

1

CHAPTER 1

INTRODUCTION

1.1 Project Background

Over the past few decades, wireless communication has been given due attention in

the field of communications. Its rapid growth is owed to the increasing demand for

tether less connectivity apart from the advancement of VLSI technology and the

success of second generation (2G) digital wireless standards [1]. In essence, wireless

communication is defined as the transmission of data without the use of wires [2].

The application of such technology is evident through its implementation, for

instance in Bluetooth, Ultra-Wide Band, satellite, cellular, wireless LAN, Wi-Fi and

WiMAX technologies. Figure 1.1 illustrates the wireless technology from the access

point to the devices.

Figure 1.1: Wireless Technology [2].

2

Bluetooth is a wireless standard design for short range wireless

communication technology. The purpose of Bluetooth design is to eliminate cable

connection between devices such as the computer, allowing for synchronisation

between printers, mobile, etc. The nearby devices will connect at Bluetooth

technology at data rate 1 Mbps. Devices that use such technology is depicted in

Figure 1.2.

Figure 1.2: Bluetooth technology [3].

Other than Bluetooth technology, the introduction of wireless network for

internet communication also has gained popularity through the introduction of Wi-Fi

or wireless fidelity and the wireless local area network (WLAN). A WLAN is an

extension of a wired LAN that connects through a wireless access point. WLANs

operate based on networking standards established by the Institute of Electrical and

Electronic Engineers (IEEE). The IEEE published a series of standards, namely

IEEE.802.11a, IEEE.802.11b, and IEEE.802.11g, that varies with respect to their

data transmission speed and distance. According to the aforementioned standards,

WLANs can transmit at a speed from 11 Mbps to 54 Mbps [2]. Conversely, the

worldwide interoperability for microwave access (WiMAX) that is also known as

IEEE.802.16 is a type of wireless technology that provides wireless internet service

over a longer distance as compared to the standard Wi-Fi. WiMAX provides

broadband wireless access to up to 30 miles and uses fixed and mobile stations to

provide users with access to high-speed voice, data, and internet connectivity.

3

Applications of WLAN and WiMAX is shown in Figure 1.3 and Figure 1.4,

respectively.

Figure 1.3: Home WLAN [4].

Figure 1.4: WiMAX [5].

In wireless communication system, there is a significant demand for higher

data rate as well as a larger bandwidth antenna at both transmitter and receiver. This

is where the utilisation of antenna plays a vital role. More than one antenna is

required for wireless communication terminals to reach higher data rate. MIMO

technology has received considerable attention over the years. Figure 1.5 illustrates

MIMO antenna system. MIMO technology that utilises multiple antennas for both at

4

the transmitting and receiving end of the communication system is considered as one

of the promising approaches to reach higher data rate in rich scattering environment

[6-8]. MIMO technology exploits multipath to provide higher data throughput and

simultaneously increases the range and reliability without consuming extra radio

frequency. MIMO can be implemented by means of beamforming, spatial

multiplexing and diversity techniques [8]. Through the introduction of multiple

antennas at both transmitter and receiver side, the diversity is achievable. The

channel capacity, bandwidth and data rate increases naturally without increasing the

transmitting power signal. The MIMO system is shown in Figure 1.5.

Figure 1.5: MIMO system.

1.2 Problem Statement

The rapid growth in wireless communication technology has led to the development

of wideband antennas and an increasing need for having higher data rate for data

signal transmission. One of the means to address this issue is by increasing the

number of antennas at the wireless communication terminals. Multiple Input and

Multiple Output (MIMO) technology has been given attention over the years as it

could provide higher data rate in rich scattering environment by having multiple

antennas. A major consideration in MIMO antenna design is to reduce correlation

between the multiple elements and in particular the mutual coupling electromagnetic

interactions that exist between multiple elements are significant. Too high mutual

5

coupling will increase the signal correlation between the channels and decrease the

antenna radiation efficiency [9]. It seems obvious that lower mutual coupling will be

ensured if the two radiating elements are spaced as far as possible. However, to

integrate multiple antennas closely in a small and compact device while maintaining

a minimum distance of 0.5λ between radiating element [10] and achieve lower

mutual coupling between antennas element is a challenging task.

Requirements for various techniques of isolation and mutual coupling

improvement for MIMO applications are constantly growing. With the rapid

development of wireless applications such as Bluetooth, WLAN, WiMAX, Wi-Fi

and LTE terminal devices, the needs to enhance the system performance and

compact antenna size have gained a lot of attention. Therefore, the design of MIMO

antenna for wireless applications carried out in this study to improve an antenna size

with lower mutual coupling less than -10 dB with minimum separated distance

between antenna elements less than 0.5λ at 2.4 GHz and good MIMO characteristics

is proposed and investigated. This study presents a number of novel wideband

printed compact MIMO antennas for wireless applications.

1.3 Aims and objectives

1.3.1 Aims

This study aims at improving the size and performance of MIMO antenna system

through the development and implementation of a wideband printed MIMO antennas

for wireless applications. The improvements in terms of mutual coupling, envelope

correlation coefficient, channel capacity, total active reflection coefficient (TARC)

and capacity loss between elements are investigated.

1.3.2 Objectives

The objectives of this study are as follows:

1. To design, model and implement several printed MIMO antennas for

wireless communication.

2. To investigate and analyze the performance of the MIMO antenna system.

6

1.4 Project Scope

The limitations of this research are:

1. Design and fabricate several two element MIMO wideband antennas

working on wireless applications such as LTE2300 (2300-2400 MHz),

LTE2600 (2500 – 2690 MHz), (Bluetooth (2.4 GHz), WLAN (2.4/5.2/5.8

GHz), WiMAX (2.5/3.5/5.5 GHz).

2. Design several two element MIMO antennas using three different techniques

to reduce mutual coupling which are neutralization line, stub structure and

defected ground plane.

3. Investigate the performance of MIMO criteria in terms of correlation

coefficient, capacity channel, total active reflection coefficient (TARC),

bandwidth, antenna gain and radiation pattern of the wideband antenna.

1.5 Thesis Organization

This thesis is organized into five chapters. Chapter 1 provides an overview of the

project that includes the project background, objectives, problem statements and

scope of the project.

Chapter 2 specifically reviews the theoretical background and previous

literature regarding the MIMO antenna. Three different techniques that can be

employed to reduce mutual coupling between antenna elements are considered.

Chapter 3 describes the details of the approach used to design the MIMO

antenna as well as the workflow for the implementation of this project. Antennas

design concept is also discussed. The measurement setup to measure S-Parameters,

radiation patterns, and the antenna gain are also explained.

Chapter 4 explains the optimization of the antennas and comparison between

the simulated and the measured performances. In this chapter, different techniques to

reduce mutual coupling are introduced. All research and published work are

discussed in detail in this chapter that includes S-Parameters, radiation patterns, and

MIMO antenna parameters. The parameters that were taken into consideration are

namely, the correlation coefficient, total active reflection coefficient (TARC),

capacity loss and channel capacity of several MIMO antennas for wireless

7

applications. The proposed MIMO antennas design using different techniques to

reduce mutual coupling are illustrated in Figure 1.6.

Figure 1.6: MIMO antenna design summary using differences technique to reduce

mutual coupling.

Finally, Chapter 5 draws the conclusion of this thesis and recommendation

for future works are also proposed in this chapter.

MIMO Antenna design

Design 1 Design 4 Design 3 Design 2

Techniques to reduce mutual coupling

Neutralization

Line (NL)

Stub

Structure

Defected

Ground Plane

NL +

Defected

Ground

Plane

FR-4

substrate

Rogers

Substrate

8

CHAPTER 2

THEORETICAL BACKGROUND OF MIMO AND ITS APPLICATIONS

2.1 Introduction

This part of the report discusses previous works conducted that are related to

different applications of MIMO antenna. Various techniques that have been

employed in the literature to achieve optimal antennas design are also reported. A

major consideration in MIMO antenna design is to reduce correlation between the

multiple elements and in particular the mutual coupling electromagnetic interactions

that exist between multiple elements are significant, because at the receiving end this

effect could largely determine the performance of the system. Lower mutual

coupling can result in higher antenna efficiencies and lower correlation coefficients

[6].

2.2 MIMO antenna system

The development of wireless communication system begins with the inception of the

single input/single output antenna system (SISO) that consists of a single antenna to

transmit the signal to the transmitter and receives the signal from a receiver. Further

development saw the birth of smart antenna system. A smart antenna system may be

categorized into a single input/multiple output (SIMO) and multiple input/single

output (MISO).

9

There is an insatiability demand for high data rate, high link quality and large

bandwidth antenna for both the transmitter as well as the receiver. These criteria may

be achieved by the inclusion of more than one antenna through MIMO technology.

The basic idea of MIMO system is to improve quality (BER) or data rate (bits/s) by

using multiple TX/RX antennas. MIMO technology using multiple antennas for both

transmitting and receiving end of the communication system is considered as one of

the most promising approaches to reach higher data rate in rich scattering

environment [6-8]. The basic notion of the SISO, SIMO, MISO and MIMO are

illustrated in Figure 2.1.

Figure 2.1: Basic model of SISO, SIMO, MISO and MIMO [11].

MIMO technology exploits multipath to provide higher data throughput and

simultaneously increase its range and reliability without consuming extra radio

frequency. MIMO can be implemented through beamforming, spatial multiplexing

and diversity techniques [12]. By introducing multiple antennas at both transmitter

and receiver side, the diversity is achievable. The channel capacity, bandwidth and

data rate increases automatically without increasing the transmitting power signal.

The MIMO concept is depicted in Figure 2.2.

Existing

Technology

Smart

Antenna

System

MIMO

System

10

Figure 2.2: The MIMO system concept.

A major challenge in designing a MIMO antenna is the mutual coupling.

Mutual coupling is the electromagnetic interaction between antenna elements due to

the small space of the antenna element. The performance of the MIMO antenna

system such as lower correlation coefficient and higher antenna efficiencies are often

influenced by low mutual coupling.

2.3 MIMO Antenna Parameters

This section reviews MIMO antenna parameters that are extended from the

traditional single element antenna parameters.

2.3.1 Total Active Reflection Coefficient (TARC)

TARC (Γ) is defined as the square root of the ratio of reflected power and incident

power, mathematically given by [12]:

=

(2.1)

For a lossless N-port antenna, the TARC can be described as:

= √

/√

(2.2)

where;

ai is the incident signal vector with randomly phased elements.

bi is the reflected signal vector.

11

Furthermore, the scattering matrix for 2×2 network can be written as;

(

) = (

) (

) (2.3)

The TARC can be computed by applying different combinations of excitation

signals to each port. The TARC value is a real number between zero and one. The

delivered power is radiated when the value of the TARC equals to zero, conversely

the power is either reflected back or goes to other ports when it is equal to one.

2.3.2 Correlation Coefficient

The correlation coefficient measures the degree of similarity amongst the received

signals. It varies from 0 to 1. Diversity systems are considered good if the correlation

coefficient of the system is zero or low by default. Ideally, the envelope correlation

coefficient (ECC) may be computed by means of; i) from far-field radiation pattern

and ii) from scattering parameters of the proposed antenna system. However,

calculating from the far field radiation pattern would require a complex and advanced

calculation.. The reason for complexity is that the correlation is actually described by

two metrics: complex and envelope correlations [13, 14]. The correlation coefficient

can be computed from S-parameters using the following equation [15]:

e=

( )( )

(2.4)

Where the terms are;

S11 = power reflected from port 1.

S12 = power transmitted from port 1 to port 2.

S21 = power transmitted from port 2 to port 1.

S22 = power reflected from port 2.

2.3.3 Capacity Loss

Theoretically, the channel capacity can be improved by increasing the number of

antennas. The capacity loss (Closs) can be calculated by using equation as follows [16

,17]

12

( ) (2.5)

Where is the receiving antenna correlation matrix given by :

[

]

With ( ( | |

)) and ( ) for i , j = 1 or 2.

2.3.4 Channel Capacity

Figure 2.3 illustrates the MIMO capacity. Multiple antennas that induces multipath

propagation at both transmitter and receiver can establish essentially multiple parallel

channels that operate simultaneously, on the same frequency band at the same total

radiated power. Channel capacity involved with the information handling capacity of

a given channel. The equation of channel capacity is given by [18]:

C= [ *

+] bits/s/Hz (2.6)

where:

SNR = Signal to noise ratio

I = Identity matrix of order

= Hermitian transpose of the H matrix (Channel Coefficient matrix)

M = number of antenna

Figure 2.3: MIMO Capacity.

13

2.3.5 Mutual Coupling and Isolation

In MIMO applications, the signals transmitted by multiple antenna elements are

generally considered to be independent or uncorrelated. However, in reality the

current induced on one antenna produces a voltage at the terminals of nearby

elements that is characterised as mutual coupling [19]. The port to port isolation

defined as the transmission power between two of the input ports of the multiport

antenna under test. It is characterised by |S21| parameter. MIMO system requires the

|S21| to be minimised as low as possible, as isolation is directly related to the antenna

efficiency.

Isolation = -10 log10 |S21|² (2.7)

2.4 Techniques to Enhance Isolation and Its Applications

Significant research efforts have been reported lately to reduce mutual coupling.

Amongst the techniques employed are neutralization line, stub on the ground plane

and defected ground plane have been proposed. In this section, a review of previous

works conducted on the aforementioned techniques will be discussed.

2.4.1 Neutralization Line (NL)

The use of neutralization line inserted between antenna elements is based on the

concept of neutralizing two antennas operating in the same frequency band to

enhance isolation. Chebihi et al. [20], drew some current from the first antenna and

re-injected it in the second antenna with a suitable phase and magnitude to mitigate

existing coupling between elements.

Huang et al. [21] suggest that the neutralization line improves the isolation

between antenna elements. This line can be seen as an inductance to neutralize the

capacitance of these two antennas. Improvements better than 15 dB was achieved.

The structure of the proposed MIMO antenna design is shown in Figure 2.4. The

simulation results of isolation with and without the neutralizing line are illustrated in

Figure 2.5. It is evident from the results that the coupling effect between antenna

element is decreased and lowers the envelope correlation coefficient (ECC) smaller

than 0.01 through the introduction of the line. The impedance bandwidth for the

measurement analysis includes 2.45 (2.4 -2.485 GHz), 5.2 (5.15 – 5.35 GHz) and 5.8

14

(5.725 - 5.825 GHz) covering Bluetooth, WLAN and WiMAX system. The measured

isolation is better than 17 dB at 2.45 GHz and 20 dB at 5.2 and 5.8 GHz,

respectively. The total dimension of this antenna design is about 80 x 40 x 1.6 mm3.

Figure 2.4: The structure of the proposed MIMO antenna (unit: mm) [21].

Figure 2.5: The simulated isolation with and without neutralizing line [21].

It was established in [22] that the antenna port isolation is effectively

enhanced by the inclusion of the neutralization line. The proposed antenna is used for

USB dongle application. This antenna occupies an area of 30 mm x 65 mm. The

configuration of detailed dimensions of the proposed antenna is illustrated in Figure

2.6. The port isolation obtained improved from 9 dB to less than about -19 dB after

the neutralization line was connected between the elements. The MIMO

characteristics such as the TARC and ECC also showed favourable results whereby

the average TARC is below -10 dB and the values of ECC remain under 0.006 at 2.4

15

GHz frequency band. The comparison of S-parameters between the proposed antenna

and the reference antenna (without neutralization line) is depicted in Figure 2.7.

Figure 2.6: Detailed dimensions of the proposed antenna [22].

Figure 2.7: Simulated reflection coefficient and isolation of the proposed antenna.

(a) With neutralization line; (b) Without neutralization line [22].

Wang et al. introduced the use of three neutralization lines inserted between

antenna elements [23]. The proposed antenna works over the frequency of 1.62 –

2.92 GHz with a mutual coupling less than -15 dB. The radiation pattern of the

proposed antenna can cover complimentary spatial region, and the antenna design

meets the requirement of low mutual and correlation. Figure 2.8 shows the geometry

(a) (b)

16

of the proposed antenna design. The antenna was fabricated on an FR-4 substrate

with a dimension of 115 x 60 mm2.

Figure 2.8: Geometry of the proposed dual-antenna with dimensions in millimeters

(the front side is in black, and the rear side is in grey); (a) Perspective view. (b) Front

view. (c) Bottom view (metal on the front is omitted) [23].

Figure 2.9: Simulated S-parameters with and without three neutralization lines.

(a) S11; (b) S21 [23].

The simulated S-parameters with and without the three neutralization line are

depicted in Figure 2.9. It can be observed that by employing the three neutralization

lines the mutual coupling is lower than -15 dB at the desired frequency band. The

ECC achieved less than 0.05 at the entire frequency band. Neutralization line was

also used to suppress the mutual coupling for 4G mobile terminals as reported in

17

[24]. The optimized neutralization line is placed in the middle of the line to fine tune

the isolation levels. An improvement of 4 dB that are from -6 dB to less than -11 dB

was achieved at the desired frequency bands. The comparison result between

simulated and measured results shows that an ECC below 0.5 between 760 MHz and

950 MHz was achieved. The isolation level with and without neutralization line is

shown, and the comparison between simulated and measured ECC are illustrated in

Figure 2.10 and Figure 2.11, respectively.

Figure 2.10: Isolation level with and without neutralization line [24].

Figure 2.11: Simulated and measured envelope correlation coefficient (ECC) [24].

Ban et al. [25], designed their antenna with a dimension of 120 x 60 mm2 that

consists of an array of elements placed symmetrically with a U-shaped neutralization

line embedded between the antenna elements. It was found that the addition of the

neutralization line reduces the mutual coupling from -6 dB to below -10 dB for the

lower band. Low correlation of less than 0.4 is achieved for the overall desired band

that meets the requirement of a MIMO antenna. The proposed antenna is used for

GSM 850/900/1800/1900/UMTS/LTE2300/2500 operations. Figure 2.12 presents the

18

geometry of the proposed antenna. The crescent-shaped radiator as shown in Figure

2.13 was proposed in [26]. It achieved an impedance bandwidth of 54.5% over 2.4-

4.2 GHz with a reflection coefficient of less than -10 dB. The two antenna elements

are separated by 0.19λ at 2.4 GHz. The neutralization line technique inserted

between the radiators resulted with a better isolation of -17 dB. It was found to be

effective enhancing the mutual coupling, correlation coefficient from 2.4 to 4.2 GHz

frequency bands significantly. Owing to the lower mutual coupling achieved, lower

capacity loss and correlation was also achieved. The capacity loss obtained did not

exceed 0.4 bps/Hz.

Figure 2.12: Geometry of the proposed antenna [25].


19

The use of neutralization line adopted together with the grounded branch

achieved a dual wideband antenna as reported in [27]. The proposed antenna design,

only reduces the mutual coupling at the lower frequency band whilst it has minor

effect on low mutual coupling at a higher frequency. The measured mutual coupling

of the antenna that is used in mobile terminal applications is less than -15 dB with an

ECC less than 0.04. The comparison of S-parameters between the reference and the

proposed antenna is illustrated in Figure 2.14 below.

Figure 2.14: Simulated S11 and S21 for the proposed antenna (the proposed dual

antenna) the case with the F-like monopole (dual-ant 1), the dual-ant without NL

(dual-ant 2) and the dual-antenna without match branch (dual-ant 3) [27].

2.4.2 Stub on the Ground Plane

Inserting stubs is one of the techniques that has been established in reducing mutual

coupling as well as to get better isolation. One or more stubs are inserted to enhance

the isolation and it deals with the ground plane instead of the radiating elements.

Zhao et al. [28] introduced one stub on the ground plane to adjust the

impedance bandwidth and to improve the isolation between the two feeding ports.

The configuration of the proposed antenna design is shown in Figure 2.15. In this

proposed design, the stub can be viewed as a reflector to reduce the mutual coupling.

After inserting stub on ground plane, high isolation which is less than -15 dB is

achieved. The proposed antenna is a suitable candidate for some portable

MIMO/diversity UWB applications.

20


Two long protruding ground stubs namely stub 1 and stub 2 are added on the

ground plane in [29]. The proposed antenna is designed for a portable UWB

application. The proposed antenna consists of two planar monopole antennas placed

perpendicularly to each other to achieve sound isolation. Stub 1 is located in parallel

with planar monopole (PM1) and is bent to minimise the space of antenna area

whilst, a simple straight stub, stub 2 is placed in parallel with PM2. The geometry of

the proposed antenna and the simulated S-parameters with and without long ground

stub are depicted in Figure 2.16 and Figure 2.17, respectively. The result shows that

an impedance bandwidth of 3.1 – 10.6 GHz, low mutual coupling (S21) less than -15

dB, low envelope correlation coefficient less than 0.2 over the frequency band were

achieved.


21

Figure 2.17: Simulated S-parameters for single element antenna with and without

long ground stub [29].

The use of metal T-shaped stub in [30] significantly reduces the mutual

coupling at the lower frequency band. The mutual coupling exists due to the near

field between the two monopoles. The two antennas radiation patterns are separated,

and the mutual coupling is reduced upon the introduction of the vertical stub that

behaves akin to a reflector. The geometry of the proposed antenna and the

comparison of S-parameter between the reference and the proposed antenna are

illustrated in Figure 2.18 and Figure 2.19, respectively. The result shows that the

isolation S21 less than -20 dB at the lower band and S21 less than -25 dB at the upper

band are achieved. The proposed antenna ECC yields less than 0.1 which in effect,

satisfies the MIMO requirement.

22


Figure 2.19: Simulated S-parameters of proposed antenna with and without

decoupling structure.(a) S11; (b) S21 [30].

Cheng et al. [31] proposed an antenna consist of two elliptical shape

monopoles. The two stubs and slot are placed between the radiators on the ground

plane. The mutual coupling and isolation higher than 20 dB were obtained. The

authors further demonstrate the effectiveness of the stub, in order to reduce mutual

coupling [32]. The isolation was improved upon the insertion of the stub i.e. S21 < -

20 dB. The proposed antenna design reported in [33] suggested two symmetrical p-

shaped radiators as the radiating element. The configuration of the proposed antenna

design is shown in Figure 2.20. By combining a notch slot at the upper centre of the

ground plane and inserting stub in the ground, good impedance matching and

improved isolation may be achieved. An impedance bandwidth of 2.9 – 11 GHz for

S11 < -10 dB and S21 < -20 dB were achieved. Figure 2.21 illustrates the simulated

and measured results of the S-parameters.

23

Figure 2.20: The geometry of the proposed antenna [33].

Figure 2.21: Simulated and measured S-Parameters. (a) S11 and (b) S21 [33].

Hong et al. also demonstrates the effectiveness of the stub to reduce mutual

coupling in [34].Three stubs were introduced in this design and is positioned between

the two Y-shaped radiators. The geometry of the proposed antenna is shown in

Figure 2.22. Stub 1, stub 2 and the radiators are placed symmetrically with respect to

the stub 3. The isolation improved with a reading of less than -20 dB upon the

insertion of the stub. In [35] the stub is connected to the ground plane via shorting

pin. The isolation obtained after inserting stub are lower than -25 dB and – 20 dB at

the 2.45 and 5.8 GHz bandwidth, respectively. The configuration of the array and the

S-parameter characteristics of the proposed antenna are shown in Figure 2.23 and

Figure 2.24, respectively.

(a) (b)

24

Figure 2.22: Geometry of the proposed diversity antenna [34].

Figure 2.23: Geometry of the proposed MIMO antenna array [35].

Figure 2.24: S-parameter characteristic of the proposed antenna [35].

99

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