Thesis

Conformal Antenna Arrays for 3G Cellular Base Stations

By

Vedran Azman

The School of Information Technology and Electrical Engineering University of Queensland Brisbane, Australia

Submitted for the degree of Bachelor of Engineering (Honours) in the division of Electrical Engineering October 18th 2002

Vedran Azman 4/26 Amelia Street Coorparoo QLD 4151 Brisbane, Australia 18th October 2002

Professor Simon Kaplan Head of School of Information Technology and Electrical Engineering University of Queensland St. Lucia, QLD 4072

Dear Professor Kaplan In accordance with the requirements of the degree of Bachelor of Engineering (Honours) in the division of Electrical Engineering at the University of Queensland, I hereby submit for your consideration the thesis titled: “Conformal Antenna Arrays for 3G Cellular Base Stations”. This work was performed under supervision of Associate Professor Marek E. Bialkowski. I declare that the work submitted in this thesis is my own, except as acknowledged in the text and has not been previously submitted for a degree at The University of Queensland or any other institution.

Yours sincerely,

Vedran Azman

Acknowledgements

I would like to thank my supervisor, Associate Professor Marek E. Bialkowski, for introducing me to the field of RF and Microwave Engineering and for giving me the chance to undertake this thesis under his supervision. Your support and valuable advice has guided me to the successful completion of this thesis that would not be possible without your help. I am also very grateful to Mr. Eddie Tsai for his support, advice and precious time spent on countless simulations performed for this thesis. I would like to thank you for gathering all simulation results and helping me to put this thesis together. Thanks also extend to Mr. Russel Clarke, the lab manager, for setting up the simulation software in the thesis laboratory. I would also like to express my gratitude to my close family for encouragement and support given throughout my life. Heartfelt thanks and appreciation goes to my parents and relatives in my homeland Bosnia and Herzegovina for their love and continuous support throughout the years. Last but not least I would like to express my appreciation to my beloved girlfriend Rekha Komala for her love, support and understanding throughout this difficult and challenging year. Thank you all for helping me achieve one of the major goals in my life.

Contents

Page

Acknowledgments

i

List of Figures

vi

List of Tables

viii

Abstract

ix

Chapter 1

Chapter 2

Introduction

1

Mobile Base Station Antennas

1

Thesis Objective

3

Thesis Overview

4

Antennas

5

Types of Antennas

5

Wire Antennas

6

Aperture Antennas

6

Microstrip Antennas

7

Array Antennas

8

Fundamental Parameters of Antenna

Chapter 3

8

Radiation Pattern

9

Radiation Pattern Lobes

9

Gain

10

VSWR

11

Bandwidth

11

Beamwidth

11

Front-to-Back Ratio

11

Cellular Base Station Antennas

12

Antenna Construction

12

Antenna Manufacturing Developments

13

ii

Chapter 4

Chapter 5

Omni-directional Antennas

14

Sector Antennas

14

Panel Arrays

16

Antennas with Downtilt

18

Diversity

18

Antenna System Selection for 3G Applications 20 Base Station Antennas

21

Performance Criteria

21

Diversity

23

Different dB Units

24

Smart Antennas

24

Cellular System Fundamentals

26

Introduction

26

Cellular Fundamentals

27

Communication using Base Stations

27

A Call from a Mobile

28

A Call to a Mobile

28

Channel Characteristics

28

Fading Channels

29

Doppler Spread

29

Delay Spread

29

Link Budget and Path Loss

30

Channel Reuse

31

Multiple Access Schemes

32

Frequency Division Multiple Access (FDMA)

32

Time Division Multiple Access (TDMA)

32

Code Division Multiple Access (CDMA)

33

Space Division Multiple Access (SDMA)

34

Comparison of Different Multiple Access Schemes 34

Cellular Configurations iii

35

Macrocell System

36

Microcell System

36

Picocell System

36

Cell Splitting and cell Sectorisation

37

Handoff

Chapter 6

Chapter 7

Chapter 8

37

Network-Controlled Handoff

38

Mobile-Controlled Handoff

38

Mobile-Assisted Handoff

39

Hard and Soft Handoff

39

Power Control

39

Mobile Communication Systems

40

Introduction – from 1G to 3G

40

First Generation Systems

41

Second Generation Systems

42

Third Generation Systems

45

Wideband CDMA (WCDMA)

53

Introduction

53

Logical Channels

56

Physical Channels

57

Spreading

60

Handover

61

Interoperability between GSM and WCDMA

62

Conformal Microstrip Antenna Arrays

65

Microstrip Patch Antennas

65

General Characteristics

65

Feeding Techniques

67

Enhancing Bandwidth

69

Conformal Arrays

72

Pattern of Circular and Cylindrical Arrays

73

Grating Lobes

76

iv

Scan Element Pattern Sector Arrays on Cylinders Pattern and Directivity

Chapter 9

Chapter 10

Chapter 11

Chapter 12

79 80 80

Comparison of Planar and Sector Arrays

82

Single Element Design

84

Overview of Ensemble CAD Software

84

Overview of HFSS CAD Software

85

Materials

86

Patch Size Calculation

87

Ensemble Simulation and Optimisation

88

Conformal Array Design and Simulations

93

PC Hardware Requirements for HFSS Software

93

Design and Simulation Results

93

Array with 0.52λ element spacing

96

Array with 0.62λ element spacing

101

Array with 0.72λ element spacing

106

Array with 0.82λ element spacing

111

Array with 0.92λ element spacing

116

Tabulated Results of Simulations

121

Discussion of Results

123

Radiation Pattern

123

Return Loss

124

Insertion Loss

125

Summary and Future Developments

126

Summary

126

Future Work

127

Conclusion

128

Bibliography

129 v

List of Figures Figure 1.1

Standard 3-sector base station with GSM and CDMA antennas.

Figure 1.2

A shared cellular base station.

Figure 1.3

Front view of a cylindrical (conformal) antenna array.

Figure 2.1

Wire antenna configurations.

Figure 2.2

Aperture antenna configurations.

Figure 2.3

Rectangular and circular microstrip patch antennas.

Figure 2.4

Typical wire, aperture and microstrip array configurations.

Figure 2.5

Coordinate system for antenna analysis.

Figure 2.6

(a) Radiation lobes and beamwidths of an antenna pattern. (b) Linear plot of power pattern and its associated lobes and beamwidths.

Figure 3.1

An omni-directional antenna constructed from a collinear array of dipoles.

Figure 3.2

A 50-ohm power divider.

Figure 3.3

WCDMA sector antenna with 17-dBi gain.

Figure 3.3a

Radiation patterns for the WCDMA panel antenna.

Figure 3.4

A panel antenna operable from 820 MHz to 960 MHz.

Figure 3.5

Antenna mounting for a 30-meter tower.

Figure 4.1

Smart Antenna Systems: (a) Switched Antennas; (b) Multiple Beam Array; (c) Steered-Beam Array.

Figure 5.1

Typical cellular system setup.

Figure 5.2

Multipath propagation leads to a multipath delay profile.

Figure 5.3

Channel reuse method in cellular systems.

Figure 6.1

The 3G spectrum.

Figure 7.1

Softer and Soft Handovers.

Figure 8.1

Performance trends of single-layered microstrip patch antenna: (a) Impedance bandwidth; (b) Directivity; (c) Surface wave efficiency

Figure 8.2

Conformal arrays: (a) Aperture dimensions much less than local radius of curvature (b) Aperture dimensions comparable with local radius of curvature.

Figure 8.3

Circular array geometry.

Figure 8.4

Coordinate system.

Figure 8.5

Grating lobe position and height vs. scan angle.

Figure 8.6

30 dB patterns for (a) θ 0 = 0, (b) θ 0 = 30 and (c) θ 0 = 60 degrees

Figure 8.7

30 dB Chebyshev patterns.

Figure 8.8

Scan element patterns for several spacings.

vi

Figure 8.9

Scan element pattern.

Figure 8.10

Arc array directivity relative to flat array across diameter.

Figure 8.11

Scan element patterns of arc arrays.

Figure 9.1

Dimensions of a single layer element.

Figure 9.2

Return loss for a single element.

Figure 9.3

Return loss for a single element.

Figure 9.4

Single patch dimensions for the return loss obtained above.

Figure 9.5

Return loss obtained for the same patch using HFSS.

Figure 9.6

Return loss for the optimised element in HFSS.

vii

List of Tables Table 3.1

WCDMA base-station panel antenna specifications.

Table 6.1

Parameters of some First-Generation Cellular Standards.

Table 6.2

Parameters of various 2nd generation communication systems.

Table 6.3

Parameters of various 2nd generation communication systems.

Table 6.4

Expected 3G data speeds.

Table 6.5

Characteristics of 3G Mobile Phones.

Table 6.6

Characteristics of 3G Mobile Base Stations.

Table 7.1

Main WCDMA Parameters.

Table 7.2

Main differences between WCDMA and GSM air interface.

Table 7.3

Main differences between WCDMA and IS-95 air interface.

Table 7.4

Logical Channels in WCDMA.

Table 10.1

Conformal array parameters for 0.52λ element spacing.

Table 10.2

Conformal array parameters for 0.62λ element spacing.

Table 10.3

Conformal array parameters for 0.72λ element spacing.

Table 10.4

Conformal array parameters for 0.82λ element spacing.

Table 10.5

Conformal array parameters for 0.92λ element spacing.

Table 10.6

Simulation results of an array with 0.52λ element spacing.

Table 10.7

Simulation results of an array with 0.62λ element spacing.

Table 10.8

Simulation results of an array with 0.72λ element spacing.

Table 10.9

Simulation results of an array with 0.82λ element spacing.

Table 10.10

Simulation results of an array with 0.92λ element spacing.

viii

Conformal Antenna Arrays for 3G Cellular Base Stations by Vedran Azman

Abstract Antennas may be one of the most taken-for-granted components in wireless and cellular communication systems. However they are critical to the operation of a cellular base station with many choices available depending upon the particular site and operating environment. Development of essentially new base station antennas has become one of the most important tasks in contemporary antenna engineering. Mobile communication systems are continuing to grow rapidly and the new millennium has seen the introduction of third-generation (3G) mobile communications systems that will offer broadband data services with high data bit rates (up to 2 Mb/s), enhanced multimedia services and Internet applications. As wireless communication markets develop very rapidly, with it the number of base station antennas has also increased. In coming years, the new generation of wireless communication systems will demand new and improved base station antennas. New base station antennas will need to be developed that will replace current sector panel antennas and reduce the overall number of antennas on cellular base stations. The purpose of this thesis is to provide an overview on third-generation mobile communication systems and conformal antenna arrays that could be used in such systems. This thesis aims to analyse the effects of curvature on the performance of cylindrical microstrip antenna arrays in the 1920 - 2170 MHz band, which is the operating frequency band for the main third-generation air interface, the WCDMA. The effects of curvature will be analysed using High Frequency Structure Simulator (HFSS) software which considers mutual coupling effects in antenna arrays. Simulation results for various curvatures are plotted for evaluation and discussion. The presented work finishes with conclusion and suggestions for possible future works. ix

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 1 Introduction Wireless communications have been one of the highest growing markets over the past two decades. The growth in the market has been continuous both in terms of the number of subscribers and number of telecommunications services offered. By April 2002 the number of world cellular subscribers reached 1 billion [26]. To connect people and improve the overall quality of life, new third generation wireless systems have been developed that offer new multimedia capabilities, better reliability, improved battery life and efficient and more cost-effective solutions. As the wireless communication continues to develop very rapidly, number of base station antennas has also increased. In coming years, the new generation of wireless communication systems will demand new and improved base station antennas. New base station antennas will need to be developed that will replace the current sectored panel antennas and reduce the overall number of antennas on a base station. They will operate in the frequency band (1920 2170 MHz) for WCDMA or may even be dual-band or multi-band and be able to cover some or all of GSM (890 - 960 MHz), GSM1800 (1710 to 1885 MHz) and CDMA (824 – 894 MHz and 1850 – 1990 MHz) frequency bands.

Cellular Base Station Antennas Outdoor cellular base stations currently utilising antennas for second generation systems (both GSM and CDMA) are becoming overloaded with antennas in same cases and with the introduction of the third generation communication systems there will be a need for new antennas for those cellular base stations. Figure 1 shows a typical 3-sector antenna configuration providing both GSM and CDMA coverage. Figure 2 shows a typical base station that is shared by several service providers. Placing new antennas for third

1

Chapter 1 - Introduction

generation mobile systems on same base stations might overload the support structures and provide visually unattractive base stations.

Figure 1.1 Standard 3-sector base station with GSM and CDMA antennas.

Figure 1.2 A shared cellular base station.

There is an increasing demand for new types of base station antennas that will replace current planar panel antennas shown in figures above. These new base station antennas will need to operate in both 2G and 3G frequency band, being either dual-band or multiband, and replace current sectored antennas by a single antenna. This will offer less overloaded and more aesthetically attractive base stations.

2

Conformal Antenna Arrays for 3G Cellular Base Stations

One of the most important innovations in modern antenna technology is a conformal antenna array. Conformal antennas are non planar and their curvature is shaped to match a given surface. Cylindrical antenna arrays have attracted the greatest attention amongst conformal antennas and their applications include mobile cellular base stations, airborne radar and mobile satellite communication terminals. Figure 3, taken from [2], shows an example of a developed four-element conformal cylindrical array. Cylindrical antennas are chosen for cellular base stations due to the 360° field of view, radiation pattern independent of azimuth pointing and a smaller number of components than in an equivalent systems made of planar arrays. The most recent demand for cylindrical array antennas concerns the Wide-band Code Division Multiple Access (WCDMA) system, which in standardisation forums, has emerged as the most widely adopted third generation (3G) air interface.

Figure 1.3 Front view of a cylindrical (conformal) antenna array [2].

Thesis Objectives The aim of this thesis will be to gain detailed understanding of characteristics and performances of cylindrical arrays for potential future use in the third-generation communication systems, in particular the WCDMA systems. In particular the effects of curvature on the radiation pattern will be studied as well as the effects of different element spacing for various curvature radii. Mutual coupling will also be investigated and its effects on the radiation pattern observed.

3

Chapter 1 - Introduction

Thesis Overview Chapter 1 gives a brief introduction into current and future cellular base station antennas with the description of the thesis objectives. Chapter 2 discusses some most common types of antennas commercially available and the fundamental parameters are described that are used to evaluate antennas performance. Chapter 3 looks at cellular base station antennas, discusses some most common types of base station antennas found in commercial markets and gives an insight into their manufacturing and performing aspects. Chapter 4 deals with antenna system selections particularly for 3G applications. It discusses their performance criteria and deals with some important aspects when implementing them in 3G cellular networks. Chapter 5 describes basic cellular system fundamentals. It gives a brief introduction into the communication using base stations. It describes then some most common channel characteristics found in cellular network environments. Various cellular configuration systems are briefly discussed and the chapter concludes with the discussion of different handoff methods available. Chapter 6 discusses all three generations of mobile communication systems and examples of most widely used systems are given. Chapter 7 introduces the Wideband CDMA which is the most common used air interface in 3G mobile communication systems. Chapter 8 describes conformal antenna array with emphasis on cylindrical arrays that forms the basis of this thesis. Some most common feeding and bandwidth enhancing techniques are discussed. The patterns of cylindrical antenna arrays are described with emphasis on the grating lobes and patterns of sector arrays on cylinders. The chapter concludes with comparison of planar and cylindrical sector arrays. Chapter 9 describes the design procedure of the single element microstrip patch which would form the basis for the antenna array design. Chapter 10 explains the design of the three-element conformal array that is based on the design results obtained in chapter 9 and the simulation results of the array are presented. Chapter 11 analyses and discusses the results obtained from simulations. Chapter 12 concludes the work performed in this thesis with objectives of future work. 4

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 2 Antennas An antenna is usually defined as the structure associated with region of transition between a guided wave and a free-space wave, or vice versa [27]. On transmission, an antenna accepts electromagnetic energy from a transmission line (coaxial cable or waveguide) and radiates it into space, and on reception, an antenna collects the electromagnetic energy from an incident wave and sends it through the transmission line. In ideal conditions it is desirable that the energy generated by the source is totally transferred to the antenna. However in practice this total transfer of energy is not possible due to conduction-dielectric losses and lossy nature of the transmission line and the antenna. Also if the transmission line is not properly matched to the antenna there will be reflection losses at their interface. Therefore it is very important that the characteristic impedance of the antenna is matched to the impedance of the antenna. In wireless communication systems the antenna is one of the most critical components. A good design of antenna can improve overall system performance and reduce system requirements. In order to meet the system requirements of today’s mobile and wireless communication systems and the increasing demand on their performances, many advancements in the field of antenna engineering have occurred in the last few decades.

Types of Antennas Many types of antennas have been developed to date that are used in radio and television broadcast, cellular and wireless phone communications, marine and satellite communications and many other applications. In this section only few common forms and various types of antennas will be briefly described.

5

Chapter 2 - Antennas

Wire Antennas Wire antennas are seen in everyday life situations- on cars, buildings, ships, aircrafts and so on. Wire antennas come in various shapes such as straight wire (dipole), loop, and helix all of which are shown in Figure 2.1. Loop antennas may take the form of a rectangle, square, ellipse or any other configuration.

Figure 2.1

Wire antenna configurations [17].

Aperture Antennas Due to the increasing demand for more sophisticated forms of antennas and utilization of higher frequencies the aperture antenna is more common today. Some forms of aperture antennas are shown in Figure 2.2. They are used for aircraft and spacecraft applications because they can be easily flush-mounted on the skin of the aircraft or spacecraft. Additionally they can be covered with suitable dielectric materials to protect them from hazardous conditions of the environment in which aircrafts and spacecrafts usually operate.

6

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 2.2

Aperture antenna configurations [17].

Microstrip Antennas Microstrip antennas became very popular in the 1970s primarily for space borne applications. Today they can be found in many other government and commercial applications. They usually consist of a metallic patch on a grounded substrate and can take many different configurations, as discussed in later chapters. Rectangular and circular patches, shown in Figure 2.3, are the most popular because of the ease of analysis and fabrication and attractive radiation characteristics. Microstrip antennas are low-profile, conformable to planar and non planar surfaces, simple and inexpensive to fabricate using modern printed circuit technology. They can be mounted on surface of high-performance aircraft, spacecraft, satellites, missiles, cars and even mobile phones. Due to these advantageous characteristics of microstrip antennas they will be further discussed and subsequently used and analysed in this thesis.

Figure 2.3

Rectangular and circular microstrip patch antennas [17].

7

Chapter 2 - Antennas

Array Antennas Many applications require radiation characteristics that can only be achieved if a number of radiating elements are arranged in a geometrical or an electrical manner that will result in the desired radiation pattern. The arrangement of such element is called an array and is used primarily to achieve a radiation pattern in a particular direction or directions. As will be discussed later, antenna arrays are used in cellular base stations to create directional patterns covering only desired area. These antennas, which are usually made up of an array of 4 to 12 elements, are referred to in cellular systems as sectored or directional antennas and take form of a panel array. Typical examples of arrays are shown in Figure 2.4.

Figure 2.4

Typical wire, aperture and microstrip array configurations [17].

8

Conformal Antenna Arrays for 3G Cellular Base Stations

Fundamental Parameters of Antennas Performance of an antenna is usually described in terms of various necessary antenna parameters. Some of the more important parameters are defined and discussed in this section.

Radiation Pattern The antenna radiation pattern is simply a mathematical function or a graphical representation of the radiation properties of the antenna. The radiation pattern is often determined in far-field region as function of space or directional coordinates. A coordinate system for antenna analysis is shown in Figure 2.5.

Figure 2.5

Coordinate system for antenna analysis [17].

Radiation properties include power flux density, radiation intensity, field strength and polarization. Two- or three-dimensional spatial distribution of radiated energy as a function of the observer’s position along a path of constant radius is of the main interest.

9

Chapter 2 - Antennas

Radiation Pattern Lobes Radiation pattern of an antenna consists of various parts referred to as lobes, which can be classified as major, minor, side and back lobes. Figure 2.6 shows a three-dimensional polar pattern with various radiation lobes. Some have greater radiation intensity then the others, which is sometimes desirable and undesirable. Figure 2.6(b) shows the same pattern characteristics in a linear two-dimensional pattern.

Figure 2.6

(a) Radiation lobes and beamwidths of an antenna pattern. (b) Linear plot of power pattern and its associated lobes and beamwidths [17].

The main beam (or major lobe) contains the direction of maximum radiation. Some antennas may produce split-beams where there are several main beams. A minor lobe is any lobe other than the major lobe. A side lobe is a radiation lobe that is in direction different to the direction of the major lobe and is usually adjacent to the main beam. A back lobe is referred to as the radiation lobe that is 180o away from the main beam. In another words, it is the minor lobe in direction opposite to that of the major lobe. Minor lobes usually represent radiation in undesired directions. The level of side lobes is usually expressed as a ratio of the power density in the minor lobe to that of the major lobe. Side lobe ratios of –20 dB or higher are desirable in most mobile communication and cellular systems application.

10

Conformal Antenna Arrays for 3G Cellular Base Stations

Gain Because most antennas are passive devices, they can achieve gain in one direction only at the expense of gain in another direction. These gain antennas cause the signal to be relatively stronger in one direction than another. For most mobile applications upward and downward radiations are not desirable, so minimizing radiation in these directions while concentrating it in the forward direction is advantageous.

VSWR Antenna with a voltage standing-wave-ratio (VSWR) of 1.0 will transmit all of the power presented to it. As VSWR rises, an increasing amount of power will be reflected. It is generally accepted that a VSWR of 1.5 - 1.7 is the highest acceptable values for a cellular antenna. So an antenna with a VSWR of 1.5 will reflect 4 percent of the total power.

Bandwidth The bandwidth of an antenna is the range of frequencies over which the VSWR remains below 1.5 - 1.7 or some other defined VSWR usually less than 2. The VSWR will vary as a function of frequency. Occasionally the bandwidth will be specified for a VSWR of 2.0 and such an antenna will not be as good a match as one specified to a VSWR of 1.5.

Beamwidth Because the gain of an antenna is a result of pattern compression, there will generally be a direction in which there is maximum gain, as seen in Figure 2.6. The beamwidth is defined by the two points that define the half-power levels (down 3dB).

Front-to-Back Ratio The front to back ratio is ordinarily measured as the ratio of the gain of the maximum lobe compared to the gain at 180 degrees to that direction. As can be seen in Figure 2.6, that number may not give a true impression of the actual power levels that are scattered in the backward direction.

11

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 3 Cellular Base Station Antennas This chapter will briefly discuss various types of cellular base station antennas currently available and employed in commercial and government cellular radio networks. Differences between omni-directional, sector and panel arrays are described following a brief discussion about issues in antenna construction and manufacturing developments.

Antenna Construction The earliest cellular antennas were simple dipoles and sector antennas derived from dipoles (dipoles surrounded by reflector). These devices served the industry well and are still the mainstay of small and rural networks. Later came the antennas with mechanical downtilt. These antennas, while simple in concept were the source of many network problems. The mechanical downtilt distorted the azimuth beam pattern and often gave unpredictable results. A further improvement in the early 1990s was electrical downtilt, which relied on phasing of the antennas and produced an undistorted downtilted pattern. Experiments showed and theory predicts that the polarization of a signal that had travelled extensively in a mobile environment was no longer vertical. Further studies showed that two cross-polarized antennas received signal that were sufficiently uncorrelated and that they provided diversity similar to that of two spatially separated vertically polarized antennas. These cross-polarized antennas could be mounted into a single radome, thus giving diversity with half the number of antennas.

Antenna Manufacturing Developments The most recent development in cellular base station antennas has been cross-polarized antenna with downtilt. These antennas come with electrical and mechanical downtilt capabilities and additional option of a remote control downtilt unit. 12

Chapter 3 - Cellular Base Station Antennas

Increasingly there is the need for dual-band and in some cases triple band antennas. One elegant solutions comes from the Australian antenna manufacturer Argus Technologies Pty Ltd which has range of dual-band antennas (900/1800 MHz) that utilise dual-slant (+/- 45o) or cross-polarization technique. They also have a new range of WCDMA (1910 – 2190 MHz) panel antennas with cross-polarization and various gain and horizontal beamwidth specifications. In Europe Kathrein Antennen currently have triple band cross-polarized panel antennas (824-960/1710-1880/1920-2170 MHz). Other major antenna manufactures are Deltec/Andrews and Andrews USA which all manufacture similar products for their respective cellular standards. Cellular panel antennas are major products but they also manufacture range of picocell, microcell and street antennas. Indoor and in-building antennas are also very popular manufacturing products.

Omni-directional Antennas An omni-directional antenna is usually collinear dipole (a number of dipoles in a line with a phasing harness). Figure 3.1 shows an omni-directional antenna. In an omnidirectional antenna, a power divider many be required to phase a number of dipoles within the one gain antenna or to connect two antennas to the one feed line.

13

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 3.1

An omni-directional antenna constructed from a collinear array of dipoles [25].

The quarter-wave transformer shown in Figure 3.2 is a simple power divider. Because the power divider is fed inside the antenna, the internal wiring harness is quite complex. A cellular omni-directional antenna is usually of this kind and it has a good wide-band performance.

Figure 3.2

A 50-ohm power divider [25].

14

Chapter 3 - Cellular Base Station Antennas

Sector Antennas Sector antennas usually have higher gain than omni-directional antennas. Typically they have a gain in the range of 14 dBi to 19 dBi. Sector antennas may combine the power gains obtained by using phased arrays with the additional gains obtained by using reflectors. Figure 3.3 shows a WCDMA sector antenna (panel antenna) with 17-dBi gain. Table 3.1 lists the specification for the antenna shown in Figure 3.3. Often antenna pattern are simplified to show only the major lobe and the minor lobes are forgotten. Horizontal and vertical radiation patterns for the WCDMA panel antenna shown below are illustrated in Figure 3.3a on the next page. Specifications Frequency Range Gain Return Loss Polarization Horizontal Beamwidth Vertical Beamwidth Electrical Downtilt Upper Sidelobe Level Front to Back Ratio Power Rating Intermodulation Impedance Lightning Protection Connector Type Antenna Dimensions Packed Dimensions Antenna Weight

1910 MHz - 2190 MHz. 17 dBi >15.5 dB Vertical 65º 7º with nullfill 0º to 10º continuously adjustable <-18 dB >30 dB 300 W continuous <-150 dBc for 2 x 40 dBm carriers 50 ohm DC grounded 7-16 DIN female 1300x185x100 mm 1840X230X150 mm 7.5kg

Environmental Figure 3.3 WCDMA base-station panel antenna with 17-dBi gain.

Temperature Humidity Rated Wind Velocity Lateral Loading Rain

-40ºC to +70ºC 95% RH @ +30ºC 200Km/h 0.36kN @ 160 km/h (Front) >140mm per hour

Table 3.1 WCDMA base-station panel antenna specifications.

15

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 3.3a

Radiation patterns for the WCDMA panel antenna.

A low-gain antenna will have much broader major lobe and far fewer minor lobes than its high-gain counterpart. This is because the lobes are the product’s interference between the radiation patterns of the various elements that make up the antenna. Because high-gain antennas have more elements, they generate more interactions. The result is that high-gain antennas, far from having nulls below them, are likely to have quite significant downward radiation. Polar diagrams that depict the vertical radiation pattern usually have a liner scale so that any lobes that appear significant on the diagram will be around 10-20 dB down from the main lobe. Back lobes in particular make even high-gain antennas susceptible to interference for distant mobiles. This interference immunity can be improved by mounting the antenna so that power in the backward direction is decreased – for example by mounting the antenna against the side of a wall.

Panel Arrays Panel arrays have recently found applications in cellular radio networks. They usually consist of a number of dipoles stacked horizontally as well as vertically. Modern panel antennas are physically smaller and less obtrusive than the conventional corner reflector. Hence they are more suitable for mounting on buildings and in places where aesthetics are important. The panel antenna shown in Figure 3.4 features both mechanical and electrical downtilt.

16

Chapter 3 - Cellular Base Station Antennas

Figure 3.4

A panel antenna operable from 820 MHz to 960 MHz [25].

Panel antennas are becoming increasingly more popular. Although they have somewhat higher wind loadings than the conventional corner reflector, they are less conspicuous and they are believed to be more reliable. This is very useful when convincing the building owner to allow the rooftop to be used for an installation. Typical panel antennas are sometimes constructed using microstrip line technology where the radiating patches are etched in a process similar to that used on printed circuit board. The patch antenna is a microstiop that is either square or rectangular shape and is mounted onto a dielectric substrate, with a conducting ground-plane backing. Microstrip antennas are described in details in later chapter. Antennas with a bandwidth sufficiently wide to permit the same unit to cover both the code division multiple access (CDMA) and the Global System for Mobile (GSM) system band are now commercially available. By the use of appropriate phasing these antennas can be designed to have half-power bandwidths from 60 degrees to 120 degrees and so meet all requirements of cellular sector antennas.

Antennas with Downtilt Antennas with built-in downtilt have become widely available and are often used in commercial cellular radio networks. Some panel antennas have combined electrical and

17

Conformal Antenna Arrays for 3G Cellular Base Stations

mechanical downtilt typically with up to 6 degrees of mechanical tilt and up to 10 degrees of electrical tilt. For the mechanical tilt there is usually a calibrated scale and the electrical tilt is given by an indicator. The difference between mechanical and electrical tilt is that electrical tilt shifts the whole transmission, whereas mechanical tilt will change pattern in different ways for different directions. Electrical tilt is accomplished by phasing the feeds of the elements that make up the antenna. The main purpose of downtilt is to reduce the coverage and hance reduce the potential interference with distant cells. They can also be employed to increase the frequency reuse factor and for this they are highly effective.

Diversity Panel antennas are usually either 9dB omni-directional or 14dB to 18dB, 60-degree to 120-degree sector antennas. Diversity reception is frequently used and antennas should be mounted as shown in Figure 3.5. Diversity results in an effective 6dB improvement in the receive path where diversity combiners are used. Due to multipath fading two receive antennas are usually used for diversity combining and this configuration ensures acceptable isolation and diversity reception. The separation for effective diversity performance depends on the height of the base station; usually 1/10 of the antenna height.

Figure 3.5 Antenna mounting for a 30-meter tower [25].

Diversity works best at right angles to the plane of the antennas. There is virtually no diversity effect in the plane of the antennas because when the antennas are in line they receive signal for the same path and so the multipath effect advantage cannot be taken. 18

Chapter 3 - Cellular Base Station Antennas

If switching diversity is used for two antennas in the plane of the received signal the second antenna will contribute virtually zero gain whereas for combining diversity the gain will be 3 dB (the power in two antennas will be added). Vertical diversity can also be used but it generally requires greater physical separation to achieve the same results. For diversity to work well it is necessary that two antennas have the same gain and thus contribute equally to the received signal. It is also important that the base station height is large compared to the antenna separation otherwise the diversity will not work well. There are two types of diversity receivers; the diversity-combining receiver which aligns the phases of the incoming signals and then adds them and the switched-diversity receiver which chooses the best of the two signal paths and switches to that path. A gain of 6 dB can be obtained using the first method and a gain of 3 dB of the second method.

19

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 4 Antenna System Selection for 3G Applications This chapter will discuss some important issues in selection of antenna systems for 3G applications. There are various technology platforms but technically the selection of the antenna type for base station is similar whether it is for a macro, micro or pico cell. There are differences related to different design issues associated with the 1G, 2G, 2.5G or 3G systems. The major difference in the antenna design issues is the need to keep the system separate or united with the new overlayed system and the underlaying technology platform. It is essential to understand and choose the right antenna system for any mobile communication platform. The antenna system is the interface between the mobile radio system and the external radio environment. The base station may have only a single antenna and one at the mobile station. Primarily the antenna is used to establish and maintain the communication link between the base station and the mobile. There are many types of antenna available, all of which perform specific functions depending on the application at hand. They may also be passive or active antennas. The active antenna usually uses some electronics to enhance its performance and passive antenna has no electronics associated with its use and it simply consists entirely of passive elements. The relative pattern of the antenna is another important factor. It indicates in what direction the energy emitted or received from it will be directed. Primarily there are two types related to the directivity; omni and directional antennas. Omni antennas are used to obtain equal coverage in 360 degree field of view. The directional pattern is usually needed to facilitate system growth through frequency reuse. The choice of which antenna to use will directly impact the performance of either the cell or the overall network. The correct antenna for the design can overcome coverage problems. Some parameters important to look for during design of the base station involve antenna gain,

20

Chapter 4 - Antenna System Selection for 3G Applications

antenna pattern, bandwidth and frequency range of the desired signals, power handling capabilities and the interface to the transmitter. There are multitude of antenna that can be used at a base station. However what compromises an antenna or antenna system is determined by the design objectives of the site. For 3G and 2.5G cellular systems most of the antenna selections will depend on the type of the base station they will be employed at. The antenna system will definitely be different between macro and pico cells. For base station there are either the omni antennas or directional antennas that were mentioned earlier. Before selecting the antenna for a base station following issues need to be looked at: Elevation and azimuth pattern meet the requirement Antenna exhibits the proper gain desired Mounting and installation capabilities Tower and wind loading effects of antenna Visual impact Antenna meets the desired performance specifications required

Antenna Performance Criteria The performance of an antenna is not restricted to its gain characteristics and physical maintenance. With introduction of 3G platforms these performance issues need to be reviewed. Some of the parameters that define the performance of an antenna are as follows: Antenna pattern

Radiation efficiency

Antenna polarization

Intermodulation suppression

Main lobe

Horizontal Beamwidth

Antenna bandwidth

Construction

Side lobe suppression

Vertical Beamwidth

Front-to-back ratio

Cost

Input impedance

Directivity

Power dissipation

Gain

21

Conformal Antenna Arrays for 3G Cellular Base Stations

With introduction of 3G platforms some 2G and existing antenna systems need to be reconfigured. This could involve the replacement or addition of more antennas in order to meet the design and performance criteria of the new system. These parameters are described below in relation to the antenna selection. The base station antenna should be chosen so that the antenna pattern matches the coverage requirements. If three-sector base station is designed then the antenna pattern should be 120 degrees which would cover only one of the three sectors. In some cases there are 6 sectors in which case the antenna pattern should be about 60 degrees each. Care must also be taken when selecting an antenna with electrical downtilt that may not be specified sometimes. One should also look at the levels of side lobes that a particular antenna might have. This is important since side lobes are able to cause interference to adjacent cells. Ideally an antenna with no side lobes is required but it is not possible in practice. Sidelobes are more important when performing downtilting because it can increase the amount of interference. The radiation efficiency for an antenna is often not referenced but it should be considered. It is given as the ratio of total power radiated by an antenna to the net power that is accepted by an antenna. This number indicates how much power is lost in the antenna itself. Ideally an antenna with close to 100 percent is required in most cases. The gain of an antenna is also of big importance. It is the ratio of the radiation intensity in a given direction to that of an isotropically radiated signal. The amount of elements in the antenna is usually related to the gain. Usually for every doubling of the amount of elements a gain of 3dB is realized. However the gain comes at the expense of the beamwidth which gets halved for 3dB increase in gain. The bandwidth defines the operating range of the frequencies for the antenna. It must be selected with great care to account for current and future configuration options with the same cell site. For example an antenna should be selected with same performance in the transmit and receive band so that in case one transmit antenna fails one of the receive antennas can be switched on internally in the cell to act as transmit antenna. 22

Chapter 4 - Antenna System Selection for 3G Applications

The front to back ratio defines how much energy is radiated in the opposite direction to the main lobe of the antenna. This is only applicable to directional antennas. The front to back ratio is not important when mounting antenna against side of buildings or walls however when antenna is mounted somewhere where there is no obstruction the front to back ratio must be taken into account.

Diversity There are several types of diversity that need to be accounted for in both the legacy systems as well as 2.5G and 3G platforms. This is usually considered on the receive path, uplink from the mobile to the base station. Transmit diversity is introduced for 2.5 and 3G platforms where the subscriber does not need a second antenna. Types of antenna diversity are listed below: Spacial

Frequency

Polarization

Vertical

Horizontal

Angle

Most 2G systems use two antennas that a separated by a physical horizontal distance. This method deploys only two antennas per sector for receive diversity. Diversity spacing is the physical separation between the receive antenna that is needed to ensure that the proper fade margin protection is available. A rule of thumb used to determine the required horizontal separation is given below. h/d = 11

where h = height above ground d = distance between antennas

This rule of thumb can be verified from Figure 3.4 where the 30 meter tower has minimum of 3 meter spatial separation between the two receive antennas. This rule is used firstly in the 800 MHz band but has been successfully applied in 1800 and 1900 MHz bands.

23

Conformal Antenna Arrays for 3G Cellular Base Stations

With introduction of UMTS, the application of transmit diversity needs to be considered into the antenna design. There are two different transmit diversity schemes possible for the 3G systems. They are the space transmit diversity (STD) and orthogonal transmit diversity (OTD) and their description is beyond the scope of this thesis.

Different dB Units The term dB is often used in radio systems and was introduced to define relative power levels logarithmically. It can be very confusing because of the large number of different units of dBs. Essentially the dB level is the log of a power ratio and dBm is the power measured compared to 1 milliwatt. e.g.

Power dBm = 10log [power (in watts)/0.001] 1 watt = 10log 1/0.001 = 30 dBm

Literature and manufactures specifications may often use different dB units and therefore it is necessary to convert from either dBi to dBd or from dBd to dBi. To convert from dBi to dBd, the following formula is used: dBd = dBi – 2.14 Similarly to convert from from dBd to dBi, the following formula is used: dBi = 2.14 + dBd

Smart Antennas Smart antennas are being introduced into commercial wireless communication systems. The smart antenna systems can be configured for either receive only or full duplex operations. With WCDMA the use of smart antenna systems is supported directly, unlike 1G and 2G, with the use of auxiliary and dedicated pilot channels.

24

Chapter 4 - Antenna System Selection for 3G Applications

Smart antennas were initially designed to provide increase to the signal to noise ratio of a sector. It is usually based on use of narrower radiation beam pattern that will provide increased gain and can be directed toward the users and at the same time offer less gain to the interferers by pointing the radiation beam nulls in their direction. Smart antennas are now also being used to uniformly balance traffic between sectors and cells and improve on the system performance through reduction in soft handoffs for CDMA systems. There are three types of smart antenna systems available that are depicted in Figure 4.1 below. They can either be receive only or full duplex configurations. The difference between the two lies in the amount of antennas and transmitting elements in the cell site itself.

Figure 4.1

Smart Antenna Systems: (a) Switched Antennas; (b) Multiple Beam Array; (c) Steered-Beam Array [23].

The beam switching antenna arrangement is simplest to implement and usually involves four standard antennas with narrow beamwidth. The appropriate antenna will be selected by the base station for use in the receive path based on the received signal levels. The multiple beam array relies on antenna matrix to perform the beam switching. The beam steering however uses phase shifting to direct the beam toward the desired user. They are located directly behind each antenna element. This direction selection will affect the entire sector. The use of electronics on the tower, such as the tower mounted amplifiers (TMA) and phase shifters, will provide better receive sensitivity and maximum transmit power for the site. 25

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 5 Cellular System Fundamentals Introduction This chapter presents fundamental concepts of cellular systems by detailing various terminology used in these systems. It also provides explanations on some common standards. The concept of cellular systems was first invented by Bell Laboratories in late 70’s and the first commercial analogue voice system was introduced in 1983 in USA. The first standard used was the Advanced Mobile Phone Systems (AMPS) which was used for the design of the first generation analogue cordless phone and cellular systems. Many similar standard were then developed around the world such as Total Access Communication System (TACS), Nordic Mobile Telephone (NMT) 450 and NMT 900 in Europe, European Total Access Communication System (ETACS) in UK, C-450 in Germany and Nippon Telephone and Telegraph (NTT), JTACS and NTACS in Japan. Second-generation systems were designed to use digital transmission in contrast to the first analogue systems. These systems include the Global System for Mobile (GSM) and DCS 1800, North American dual-mode cellular system Interim Standard (IS-54) and IS95 and the Japanese Personal Digital Cellular system (PDC). The third-generation (3G) mobile communication systems are commonly associated under the names of Universal Mobile Telecommunications System (UMTS) and International Mobile Telecommunications (IMT-2000). These systems should provide advanced communication services (video, sound and data), having wideband capabilities, using a single standard.

26

Chapter 5 - Cellular System Fundamentals

Cellular Fundamentals A cell is a term known as the area that is served by mobile phone system. Each cell contains one base station that is used to communicate with mobiles in that cell. It does so by transmitting and receiving signal on two radio link; one from base station to the mobile (down-link) and one from mobile to the base station (up-link). Each base station is connected to a mobile switching centre (MSC) that connects calls to and from the base station to mobiles in other cells. The MSC is associated with a Public Switching Telephone Network (PSTN). The figure below depicts a typical setup using base station and switching centres.

Figure 5.1

Typical cellular system setup [24].

Communication Using Base Stations A base station communicates with mobile using control channels that carry control information and traffic channels that carry messages. Control channels are continuously used by the base station to transmit control information. When a mobile phone is switched on, it first scans the control channels and tunes to the one with the strongest signal. It then exchanges identification and authorization information with the base station and is ready to receive or send calls.

27

Conformal Antenna Arrays for 3G Cellular Base Stations

A Call from a Mobile When a mobile phone initiates a call it first sends the required number to the base station which then sends this information to the MSC. The MSC assigns a traffic channel to this call and this information is send to the mobile via the base station. The mobile switches to this channel and the switching centre then completes the rest of the call.

A Call to a Mobile When a mobile is being called, the call first arrives at the MSC. The MSC then sends a paging message to all base stations it is associated with. A mobile tuned to a control channel detects its number in the paging message and responds in similar way to the nearest serving base station. The base station informs the MSC about the location of the mobile. The MSC then assigns a traffic channel to the call and relays this information to the mobile via the base station. The mobile tunes to this traffic channel and the call is complete. The paging process can become very impractical and costly if there is large number of base stations associated with the MSC. This is usually avoided by registration procedure where a roaming phone registers with the nearest MSC. This information is then stored with the switching centre of the area or the home switching centre of the mobile where it is permanently registered. Once a call is received for this mobile, its home switching centre contacts the switching centre where the mobile is currently roaming. Paging in the vicinity of the previous known location helps to locate the mobile.

Channel Characteristics It is important to have a solid understanding of channel characteristics and propagation conditions in order to efficiently use the transmission medium. This section will discuss issues relating to channel characteristics such as fading channels, Doppler spread and delay spread and also link budget and path loss.

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Chapter 5 - Cellular System Fundamentals

Fading Channels The signal arriving at a base station is combination of many signals arriving from different direction as result of multipath propagation. This is usually due to different terrain conditions, buildings and structures which cause the received signal to fluctuate randomly as a function of distance. This is called fading. One can consider the signal to be made up of two components, short-term and long-term fading. The short-term components changes faster than the long-term and has a Rayleigh distribution. The long-term component or slow-varying has lognormal distribution. A movement in a mobile receiver causes it to encounter fluctuations in the received power. This directly depends on the frequency of the transmission and the speed of the mobile.

Doppler Spread Doppler shift occurs due to the relative movement in a mobile which also causes the transmitted frequency to differ from the received frequency. Doppler Spread can be viewed as the spreading of the transmitted frequency. The rate of fluctuations in the observed signal is associated with the Doppler spread in frequency domain.

Delay Spread Due to the multipath effects in the propagation environment the mobile may receive multiple and delayed copies of the same signal resulting in the spreading of the signal in time, as shown in Figure 5.2. For example, the rms delay spread may be in order of nanoseconds in urban areas and 100 microseconds in hilly areas. This would restrict the maximum signal bandwidth between 40 and 250 kHz. This bandwidth is called coherence bandwidth over which the channel has constant gain and linear phase.

29

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 5.2 Multipath propagation leads to a multipath delay profile [21].

For signals which have bandwidth larger than the coherence bandwidth the gain and phase characteristics become frequency selective. This is when intersymbol interference (ISI) occurs in digital communications. It usually happens when the rms delay spread is larger than the symbol duration and the channel becomes frequency selective.

Link Budget and Path Loss Link budgets are the main process of estimating the required power at the receiver and taking into account the losses and attenuation in signal caused by the transmission medium and distance between the receiver and the transmitter. These losses are referred to as the path loss. In free space the path loss is proportional to the square of the distance, i.e. the received power drops by one quarter by doubling the distance between the receiver and transmitter. For mobile communication environment with fading channels the distance power varies and depends on propagation conditions. In indoor areas it ranges from less than factor of two to about six in metal buildings. For urban areas the path loss between the base station and the cell site is often taken as the forth power of the distance between the base and the cell. Link budgets are normally done by calculating the carrier to noise ration (CNR). In mobile communication environments the noise created by other mobile units is more dominant than the background and thermal noise of the system. These systems are therefore more limited by the amount of total interference present than the background noise. For mobile communications the signal to interference (SIR) is limiting factor compared to the signal to noise ratio (SNR) in other communication systems. 30

Chapter 5 - Cellular System Fundamentals

Channel Reuse Channel reuse can be understood from figures below that show cluster of three cells. Channel is normally used to denote a frequency, time slot or a code. The number of channels in a system is limited which limits the capacity of the system. One way of increasing this capacity is by using each channel to carry many calls simultaneously. Using the same channel again and again is option that is normally referred to as channel reuse. In the figure below cells use three separate sets of channels which is indicated by a letter. In Figure 5.3 this cluster is repeated to indicate that three sets of channels are being reused in different cells.

A

C

A

B

C

B

A

C

Figure 5.3

A

C

B

B

Channel reuse method in cellular systems.

If the number N cells in a cluster and it is repeated X times over the same area then the system capacity is increased to XF where F is the total number of channels in the system. The cluster size N determines the frequency reuse factor 1/N. It is sometimes referred to as N frequency reuse plan. The cluster size is an important parameter. For a given cell size as the cluster size is decreased more clusters and cells are needed to cover the given area. This leads to more reuse of channels and hence the system capacity increases. Maximum capacity is achieved when cluster size is one and

31

Conformal Antenna Arrays for 3G Cellular Base Stations

therefore all channels are reused in each cell. CDMA systems have frequency reuse factor of 1 and so no frequency planning is required. The cells using the same set of channels are known as co-channel cells. The distance between co-channel cells is called the co-channel distance and the interference caused by these cells is called the co-channel interference. This interference needs to be minimized by decreasing the power transmitted by the base stations and increasing the co-channel distance. Transmitted power depends on the cell size so the minimum interference is attained by minimum co-channel distance. To have a proper functioning system a trade-off between the system capacity and co-channel interference needs to be made.

Multiple Access Schemes There are basically four different multiple access schemes that are used to share the available spectrum bandwidth. These are frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) and space division multiple access (SDMA).

Frequency Division Multiple Access (FDMA) In FDMA the available spectrum is divided into a number of channels each with a certain bandwidth and individual users use the entire channel bandwidth for the duration of the call. All first-generation systems have used FDMA scheme.

Time Division Multiple Access (TDMA) In TDMA a particular frequency band is shared by many users, each allocated a given timeslot during the call. Each user uses the allocated bandwidth only for the duration of the time slot after which a new time slot is assigned to the user. Based on its data rate within the frame for uplink and downlink the call is allocated number of time slots.

32

Chapter 5 - Cellular System Fundamentals

Each time slot also carries other data for synchronisation, guard times and control information. Time division multiplex (TDM) is used on the downlink and on the uplink each mobile transmits in its own allocated time slot. Guard times, similar to guard bands in FDMA are used to prevent overlap between different time slots that may suffer from propagation delays. Very precises slot synchronisation schemes are also employed to prevent time slots from overlapping. As mentioned above the TDMA scheme is used along with the FDMA scheme because there are different frequency bands allocated to each cell. The traffic in two directions is separated using either frequency division duplex (FDD) or time division duplex (TDD). The FDD scheme uses less bandwidth than TDD scheme and does not require the precise synchronisation of data normally required in TDD schemes. TDD however offers more flexibility it terms of bandwidth allocation in uplink and downlink.

Code Division Multiple Access (CDMA) This scheme is also known as direct sequence (DS), spread-spectrum. It enables multiple users to occupy the same radio channel (frequency spectrum) at the same time. Each of the users utilizes a unique code to differentiate themselves from the other users. These unique codes are generated with wideband pseudo noise (PN) sequence generators. They are used to spread the spectrum of the modulated signal over a large bandwidth. Therefore various CDMA signals occupy the same bandwidth and appear as noise to each other. For each different call the user is allocated an individual code that is used both for spreading the signal during transmission and used to recover the signal at the receiver. The only way the receiver can distinguish between the desired signal and the noise is by knowing the code that was used at the receiver to spread the signal. All the other signals at the receiver will appear as pure noise. CDMA uses FDD for uplink and downlink traffic. On downlink the base station transmits to all users synchronously to preserve the orthogonality of different codes assigned to different users. However on the uplink each

33

Conformal Antenna Arrays for 3G Cellular Base Stations

user transmits independently from other users and thus the transmission is asynchronous. PN sequence codes are designed to be orthogonal to each other so that only the desired receiver can decode the signal and to appear as noise to the other receivers. This is the case on the downlink and on the uplink the situation is different. On the uplink signals arriving from different mobiles are not orthogonalized because of the asynchronous transmission. This causes problems to the base station when it is trying to receiver a weak signal from a mobile that is located far away in the cell if there is presence of a strong signal from a nearby mobile. This is known as “near-far” problem and occurs when the DS signal from a nearby mobile is much stronger than the DS signal from a distant mobile and makes detection difficult. This is usually prevented using power control methods by controlling the power transmitted from various mobile so that received signals at the base station are almost of equal strength. A significant advantage of CDMA is the fact that it practically eliminates frequency planning since it uses a frequency reuse factor of 1. It allows a given RF carrier to be reused in every cell and therefore there is no need for expensive retuning of the network when a new cell is added.

Space Division Multiple Access (SDMA) The SDDMA schemes controls the radiated energy for each user in space. It serves different users by using spot beam antennas. It exploits the directivity and beam forming capability of an antenna array to reduce co-channel interference. So by employing space diversity it is possible that simultaneous calls in a cell could be established. This method would allow the increase in capacity of a cellular system. Signal arriving from a distant source reaches different antennas in an array at different times as a result of their spatial distribution. This delay would be used to differentiate one or more users in one area from those in different area. This scheme would allow effective transmission to take place between a base station and a mobile without disturbing the transmission to other mobiles. Therefore instead of using a fixed cell size a more dynamic cell can be shaped to reflect the user hot spots.

34

Chapter 5 - Cellular System Fundamentals

Comparison of Different Multiple Access Schemes In this section some advantages and disadvantages of different multiple access schemes are discussed. TDMA has number of advantages: base station only requires one set of common radio equipment variable date rates easily achieved by changing the number of time slots allocated to the user does not require tighter power control as CDMA because the interference is controlled by time slot and frequency allocations Some disadvantages are: requires complex time synchronization of different user data complex portable RF unit CDMA has characteristics that give distinct advantages over other: ability to reject delayed multipath signals and thus reduce the multipath fading uses RAKE receiver to combine different multipath components to reduce multipath fading frequency reuse factor of 1, therefore same frequency channel used in neighbouring cells and thus increase system capacity speech activators used to increase capacity and allow efficient use of the spectrum during non-active periods in a speech

Cellular Configuration Depending on the cell size a cellular configuration are commonly knows as a macrocell, a microcell or a picocell. Some characteristics of these configurations are described below. 35

Conformal Antenna Arrays for 3G Cellular Base Stations

Macrocell System A macrocell system is a cellular system having cell size of several kilometres. Base stations in these systems transmit several watts of power from usually high towers. There is no line of sight between the base station and mobiles and thus a typical received signal suffers form multipath fading. The signals also experiences spreading of several microseconds due to this propagation condition.

Microcell Systems In microcell systems cells are usually split up with a radius of about one kilometre. Base stations typically transmit less than 1W of power from antennas mounted on smaller towers or on side of buildings. Due to the small cell size and the line of sight between the base station and mobiles the rms delay spread is only few tens of nanoseconds compared with a few microseconds in macrocell systems. Therefore data rates and maximum bit rates in microcell systems are higher than in macrocell systems, about 1Mbps compared to about 300 kbps. Microcell systems are usefull in providing coverage along roads and highways. Depending on how antennas are mounted on intersections and corners, various cell plans are possible.

Picocell Systems Picocell systems usually have a cell size of less than 100 m. These systems usually cover large rooms, shopping centres, underground stations or small inner city streets. Antennas are usually mounted below rooftop levels or in buildings. In-building areas have different propagation conditions than those covered by microcell or macrocell and thus require different considerations. Picocell and microcell systems are sometimes referred to as cordless systems and cellular systems are usually associated with macrocell systems. It is suggested to use radio frequencies in 18 GHz band for inbuilding systems because they don’t penetrate concrete walls and steel structures thus eliminating the problem of co-channel interference. These systems offer very large bandwidth and require millimeter size antennas that are easily manufactured and installed. 36

Chapter 5 - Cellular System Fundamentals

Cell Splitting and Cell Sectorisation Each cell has a limited channel capacity and thus can only serve a limited mobiles at a given time. Once the limit is exceeded the cell is further subdivided into smaller cell, each with its own base station and new frequency. Therefore new power levels are adjusted normally less than the previous transmitted power. Cell splitting can be very costly and time consuming for cellular operators. It requires a new frequency planning assignment which affects the neighbouring cells. Smaller cell sizes will cause the number of handoffs to increase which affects the traffic in control channels. Cell sectorization is referred to cell being subdivided into sectors which are served by the same base station. This is normally done by employing directional antennas which only radiate the energy in given sector. This method also increases the system capacity like the cell splitting method. However it is a cheaper options because only one base station is required to serve all sectors. It helps in reducing the co-channel interference because the energy is directed only in the direction of the sector and does not interfere in co-channel cells that are in the opposite direction to the sector. As with the case of cell splitting, cell sectorization also affects the handoff rate.

Handoff For moving mobiles it is very common for it to move away from the serving base station and approach the cell boundary. At this point the strength and quality of the received signal is decreasing and it may receive a stronger signal from a neighbouring base station. The control of the mobile is then handed over to the new base station by assigning a channel belonging to the new cell. This process is referred to as handoff or handover. Intercell handoffs occur between two base stations and intracell handoffs occurs between two channels belonging to the same base station. This usually happens when the network that monitors the channel finds one of better quality than the one that is currently used by a mobile. It then decided to move the mobile to this new channel. It is not desirable to have forced terminated calls and to avoid it there are number of techniques used. They are reserving channels for handoff, handoff priority schemes and 37

Conformal Antenna Arrays for 3G Cellular Base Stations

queuing the handoff request. Thus there is the trade off between probability of forced termination and number of blocking new calls. The queuing method is effective when handoff requests arrive in groups and there is reasonable likelihood of channel availability in the future. The handoff is initiated when the quality of the current channel deteriorates below an acceptable threshold or a better channel is available. The channel quality is commonly measured in terms of bit error rate (BER) and received signal strength. The signal strength usually gives an indication of the distance between the base station and the mobile. The measurement of various parameters is carried out either at the base station or at the mobile. Depending who initiates the handoff, there are various possibilities of implementing handoffs such as network-controlled handoff, mobile-controlled handoff and mobile-assisted handoff.

Network-Controlled Handoff Each base stations monitors the strength of the received signal from mobiles in its cell and also periodic measurements of mobiles in neighbouring cells. The MSC initiates and completes the handoff. The decision is usually based on the received signal strength at the base station and neighbouring base station. This method takes few seconds to complete and is not desirable method in microcellular systems where quick handoffs are desirable.

Mobile-Controlled Handoff This method is independent of the MSC. The mobile monitors the signal strength on its current channel and measures the signal from neighbouring base station. It initiates the handoff based on this information and the BER it receives from serving base station. It requests the neighbouring base station to allocate a new channel and the total handoff process takes around 100 ms which is suitable for microcell systems.

38

Chapter 5 - Cellular System Fundamentals

Mobile-Assisted Handoff In this method the mobile assists the network in making the handoff by sending the information about received signal strength to the MSC via the base station. The handoff is initiated and completed by the network in order of 1 s.

Hard Handoff and Soft Handoff Hard handoff occurs when the communication link is broken with the current base station and is established again with a new station. There is normally a gap in the transmission. Hence in hard handoffs the mobile communicates with only one base station. In soft handoff the mobile communicates and receives the signal from more than one base station. The network combines the different signals received from the mobile. This method is mostly used in CDMA systems. Hard handoff is simpler to implement than the soft handoff and is more appropriate for TDMA and FDMA systems. However it can cause unnecessary handoffs between two base stations when the received signal fluctuates. This is avoided by using a hysteresis margin such that the handoff is not initiated until the difference between the received signals is more that the margin.

Power Control Power control in cellular systems is very important process and allows that the mobile functions properly with received signal being large enough but not too high to cause any interference to other receivers. This is done by maintaining constant power level at the receiver by transmitter power control. The receiver tells the transmitter how much power is required, eg. The base station would control the power level transmitted by the mobile phone and the mobile phone would control the received power by telling the transmitter the required level of power it needs to receive. This is done by mobile monitoring its received power and sending this information to the base station to control its power transmission. Power control minimizes the co-cell interference and reduces the near-far problem in CDMA systems.

39

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 6 Mobile Communication Systems The chapter describes some common first-generation, second-generation and the next generation, 3G, communication systems used in today’s cellular and mobile markets

Introduction – from 1G to 3G First generation (1G) mobile communications systems started in the early to mid 1980’s, offering simple wireless voice services based on analogue technology. These 1G systems which provided low quality voice services, were very limited in capacity and did not extend across geographic areas. Digital second generation (2G) systems were developed in Europe (mainly GSM, based on TDMA technology) and the US (mainly IS-95, based on CDMA technology) to provide better voice quality, higher capacity, global roaming capability as well as lower power consumption. 2G systems also offer support for simple non-voice services like Short Messaging Service (SMS). However, different 2G technologies do not interoperate. There are also difficulties with roaming between GSM and IS-95 countries. In addition, the low bit rate of 2G systems (9.6kbps for GSM) cannot meet subscriber demands for new and faster non-voice services on the move. Third generation (3G) systems aim to solve these problems encountered with 2G, by promising global roaming across 3G standards, as well as support for multimedia applications. With the advent of 3G systems, and it’s accompanying mobile applications and services, mobile devices will become more than just a handheld phone or a basic electronic organiser. Hybrid devices will appear in the near future, supporting traditional voice, video streaming and downloads, as well as Internet and Intranet access. 3G’s high bit rate capabilities will allow the convergence of value-added data and voice services on the same mobile device. This will dramatically change the way people communicate, work and carry out their daily lives. 40

Chapter 6 - Mobile Communication Systems

First-Generation Systems The first generation communication systems are based on the analogue transmission. They use frequency modulation for speech services and frequency shift keying (FSK) for signalling. It uses FDMA to share the allocated spectrum. Some popular 1st generation standards developed around world are: Advanced Mobile Phone Service (AMPS), Total Access Communication Systems (TACS), Nordic Mobile Telephone (NMT), Nippon Telephone and Telegraph (NTT) and C450. In these systems two separate frequency channels are used, one for downlink and one for uplink.

AMPS Characteristics AMPS uses two bands, one for uplink (824 to 849 MHz) and one for downlink (869 to 894 MHz) transmission. Each channel has 30 kHz bandwidth. Uplink and downlink channels during a two-way connection are separated by 45 MHz. This separation allows for use of inexpensive duplexers. AMPS typically utilised a cluster of 12 omnidirectional cells or 7 cell cluster with three sectors per cell. Out of 842 available duplex channels 42 are used as control channels and remaining 790 are used as voice channels. They are grouped into forward/reverse control or voice channels (FCC/RCC or FVC/RVC respectively.) Data is sent on FCC and received on RCC by the base station and mobiles have to be locked on an FCC with strongest signal to receive and send calls.

N-AMPS, ETACS and Other Systems The narrowband AMPS (N-AMPS) provides three 10 kHz channels using FDMA in a 30 kHz AMPS channel. Using this system the capacity of the system was increased by a factor of three. Due to the lower channel bandwidth the signal results in degradation of audio quality. The European Total Access Communication Systems (ETACS) is same as the AMPS except that it uses 25 kHz channels instead of the 30 kHz channels used by AMPS. Parameters of some other analogue systems are shown in the Table 6.1.

41

Conformal Antenna Arrays for 3G Cellular Base Stations Parameters Tx Frequency (MHz) Mobile Base Station Channel Bandwidth (kHz) Channel Spacing (MHz) Control signal data rate (kbps) Handoff decision is based on

AMPS

C450

NMT 450

NTT

TACS

824-849 869-894 30

450-455.74 460-465.74 20

453-457.5 463-467.5 25

925-940 870-885 25

890-915 935-960 25

45

10

10

55

45

10

5.28

1.2

0.3

8

Power received at base station

Round-trip delay

Power received at base station

Power received at base station

Power received at base station

Table 6.1

Parameters of some First-Generation Cellular Standards [30].

Second-Generation Systems 2nd generation communication systems are designed to use digital transmission and to employ TDMA or CDMA multiple access schemes. Some popular 2nd generation standards are: North American dual-mode cellular system IS-54, North American Interim Standard IS-95, Japanese Personal Digital Cellular system PDC and European GSM and DCS 1800 systems. IS-95 uses CDMA access scheme whereas the other standards use the TDMA and all of them employ the FDD duplexing technique. This section briefly describes some of these systems and other parameters for these systems are shown in Table 6.2. Parameters

IS-54

GSM

IS-95

PDC

Mobile

824-849

890-915

824-849

Base Station

869-894

935-960

869-894

30 kHz

200 kHz

1250 kHz

25 kHz

45

45

45

30/48

π/4 DQPSK 3 832 40

GMSK 8 124 4.615

BPSK/QPSK 64 9 and 10 20

π/4 DQPSK / / 20

Tx Frequencies (MHz)

Channel Bandwidth (kHz) Spacing between channels (MHz) Modulation Users/Channel Number of Channels Frame duration (ms)

Table 6.2

940-956 and 1429-1453 810-826 and 1477-1501

Parameters of various 2nd generation communication systems [32].

42

Chapter 6 - Mobile Communication Systems

Parameters

IS-136

IS-95

DCS1800 (GSM)

DCS1900 (GSM)

Tx Frequencies (MHz) Mobile

1850-1910

1850-1910

1710-1785

1850-1910

Base Station

1930-1990

1930-1990

1805-1880

1930-1990

30 kHz

1250 kHz

200 kHz

200 kHz

TDMA/FDMA

CDMA/FDMA

TDMA/FDMA

TDMA/FDMA

π/4 DQPSK 3 166/332/498 40

QPSK 64 4 -12 20

GMSK 8 325 4.615

GMSK 8 25/50/75 20

Channel Bandwidth (kHz) Multiple Access Method Modulation Users/Channel Number of Channels Frame duration (ms)

Table 6.3

Parameters of various 2nd generation communication systems [32].

United States Digital Cellular (Interim Standard-54) IS-54 is a digital system which uses TDMA as multiple access technique. It is a dualmode system because it shares the same frequency and base stations with AMPS. This was done to increase system capacity and enable the migration from analog to digital system. In this system each frequency channel of 30 kHz is divided into six time slots in each direction. Each user is allocated two time slots for full-rate speech or one time slot for half-rate speech. IS-54 uses FSK signalling technique for control and π/4 DQPSK for the voice. It has twice the number of control channels than the AMPS thus being able to carry twice as much traffic in a given area. Each time slot consists of digital traffic channel for user data and digitised speech and three channels to carry control information. The three control channels are digital verification colour code (CDVCC), slow associated control channel (SACCH) and fast associated control channel (FACCH). SACCH is used to carry control information between base station and mobile while a call is in progress. It carries information about power level change, handoff etc. Mobile uses this channel to send signal strength measurement of neighbouring base stations so that the base station can implement a mobile-assisted handoff.

43

Conformal Antenna Arrays for 3G Cellular Base Stations

Personal Digital Cellular System The Japanese PDC system employs the TDMA technique. Each channel has three time slots with frame duration of 20 ms. Similarly it supports full-rate and half-rate speech like IS-54. Channel spacing of 25 kHz is used with π/4 DQPSK modulation technique. Frequency-reuse plan of four is supported and it uses mobile-assisted handoffs.

Code Division Multiple Access Digital Cellular System (Interim Standard-95) The IS-95 standard uses CDMA as a multiple access technique and occupies the same frequency band as AMPS. The uplink and downlink channels are separated by 45 MHz. The carrier occupies a 1.25 MHz bandwidth which is shared among many users. Users are separated from each other by the use of orthogonal Walsh spreading sequences. There are total of 64 of these Walsh functions. The user data are grouped into 20 ms frames and are transmitted at a basic user rate of 9.6 kbps. The chip rate of 1.2288 Mchip/s is used for spreading the signal giving a spreading factor of 128. CDMA is very resistant to multipath fading due to the use of RAKE receivers at both base stations and mobiles. This standard allows for soft handover in which the mobile keeps link with both base stations and combines signals from both the stations to improve signal quality and combine multipath signals. The transmission on the downlink is simultaneous to all users. All signals in cell are decoded using a PN sequence of length 215 to reduce the co-channel interference. During this process the orthogonality between the users is preserved. The forward channel consists of one pilot channel, one synchronisation channel, up to 7 paging channels and up to 63 traffic channels. The pilot channel transmits higher power than other channels and is used by mobiles to acquire timing for forward channel and to compare signal strength of different base stations. The synchronisation channel is used to broadcast synchronisation messages to mobile at a rate of 1200 bps. Similarly paging channels are use to send paging messages from base station to mobiles and operates at three different bit rates, namely. Traffic channel supports variable data rates and operates at 9600, 4800, 2400 and 1200 bps.

44

Chapter 6 - Mobile Communication Systems

On reverse channels a strict power control is applied so that the base station receives constant power from each mobile and thus avoiding the near-far problem. Power control is done on the downlink at the rate of 800 bps. Reverse channels operate at 4800 bps for paging purposes by mobiles to initiate calls with base station and respond to paging messages. For traffic channels on the reverse link same rates as for forward channels are used and variable data rates are supported.

Pan European Global System for Mobile (GSM) GSM communication system operates at two frequency bands, 900 and 1800 MHz. GSM mainly refers to the primary 900 MHz band. It uses two subbands, one for uplink and one for downlink, each with bandwidth of 25 MHz and separated from each other by 30 MHz. The carriers are separated by a guard band of 200 kHz thus giving total of 124 frequency channels. In the secondary band, 1800 MHz, there are similarly 374 different frequency channels allocated. GSM employs both TDMA and FDMA multiple access techniques in combination with slow frequency hopping. Transmission takes place by allocating a specific time slot of particular duration during which burst of data will be transmitted. Many types of channels are defined by specifying time slots in GSM. Transmission on the uplink follows the downlink reception and similarly hopping frequencies are also related. The hopping frequency in the uplink direction is derived by adding 45 MHz to the one in the downlink direction. Each carrier is divided into eight timeslots and transmitted in a frame structure. Each frame last about 4.62 ms such that each time slots last about 576.9 microseconds. Depending on the number of carriers in a given cell all eight timeslots could be used to carry user traffic. However there must be at least one timeslot allocated for control channel purposes and therefore there are maximum of seven simultaneous traffic channels.

Third-Generation Systems The aims of the third-generation communication system is to provide seamless network that can provide users voice, data, multimedia and video services anywhere anytime on the network. It will support global roaming while providing high-speed data and 45

Conformal Antenna Arrays for 3G Cellular Base Stations

multimedia applications of up to 144 kbps for moving mobiles and up to 2 Mbps in indoor area. Third-generation

communication

systems

are

defined

by

the

International

Telecommunications Union (ITU). This work was done through the IMT-2000 and the European

proposal

for

IMT-2000

is

known

as

the

Universal

Mobile

Telecommunications Systems (UMTS). UMTS will provide significant changes for customers and technologies. Japan already launched its UMTS network using the wideband-CDMA (WCDMA) technique. Europe will follow shortly after Japan and third-generation networks are expected in United States between 2003 and 2005. Several different radio environments are utilized to provide the required layers of coverage. These range from vary small indoor picocells with high capacity, through to terrestrial micro- and macrocells, to satellite megacells. Key features and objectives of IMT-2000 [21]: -

Integration of current first and second generation terrestrial and satellite-based communications systems into a third-generation

-

Ensuring a high degree of commonality of design at a global layer

-

Compatibility of services with IMT-2000 and with fixed networks

-

Ensuring high quality and integrity of communications, comparable to the fixed networks

-

Accommodation of a variety of types of terminals including pocket-size terminals

-

Use of terminals worldwide

-

Provision for connection of mobile users to other mobile users or fixed users

-

Provision of services by more than one network in any coverage area

-

Availability to mobile users of a range of voice and non-voice services

-

Provision of services over a wide range of user densities and coverage areas

-

Efficient use of the radio spectrum consistent with providing service at acceptable cost

-

Provision of a framework for the continuing expansion of mobile network services and for the access to services and facilities of the fixed network

-

Number portability independent of service provider 46

Chapter 6 - Mobile Communication Systems

-

Open architecture that accommodates advances in technology and different applications

-

Modular structure that allows the system to grow as needed

Radio Access IMT-2000 will provide a wide range of services in a wide variety of operating environments. For multimedia communications high data rates are required ranging from a few kbps for images to about 2 Mbps for video. Therefore two coverage areas are defined for IMT-2000, full area coverage with 384 kbps and 2Mbps for local area coverage. To provide these services two radio links will be employed, either terrestrial or satellitebased. Due to practical difficulties of spectral and power efficiency design constraints it would be hard to provide common radio interface for both terrestrial and satellite components. Therefore terminals will most likely be required to operate over more than one type of interface. Dual-mode handsets already exist to combine GSM at different frequencies. The UMTS radio interface, called UMTS terrestrial radio access (UTRA) will consist of a number of hierarchical layers. The higher layer will use W-CDMA where each user will be given a special CDMA code and full access to the allocated bandwidth. The macro layer will provide basic data rates to 144 kbps. The lower layers will provide higher data rates of 384 kbps and 2Mbps, through the use of FDD. It may also be possible to use TDD through time division CDMA (TD-CDMA) for higher data rates by dividing the frequency allocation into time slots the lower layers. This compromise between the two competing standards of W-CDMA and TD-CDMA means that Europe will have a group of standards. TD-CDMA provides greater efficiency than GSM and offers reuse of the existing GSM network structure as well as efficient inter-working with GSM. TD-CDMA has the same basic frame structure as GSM, each having eight time slots per frame length, but provides higher data rates, up to 2 Mbps indoors. Figure 6.1 below shows the break up of the 3G spectrum.

47

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 6.1

The 3G spectrum.

The combination of different access methods is intended to provide flexibility and network efficiency, with the UMTS terminal adopting the access method that best seeks its environment. The work of ITU on the IMT-2000 is aimed at the establishment of advanced global communication services within the frequency bands, 1885 to 2025 MHz and 2110 to 2200 MHz. Within these bands, 1980 to 2010 and 2170 to 2200 MHz will be used by the satellite component. The table below summarises the expected data speeds for various 3G radio access schemes. Expected 3G data speeds Peak Network Speed

Peak Device Speed

Average PC Browser Speed (loaded network)

Average Streaming Media Speed (loaded network)

GPRS

115 kbps

53 kbps

20-30 kbps

10-20 kbps

EDGE

470 kbps

237 kbps

80-130 kbps

20-40 kbps

2 mbps

2 mbps

200-300 kbps

up to 384 kbps

CDMA 1xRTT

153 kbps

153 kbps

40-60 kbps

~64 kbps

CDMA 1xEVDO

2.4 mbps

2.4 mbps

120-300 kbps

50-100 kbps

WCDMA

Table 6.4

Expected 3G data speeds.

48

Chapter 6 - Mobile Communication Systems

Characteristics of 3G Mobile Stations CDMACDMA- UWC-136 UWC-136 (TDMA)

Carrier Spacing Transmitter Power Antenna Gain Antenna Height Body Loss Access Techniques Data Rates Supported Modulation Type

WCDMA

2000

2000

(TDMA)

GPRS/EDGE

1.25 MHz

3.75 MHz

30 kHz

200 kHz

5 MHz

100 mW

100 mW

100 mW

100 mW

100 mW

0 dBi 1.5 m 0 dB

0 dBi 1.5 m 0 dB

0 dBi 1.5 m 0 dB

0 dBi 1.5 m 0 dB

0 dBi 1.5 m 0 dB

CDMA

CDMA

TDMA

TDMA

CDMA

144 kbps

384 kbps

384 kbps

384 kbps

QPSK/BPSK

QPSK/BPSK

GMSK 8-PSK

QPSK

0.03 MHz 0.03 MHz 0.04 MHz

0.18 MHz 0.22 MHz 0.24 MHz

3 GPP TS25.101

9 dB

9 dB

9 dB

-109 dBm -100 dBmb

-121 dBma

-113 dBma

-109 dBm at 384 kbps

1.10 MHz 1.6 MHz 3.7 MHz 6.6 dB

3.30 MHz 4.7 MHz 11 MHz 6.6 dB

0.03 MHz 0.04 MHz 0.09 MHz 7.8 dB

0.18 MHz 0.25 MHz 0.58 MHz 8.4 dB

? ? ? 3.1 dBf

-107 dBm

-103dBm

-113 dBm

-104 dBm

-106 dBm

-119 dBm

-115 dBm

-127 dBm

-119 dBm

Not Needed

-104 dBm

-100 dBm

-111 dBm

-103 dBm

Not Needed

30 kbps & 44 kbps /4-DQPSK 8PSK

Emission Bandwidth -3 dB -20 dB -60 dB Receiver Noise Figure Receiver Thermal Noise Level

1.1 MHz 1.4 MHz 1.5 MHz

3.3 MHz 4.2 MHz 4.5 MHz

9 dB

9 dB a

-113 dBm -105 dBmb

a

Receiver Bandwidth -3 dB -20 dB -60 dB Eb/N for Pe = 10-3 Receiver Sensitivityc Interference Thresholdd Interference Thresholde

Table 6.5

Characteristics of 3G Mobile Phones.

Characteristics of 3G Base Stations CDMACDMAUWC-136 UWC-136 (TDMA)

Operating Bandwidth Transmitter Power Antenna Gain Antenna Height Tilt of Antenna Access Techniques Data Rates Supported

WCDMA

2000

2000

(TDMA)

GPRS/EDGE

1.25 MHz

3.75 MHz

30 kHz

200 kHz

5 MHz

10 W

10 W

10 W

10 W

10 W

o

17 dBi per 120o sector 40 m 2.5o down

17 dBi per 120o sector 40 m 2.5o down

17 dBi per 120 sector 40 m 2.5o down

17 dBi per 120o sector 40 m 2.5o down

17 dBi per 120o sector 40 m 2.5o down

CDMA

CDMA

TDMA

TDMA

CDMA

144 kbps

384 kbps

30 kbps & 44 kbps

384 kbps

384 kbps

49

Conformal Antenna Arrays for 3G Cellular Base Stations Modulation Type

QPSK/BPSK

/4-DQPSK 8PSK

GMSK 8-PSK

QPSK

3.3 MHz 4.2 MHz 4.5 MHz

0.03 MHz 0.03 MHz 0.04 MHz

0.18 MHz 0.22 MHz 0.24 MHz

3 GPP TS25.104

5 dB

5 dB

5 dB

-113 dBm -104 dBmb

-125 dBma

-117 dBma

-113 dBm at 384 kbps

1.10 MHz 1.67 MHz 3.7 MHz 6.6 dB

3.3 MHz 4.7 MHz 11 MHz 6.6 dB

0.03 MHz 0.04 MHz 0.09 MHz 7.8 dB

0.18 MHz 0.25 MHz 0.58 MHz 8.4 dB

? ? ? 3.4 dBf

-111 dBm

-107 dBm

-117 dBm

-108 dBm

-110 dBm

-123 dBm

-119 dBm

-131 dBm

-123 dBm

Not Needed

-108 dBm

-104 dBm

-115 dBm

-107 dBm

Not Needed

QPSK/BPSK

Emission Bandwidth -3 dB -20 dB -60 dB Receiver Noise Figure Receiver Thermal Noise Level

1.1 MHz 1.4 MHz 1.5 MHz 5 dB

5 dB a

-117dBm -109dBmb

a

Receiver Bandwidth -3 dB -20 dB -60 dB Eb/N for Pe = 10-3 Receiver Sensitivityc Interference Thresholdd Interference Thresholde

a - In bandwidth equal to data rate b - In receiver bandwidth c - For a 10-3 raw bit error rate, theoretical Eb/No d - Desired signal at sensitivity, I/N = -6 dB for a 10 percent loss in range e - Desired signal 10 dB above sensitivity, S/(I+N) for a 10-3 BER f - Assumes Eb/No for Pe = 10E-6 without diversity

Table 6.6

Characteristics of 3G Cellular Base Stations.

3G Standards To ensure a smooth transition towards 3G, the IMT-2000 was set up by the International Telecommunication Union (ITU) to harmonise the different proposed 3G standards. To date, the ITU has decided on a single flexible standard with a choice of multiple access methods (CDMA, TDMA and a hybrid TDMA/CDMA). CDMA is perceived to be the predominant air interface. Two 3G standards - Wideband CDMA (W-CDMA, supported by current GSM-centric countries) and cdma2000 (supported by current CDMA-centric countries) - have emerged as the most prominent contenders. Although both technologies are CDMAbased, major differences exist between them. W-CDMA systems work on a RF bandwidth of 5MHz, much wider than the cdmaOne carrier size of 1.25MHz. The wider bandwidth serves to enhance performance under multipath environments (the receiver

50

Chapter 6 - Mobile Communication Systems

can better separate the different incoming signals) and increase diversity. It’s carriers may be spaced 4.2 to 5.4MHz apart in 200MHz increments. The larger spacing is more likely to be applied between operators than within one operator’s spectrum. This will help to reduce inter-operator interference. W-CDMA also offers seamless interfrequency handover, a useful feature in high-subscriber-density areas. A major difference between W-CDMA and cdma2000 is that cdma2000 base stations are network synchronous. In cdma2000, base stations receive a common reference timing to align their clocks with one another. This is usually obtained using global positioning systems (GPS). In cdma2000, there are two main alternatives for the downlink: multicarrier (MC) or direct sequence (DS). The MC approach involves setting up three carrier frequencies, each with a spreading bandwidth of 1.25MHz. This approach allows co-existence with existing IS-95B systems. In the DS option, only a single carrier is set up, with a spreading bandwidth of 4.75MHz. The advantage of DS over MC is better multipath mitigation. Whichever the standard that is chosen by an operator, IMT-2000 aims to ensure that in the evolution / migration towards 3G, operators can continue to leverage on existing infrastructure. In addition, all 3G systems will support the following bit rates: up to 144kbps in macro-cellular environments (e.g. in moving vehicle), up to 384kbps in micro-cellular environments (e.g. walking pedestrian) and up to 2Mbps in indoor/pico-cellular environments (e.g. in office buildings). IMT-2000 has also been designed from the outset to link both terrestrial and satellite components, so that subscribers roaming between terrestrial and satellite networks can expect smooth communication.

51

Conformal Antenna Arrays for 3G Cellular Base Stations

Migration from 2G to 3G Systems In the evolution from 2G to 3G systems, different migration paths have been identified for GSM and CDMA systems. The objective is to enhance spectral efficiency and network capacity. Mobile operators around the world will be migrating their networks towards 2.5G (e.g. General Packet Radio Service) or even 3G systems in the near future. Unlike 2G systems, 2.5G and 3G systems will feature packet-switched technology. Packet-switching means that dedicated circuits do not need to be established between communicating devices, and network resources are used only when actual data is transmitted. This means "always-on" connectivity for subscribers. Billing for 2.5G and 3G services could, in the future, be packet-based, time-based or a mixture of the two. GSM operators have the option to implement General Packet Radio Service (GPRS) or Enhanced Data Rates for Global Evolution (EDGE) prior to 3G rollout. GPRS provides a relatively easy upgrade of existing 2G networks to support higher bit rates. Commonly considered a 2.5G technology, GPRS offers a theoretical maximum 171.2kbps bit rate, when all eight timeslots are utilised at once. However, it is more likely that subscribers would only be allocated 2-4 time slots, significantly lowering the actual bit rate. In addition, initial GPRS deployments would only provide point-to-point support, meaning that subscribers can only communicate with one party at any one time. At present, some European operators have announced commercial GPRS rollouts this year. GPRS roaming trials have also been conducted in Asia. Mobile data services are likely to take off with the advent of higher bit rates offered by GPRS. Beyond GPRS, operators have the option of implementing EDGE or migrating directly to W-CDMA. EDGE enhances GPRS and offers bit rates of up to 384kbps through the use of a more efficient modulation technique. Another advantage of EDGE over GPRS is support for point-to-multipoint communication. Operators without 3G licenses may be able to offer GPRS or EDGE instead. However, some operators may prefer a direct 3G implementation over additional infrastructure costs in association with EDGE. Also, a significant challenge facing GSM migration is handset compatibility. New handsets will be required for every migration step, GPRS, EDGE, as well as W-CDMA.

52

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 7 Wideband CDMA (WCDMA) WCDMA Parameters The main objective of this thesis is to design a conformal array for WCDMA systems and hence it important to understand the main operational principles of this third generation air interface. This chapter will therefore introduce the principle of the WCDMA air interface and main system design parameters of WCDMA will be presented in the following section. Some of the parameters that characterise WCDMA air interface are [21]: •

WCDMA is a wideband Direct-Sequence Code Division Multiple Access (DSCDMA) system. The user information bits are spread over a wide bandwidth by multiplying the user data with pseudo-random bits generated from CDMA spreading codes. In order to support high bit rates up to 2Mbps, variable spreading factor is used.

•

The chip rate of 3.84 Mcps is used to obtain the wide carrier bandwidth of 5 MHz. DS-CDMA systems such as the IS-95 (Interim Standard 95) are commonly known as narrowband CDMA systems. They have carrier bandwidth of 1.25 MHz due to a lower chip rate of 1.288 Mcps. WCDMA supports high user data rates due to its wide carrier bandwidth and also features increased multipath diversity.

•

WCDMA supports bandwidth on demand i.e. highly variable user data rates. Frames of 10ms are allocated to each user during which the user data rate is kept constant. However, the data capacity among the users can change from frame to frame. This capacity allocation is used to achieve optimum throughput for data packet services.

•

WCDMA supports two basic modes of operation; Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In the FDD mode, two carrier 53

Chapter 7 - Wideband CDMA

frequencies of 5 MHz each are used, one for uplink (from the mobile to base station) and one for downlink (from base station to the mobile). •

WCDMA supports the operation of asynchronous base stations. Narrowband DS-CDMA systems such as IS-95 require that the transmitter and receiver are synchronised so that the signals are correctly despread at the receiver. This is normally achieved by the use of global time reference such as a GPS. Therefore GPS antennas are used on outdoor IS-95 base stations so that there is a direct line of sight between the satellite and the base station. Therefore with WCDMA deployment of indoor and micro base stations is easier because there is no need for GPS signals to provide synchronous operation.

•

WCDMA employs coherent detection coherent detection on uplink and downlink. Coherent detection is already used in IS-95 on the downlink but it is not very common on uplink. This will provide an overall increase in coverage and capacity on the uplink.

•

Multiuser detection and smart adaptive antennas can be deployed in WCDMA networks to increase capacity and coverage. WCDMA can be integrated into GSM systems so the handovers between GSM and WCDMA are supported.

Table 7.1 below summarises the main parameters related to the WCDMA air interface. Multiple access method

DS-CDMA

Duplexing method

FDD/TDD

Base station synchronisation

Asynchronous operation

Chip Rate

3.84 Mcps

Frame Length

10ms

Service Multiplexing

Multiple services with different quality of service requirements multiplexed on one connection

Multirate concept

Variable spreading factor and multicode

Detection

Coherent using pilot symbols

Multiuser detection, smart antennas

Supported by the standard

Table 7.1

Main WCDMA Parameters [32].

54

Conformal Antenna Arrays for 3G Cellular Base Stations

Differences between WCDMA and 2nd Generation Air Interfaces Main differences between the third and second-generation air interfaces are described in this following section. GSM and IS-95 are second-generation air interfaces discussed here. The second-generation systems were built mainly to provide speech services. However, new requirements of the third generation systems need to be considered first which are listed below [21]: Bit rates up to 2 Mbps Variable bit rate to offer bandwidth on demand Multiplexing of services with different quality requirements on a single connection (eg. speech, video and packet data) Delay requirements from delay-sensitive real-time traffic to flexible besteffort packet data Quality requirements from 10% frame error rate to 10-6 bit error rate Co-existence of second and third generation systems and inter-system handovers from coverage enhancements and load balancing Support of asymmetric uplink and downlink traffic High spectrum efficiency Co-existence of FDD and TDD modes Table 7.2 lists the main differences between WCDMA and GSM, and Table 7.3 lists those between WCDMA and IS-95. In this comparison only the air interface is considered. System

WCDMA

GSM

Carrier Spacing

5 MHz

200 kHz

Frequency reuse factor

1

1-18

Power control frequency

1500 Hz

2 Hz or lower

Quality control

Radio

Frequency diversity

resource

management

Network Planning (frequency

algorithms

planning)

5 MHz bandwidth gives multipath

Frequency hopping

diversity with Rake receiver

55

Chapter 7 - Wideband CDMA Packet Data

Load-based packet scheduling

Time slot base scheduling with GPRS

Downlink transmit diversity

Supported for improving downlink

Not supported by the standard

capacity

Table 7.2

Main differences between WCDMA and GSM air interface [21].

System

WCDMA

IS-95

Carrier spacing

5 MHz

1.25 MHz

Chip Rate

3.84 Mcps

1.2288 Mcps

Power control frequency

1500 Hz, both uplink and

Uplink: 800 Hz, Downlink:

downlink

slow power control

Base Station Synchronisation

Not needed

Yes, typically obtained via GPS

Inter-frequency handovers

Yes, measurements with slotted

Possible,

mode

method not specified

Yes, provide required quality of

Not needed for speech only

management algoriths

service

networks

Packet data

Load-based packet scheduling

Packet data transmitted as short

Efficient

radio

resource

but

measurement

circuit switched calls Downlink transmit diversity

Supported

for

improving

Not supported by the standard

downlink capacity

Table 7.3

Main differences between WCDMA and IS-95 air interface [21].

Logical Channels WCDMA uses a single type of radio carrier frequency waveform to transfer data between the base station and mobile telephone. The data on this radio channel is divided into logical (transport) channels that perform specific functions. The transport channels carry control and user data information. Control channels transfer broadcast, paging and access control. Data channels transfer voice and data (e.g. fax) information. These logical channels are assigned to physical channels. Following logical channels are defined for WCDMA:

56

Conformal Antenna Arrays for 3G Cellular Base Stations Channel Name

Abbreviation

Function Carries system and cell-specific information that can be

Broadcast Channel

BCH

used to identify and assist mobile telephones that are operating with their system and is always transmitted over the entire cell with a low fixed bit rate

Paging Channel

PCH

Carries messages that alert mobile telephones of an impending event, often a call page or a short message Carries control information on the downlink in one cell when the system knows the location cell of the mobile. It

Forward Access Channel

FACH

may carry short user packets and is transmitted over the entire cell or over only a part of the cell using lobeforming antennas Uplink channel used to carry control information and

Random Access Channel

RACH

request for service from the mobile station to the base station when they begin to setup a call. It may carry short user packets and is always received from the entire cell Uplink channel used to carry small and medium-sized

Common Packet Channel

CPCH

packets for transmission of burst data traffic. It is associated with a dedicated channel on the downlink, which provides power control for the uplink CPCH Downlink or uplink channel used to carry user or control information between the network and the UE. It is

Dedicated Channel

DCH

transmitted over the entire cell or over only a part of the cell using

lobe-forming

antennas.

The

DCH

is

characterized by the possibility of the fast rate change (every 10ms), fast power control.

Table 7.4

Logical Channels in WCDMA [32].

Physical Channels Physical channels consist of a three-layer structure of superframes, radio frames and time slots. Depending on the symbol rate of the physical channel, the configuration of radio frames or time slots varies. A superframe has a duration of 720 ms and consists of 72 radio frames. A radio frame is a processing unit that consists of 15 time slots. A time slot is a unit that consists of the set of information symbols. The number of symbols per

57

Chapter 7 - Wideband CDMA

time slot depends on the physical channel which corresponds to a specific carrier frequency, code and relative phase. There are different types of physical channels used for specific purposes. They are designed so they cycle through a prescheduled sequence of operations and different types of information are transmitted on each time slot during this cycle. These physical channels include shared and dedicated control channels. Some physical radio channels are exclusively used as control channels and other data channels share control and user information on the same physical channel. For the dedicated physical control channels, a specific fixed spreading sequence is typically used to uniquely identify each channel. This allows the mobile phone to more easily discover and decode the control channel.

Uplink Physical Channels There are two dedicated channels and one common channel on the uplink. User data is transmitted on the dedicated physical-data channel (DPDCH) and control information is transmitted on the dedicated physical-control channel (DPCCH). In most cases, only one DPDCH is allocated per connection and services are jointly interleaved sharing the same DPDCH. However, multiple DPDCHs can also be allocated to avoid a too-low spreading factor at high data rates. The dedicated physical-control channel (DPCCH) is needed to transmit pilot symbols for coherent reception, power control signalling bits and rate information for rate detection. Two basic solutions for multiplexing physical control and data channels are time multiplexing and code multiplexing. Dual-channel QPSK is used in WCDMA uplink to avoid electromagnetic compatibility (EMC) problems with DTX. EMC problems arise when DTX is used for user data. Speech is one of a DTX services. During silent periods, no information bits are transmitted in any case. Because the rate of transmission of pilot and power control symbols is on the order of 1 to 2 kHz, they cause severe EMC problems to both external equipment and terminal interiors. This EMC problem is more difficult in the uplink direction since mobile stations can be close to other electrical equipment, like hearing aids. The WCDMA random access scheme is based on a slotted ALOHA technique with fast acquisition indication. The mobile station can start the transmission at a number of well defined time-offsets, relative to the frame boundary of every second frame of the

58

Conformal Antenna Arrays for 3G Cellular Base Stations

received BCH of the current cell. The different time offsets are denoted access slots. There are 15 access slots per two frames and they are spaced 5,120 chips apart. Information on what access slots are available in the current cell is broadcast on the BCH. Before the transmission of a random access request, the mobile terminal should carry out the following tasks: -

Achieve chip, slot and frame synchronization to the target base station from the SCH and obtain information about the downlink scrambling code, also from the SCH;

-

Retrieve information from BCCH about the random access codes used in the target cell or sector;

-

Estimate the downlink path loss, which is used together with a signal strength target to calculate the required transmit power of the random access request

Downlink Physical Channels In the downlink, there are four common physical channels. The common pilot channel (CPICH) is used for coherent detection; the primary and secondary common control physical channels (CCPCH) are used to carry the BCH; the SCH provides timing information and is used to handover measurements by the mobile station. The primary CCPCH carries the BCH channel. It is of fixed rate and is mapped to the DPDCH in the same way as dedicated traffic channels. The primary CCPCH is allocated the same channelization code in all cells. A mobile terminal can thus always find the BCH, once the base station’s unique scrambling code has been detected during the initial cell search. The secondary physical channel for common control carries the PCH and FACH in time multiplex within the super-frame structure. The rate of the secondary CCPCH may be different for different cells and is set to provide the required capacity for PCH and FACH in each specific environment. The channelization code of the secondary CCPCH is transmitted on the primary CCPCH.

59

Chapter 7 - Wideband CDMA

The SCH consists of two subchannels, the primary and secondary SCHs. The SCH applies short code masking to minimize the acquisition time of the long code. The SCH is masked with two short codes (primary and secondary SCH). The unmodulated primary SCH is used to acquire the timing for the secondary SCH. The modulated secondary SCH code carries information about the long code group to which the long code of the BS belongs. The primary SCH consists of an unmodulated code of length 256 chips, which is transmitted once every slots. The primary synchronization code is the same for every base station in the system and is transmitted time aligned with the slot boundary. The secondary SCH consists of one modulated code of length 256 chips, which is transmitted in parallel with the primary SCH. The secondary synchronization code is chosen from a set of 16 different codes, depending on which of the 32 different code groups the base station downlink scrambling code belongs. There is only one type of downlink dedicated physical channel, the downlink dedicated physical channel (DPCH). Within one downlink DPCH, data is transmitted in timemultiplex with control information generated at layer 1, such as known pilot bits, TPC commands and an optional transport format combination indicator (TFCI).

Spreading The WCDMA scheme employs long spreading codes. Different spreading codes are used for cell separation in the downlink and user separation in the uplink. In the downlink, Gold codes of length 218 are used, but they are truncated to form a cycle of a 10-ms frame. The total number of available scrambling codes is 512, divided into 32 code groups with 16 codes in each group to facilitate a fast cell search procedure. In the uplink, either short or long spreading (scrambling codes) is used. The short codes are used to ease the implementation of advanced multiuser receiver techniques; otherwise long spreading codes are used. Short codes are VL-Kasami codes of length 256 and long codes are Gold sequences of length 241.

60

Conformal Antenna Arrays for 3G Cellular Base Stations

Handover Base stations in WCDMA need not be synchronized and therefore no external source of synchronisation, like global positioning system (GPS), is needed for the base stations.

Soft and Softer Handover During softer handover, a mobile station is in the overlapping cell coverage are of two adjacent sectors of a base station. The communications between mobile station and base station take place concurrently via two channels, one for each sector. The two signals are received in the mobile station by means of Rake processing. Figure 7.1 shows the softer and soft handover scenarios. During soft handover, a mobile station is in the overlapping cell coverage are of two sectors belonging to different base stations. Before entering soft handover, the mobile station measures observed timing differences on the downlink SCHs from the two base stations. The structure of SCH was discussed earlier in the chapter. The mobile station reports the timing differences back to the serving base station. The timing of a new downlink soft handover connection is adjusted with a resolution of one symbol. That enables the mobile RAKE receiver to collect the macro diversity energy from the two base stations. Soft and softer handover can take place in combination with each other.

Figure 7.1

Softer and Soft Handovers [21].

61

Chapter 7 - Wideband CDMA

Inter-frequency Handovers Inter-frequency handovers are needed for utilization of hierarchical cell structures; macro, micro and indoor cells. Several carriers and inter-frequency handovers may also be used for taking care of high capacity needs in hot spots. Inter-frequency handovers are also needed for handover to second generation systems, like GSM and IS-95. In order to complete inter-frequency handovers, an efficient method is needed for making measurements on other frequencies while still having the connection running on the current frequency. One methods considered for inter-frequency measurements in WCDMA is dual receiver. The dual receiver approach is considered suitable, especially if the mobile terminal employs antenna diversity. During the inter-frequency measurements, one receiver branch is switched to another frequency for measurements, while the other keeps receiving from the current frequency. The loss of diversity gain during measurements must be compensated for with higher downlink transmission power. The advantage of the dual receiver approach is that there is no break in the current frequency connection. Fast closed loop power control is running all the time.

Interoperability Between GSM and WCDMA When WCDMA was standardized a key aspect was to ensure that existing investments could be reused as much as possible. One example is handover between the new WCDMA network and the existing (GSM) network, which can be initiated by coverage, capacity or service requirements. Handover from WCDMA to GSM, for coverage reasons, is initially expected to be important since operators are expected to deploy WCDMA gradually within their existing GSM networks. When a subscriber moves out of the WCDMA coverage area, a handover to GSM has to be conducted in order to keep the connection. Handover between GSM and WCDMA can also have positive effect on capacity through the possibility of load sharing. If for example the numbers of subscribers in the GSM 62

Conformal Antenna Arrays for 3G Cellular Base Stations

network is close to capacity limit one area, handover of some subscribers to the WCDMA network can be performed. When performing handover to GSM, measurements have to be made in order to identify the GSM cell to which the handover will be made. Measurement for the handset are created using the compress mode in which all the information is send during the first half of the frame and the second half is used for measurements on the other systems. The GSM compatible multiframe structure, with superframe being a multiple of 120 ms, allows similar timing for intersystem measurements as in the GSM system itself. Apparently, the needed measurement interval does not need to be as frequent as for GSM terminal operating in a GSM system, as intersystem handover is less critical from an intra-system interference point of view. This way, the relative timing between GSM and WCDMA carriers is maintained similar to the timing between two asynchronous GSM carriers. GSM traffic channel and WCDMA channels use similar 120ms multiframe structure. The GSM frequency correction channel (FCCH) and GSM SCH use one slot out of the eight GSM slots in the frames, with the FCCH frame with one time slot for FCCH always preceding the SCH frame with one time slot for SCH. A WCDMA terminal can do the measurements either by requesting the measurement intervals in a form of a slotted mode, where there are breaks in the downlink transmission, or then it can perform the measurements independently with a suitable measurement pattern. With independent measurements, the dual receiver approach is used instead of the slotted mode, since then the GSM receiver branch can operate independently of the WCDMA receiver branch. For smooth interoperation between the systems, information needs to be exchanged between the systems to allow the WCDMA base station to notify the terminal of the existing GSM frequencies in the area. In addition, more integrated operation is needed for the actual handover where the current service is maintained, taking naturally into account the lower data rate capabilities in GSM, when compared to maximum UMTS data rates of 2 Mbps. The GSM is likewise expected to also indicate the WCDMA spreading codes in the area to make the cell identification simpler. After that, the existing measurement practices in 63

Chapter 7 - Wideband CDMA

GSM can be used for measuring the WCDMA when operating in GSM mode. WCDMA does not rely on any superframe structure as with GSM to find out synchronisation, so the terminal operating in GSM can obtain the WCDMA frame synchronization once the WCDMA base station scrambling code timing is acquired. The base station scrambling code has a 10 ms period and its frame timing is synchronized to WCDMA common channels.

64

Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 8 Conformal Microstrip Antenna Arrays

Microstrip Patch Antennas Microstrip patch antennas were first introduced in 1970s and since then it has become the main field of antenna research and development. Several well-known advantages of microstrip antennas over other conventional antenna structures include their low profile and hence conformal nature, low weight and cost of production, compatibility with microwave monolithic integrated circuits (MMICs) and optoelectronic integrated circuits (OEICs) technologies. Due to these advantages microstrip antennas have found many

applications

including

mobile

communication

base

stations,

satellite

communication systems and even mobile cellular phones. However there are some disadvantages of microstrip antennas that need to be mentioned. They have small bandwidth and relatively low radiation efficiency due to surface wave excitation and conductor and dielectric losses. Vast efforts of many universities and research institutions have been done to address and solve these issues. Still the area of microstrip patch antennas is a thriving technology and it continues to be so in following years to come.

General Characteristics A microstrip patch antenna is usually etched on a grounded dielectric laminates of some common shape; rectangular, square, circular, elliptical, triangular etc. Properties of the substrate such as its height and its dielectric constant play an important role in the performance of the printed microstrip antenna. Depending on its application it is essential that the right substrate type is selected for microstrip patch antenna design.

65

Chapter 8 - Conformal Microstrip Antenna Arrays

Figures below show some very important performance trends of a single-layer rectangular microstrip patch antenna, with a simple excitation method as a function of the substrate thickness. Figure 8.1a shows the bandwidth for various dielectric constant values as a function of substrate thickness. The thicker the material is, the greater the bandwidth. Also to note is that the lower the dielectric constant the greater the bandwidth that can be achieved. Figure 8.1b shows the directivity of the patch antenna. Greater directivity is obtained for antennas mounted on lower dielectric constant since they appear physically larger and hence have larger collecting area then the patch antennas mounted on higher dielectric constant. As the thickness of the substrate increases so does the directivity due to the increasing volume of the antenna. Figure 8.1c shows the surface wave efficiency of a microstrip patch antenna. It can be observed that the higher the dielectric constant the more power is lost to the surface wave and therefore the antenna is less efficient.

(a)

66

Conformal Antenna Arrays for 3G Cellular Base Stations

(b)

(c) Figure 8.1

Performance trends of single-layered microstrip patch antenna: (a) impedance bandwidth; (b) directivity; (c) surface wave efficiency [15].

Feeding Techniques There are four fundamental techniques to feed a microstrip patch antenna; edge fed, probe fed, aperture coupled and proximity coupled. Some properties of each feeding method are described below.

67

Chapter 8 - Conformal Microstrip Antenna Arrays

Edge-Fed Patches Edge feeding or microstrip-line feeding technique is one of the original methods for exciting the microstrip patch antenna. In this method a microstrip feed line of width Wf is in direct contact with a rectangular patch of length L and width W. They have some advantages over other feeding techniques. They are easier to fabricate because the feed line and the patch can be etched on the same board. Input impedance can be easily controlled by adjusting the point at which the feed line comes into contact with the patch. Low impedance down to few ohms is obtained if the contact point is near the centre of the patch. This feeding technique is also easier to model if thin material is used. Simple transmission line models can be used. This form of feeding suffers from high spurious feed radiation because the feed network is not separated from the antenna and thus the feed network radiates too when the patch radiates.

Probe-Fed Patches This method is another method that was originally proposed in 1970s when microstrip patch antennas were introduced. In this method a coaxial probe of radius r extends through the ground plane and is connected to the patch conductor. The method is also referred to as coaxial feed since the inner conductor of the coaxial cable is used as a feeding pin. The probe fed patch has several key advantages. The feed network, including phase shifters and filters, is isolated from the radiating elements via the ground plane. Due to this feature the spurious radiation is minimized and its most efficient feed method because the probe is in direct contact with the element. However, as with edge-fed patches, probe-fed patches have small bandwidth and are somewhat difficult to analyse. If electrically thick substrates are used the probe can generate high cross-polarized fields.

Aperture-Coupled Patches The aperture coupling method is the first non-contact feed mechanism that was introduced to try to improve on shortcomings of direct feed techniques, namely the 68

Conformal Antenna Arrays for 3G Cellular Base Stations

small bandwidth and the effects of surface waves. Separate substrates are used for feed network and patch antennas. The coupling between the feed and the patch antenna is achieved using a small slot in the ground plane which separates the substrates. This configuration has some advantages over the direct contact techniques. Unlike the probefed configuration, no vertical feeds are required, simplifying the fabrication and allowing independent optimisation of the feed and antenna substrates. Due to its multiplayer configuration the alignment issues arise as well as the multilevel fabrication problems. The performance of the antenna depends on the small gaps between the layers of dielectric as well as the bonding material. The efficiency of the antenna is reduces if the bonding material is lossy and located near the slot. Aperture-coupled patches can be easily and accurately modelled using full-wave analysis. They are the most utilized microstrip patch antennas in today’s global market.

Proximity-Coupled Patches Proximity-coupled patches are the second form of non-contact fed patches that were created to overcome the shortcoming of the direct contact fed patches. The microstrip feed line is located on the grounded substrate and the microstrip patch is etched on top of second substrate that is located above. Because the two substrates are separated certain distance the power from the feed line is electromagnetically coupled to the patch. This coupling mechanism is capacitive in nature as opposed to inductive coupling for direct contact techniques. Therefore the bandwidth of proximity-coupled patches is usually greater. These antennas have high spurious feed radiation because the feed and antenna layers are not fully independent. As with aperture-coupled patches, small gaps between the two substrates can affect the coupling efficiency to the patch and so must be taken into consideration during fabrication.

Enhancing Bandwidth The simple microstrip patch cannot satisfy the bandwidth requirements for most wireless communication systems. Over the years a lot of research was undertaken in order to investigate bandwidth enhancement techniques. Generally to improve the 69

Chapter 8 - Conformal Microstrip Antenna Arrays

bandwidth one or more resonant antennas are added to the patch configuration. Some more recognized methods for enhancing the bandwidth are discussed below. Parasitically Coupled (or Gap-Coupled) Patches This technique was proposed in 1980s and consists of two parasitic patches positioned on either side of the excited patch. If the resonant frequency of the coupled elements is slightly different to that of the driven patch, then the bandwidth of the entire antenna may be increased. The critical parameters in this configuration are the lengths and widths of each patch for control of resonant frequency and bandwidth as well as the element spacing. The element spacing controls the coupling between the elements and therefore the tightness of the resonant loops in the impedance locus of the antenna. This bandwidth enhancement technique has been used to achieve bandwidths on the order of 20% (bandwidth of approx. 5% for single layer microstrip patches). However to achieve such wide bandwidths, wide parasitic elements are required which make the overall size of the antenna electrically large and therefore introducing grating lobe problems. Stacked Microstrip Patches Stacked microstrip patch is the most common procedure used to enhance the bandwidth of a microstrip antenna. Direct contact edge-fed stacked patches or aperture-coupled stacked patches can used in this method. Bandwidths of almost 30% have been achieved using these techniques. Advantages of edge-fed stacked patches over aperture-coupled stacked patches include ease of fabrication and a minimal backward-directed radiation. The stack patch geometry has several advantages over other bandwidth enhancement techniques. They are relatively easy to design and can be easily accommodated into an array environment. Large Slot Aperture-Coupled Patches Increasing the size of the slot of an aperture-coupled patch is a simple way of enhancing the bandwidth. This will ensure that the power is coupled to the patch that is located on a thick dielectric layer of substrate. Bandwidths of 40% have been achieved using this technique, however there are two problems with using a large slot aperture-coupled 70

Conformal Antenna Arrays for 3G Cellular Base Stations

patch. Firstly the front to back ratio tends to be poor. This leads to increased level of interference in sectored wireless communication systems and mobile communication systems. Base stations utilizing three 120 deg sectors need to have antennas with minimum back radiation levels. Ways of minimizing back radiation include the addition of reflector elements or use of cavity-backed configurations. Secondly the large slot can cause deformation of the radiation pattern. One way of avoiding this problem is to ensure that the ground plane extends a relatively large distance with respect to the centre of the patch and the slot. Despite these problems large slot aperture-coupled patches are currently utilized as the antenna for several mobile communication base stations throughout the world. Aperture-Stacked Patches This printed antenna, referred to as an aperture-stacked patch (ASP) consists of a large slot and two directive patches. Impedance bandwidths in excess of an octave have been achieved using this printed antenna configuration. The front to back ratio is not as poor as for that of a large slot aperture-coupled patch because of the additional directive patch. Currently ASPs are the ultimate wideband printed antennas based on microstrip patch technology. They are very suitable for wideband operation because they have suitable characteristics; good impedance and gain bandwidth, good polarization control, compactness and relatively simple development. Despite it’s electrical thickness, it does not suffer from surface wave problems because the surface wave power is coupled to the adjoining patches and radiated into space. Other types of printed antenna can provide very wide bandwidths as well and they include: printed spirals, tapered slots, printed bow-tie antennas, L-shaped excited stacked patch antennas and printed quasi yagi antennas.

71

Chapter 8 - Conformal Microstrip Antenna Arrays

Conformal Arrays The essential part of a conformal array is its curvature. Many applications exist where the conformal array occupies only a small part of a curved body and where the conformal effects are mainly on patterns and excitation rather than on impedance. For these applications the array can be designed as a planar array but with proper conformal phasing. Flush-mounted antennas for aircraft and missiles are some common examples. The conformality may be required for aerodynamic reasons or to reduce the antenna’s radar cross section. Sometimes arrays are conformal to a stationary shaped surface in order to increase the angular sector served by a single array. Arrays required to provide 180 or 360 deg azimuth coverage may be conformal to a cylinder, depending on the elevation coverage required while a spherical surface may be required to provide full hemispherical coverage. Arrays on curved surfaces may be divided into two categories as shown in Figure 8.2. If the array dimensions are small compared to the radius of the curvature then the array is treated as locally planar. Such nearly planar arrays also have coverage limited by the field of view of the planar array. Arrays that are large with respect to the radius of curvature conform to the surface and may be used to scan over a far larger sector if the illuminations are somehow commuted around the surface. This commutation is accomplished by several means which are discussed briefly in this section.

Figure 8.2

Conformal arrays: (a) aperture dimensions much less than local radius of curvature (b) aperture dimensions comparable with local radius of curvature [19].

72

Conformal Antenna Arrays for 3G Cellular Base Stations

For conformal arrays, the analysis and synthesis is significantly more complex than for a nearly planar or conventional planar array. They differ from planar arrays in several aspects. Pattern synthesis is complicated because the element positions are in one plane and the element spacings are not always equal. For these arrays, the array factor and element patterns are not separable and the array factor is not always a polynomial. To produce a low-sidelobe pattern with an array that is large with respect to the radius of curvature, one must commutate the illumination around the radiating surface in order to utilize the elements that radiate efficiently in the direction of desired radiation. A third aspect is that the polarization radiated by elements on surfaces that are not parallel to one another will not generally be aligned and therefore may cause high cross polarization.

Patterns of Circular and Cylindrical Arrays Circular and cylindrical arrays possesses the advantage of symmetry in azimuth, which makes them ideally suited for full 360 deg coverage. This advantage has been exploited for the development of broadcast antennas and direction-finding antennas. “Conformal Array Antenna Design Handbook” edited by R.C. Hansen, presents an extensive literature search and practical pattern results for both circular and cylindrical arrays. Figure 8.3 shows a group of elements in a circular or ring array. The array pattern for the ring array of radius a with N elements at locations Φ’ = n∆Φ is given by the usual array expression with rn = R0 – a sin θ cos(Φ - n∆Φ) The resulting pattern is N −1

F (θ , φ ) =

∑I n =0

n

f n (θ , φ )e + jka sin θ cos(φ −n∆φ )

Because of the symmetry, the element patterns are dependent on the element location and have the form:

73

Chapter 8 - Conformal Microstrip Antenna Arrays

fn (θ ,φ ) = f (θ , φ − n∆φ ) and generally include mutual coupling and effects of ground plane curvature. The ring array is of particular importance because it is also the basic element of cylindrical arrays.

Figure 8.3

Circular array geometry [19].

The axial distance of the cylindrical array allows elevation pattern control and higher directivity. Elements are more direct due to the metallic cylinder on which elements are placed. The influence of the curved surface on the radiation pattern is examined next. The cylindrical array can be considered to consist of a stack of identical ring arrays. The coordinate system is shown in Figure 8.4. For simplicity the complex excitation of the pth elements in the qth ring can be denoted by Ipq = I(βp,zq), where βp is the angular location of the pth and zq is the z-axis location of the qth ring.

74

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 8.4

Cylindrical coordinate system [18].

All elements are assumed identical, symmetrical, equally spaced, and pointed along the radius. Thus, the azimuth element pattern can be expressed as a function of |Φ – β|. The azimuth pattern depends on the elevation angle θ. Assuming that the phase centre is at the element,

G(φ − β ,θ ) = G(φ − β ,θ ) exp[ jkp cosθ cos(φ − β )]. The far field is E (φ ,θ ) =

∑∑I p

pq

G (φ − β p ,θ )exp( jqu ),

q

where u = kd sinθ and d = spacing between elements in the axial direction. A beam can be formed in the direction φ = 0 and θ = θ

0

by exciting all elements to add in

phase in that direction (beam co-phase excitation). The azimuth distribution depends on the beam-pointing angle in both azimuth and elevation. Analysis can be simplified by considering the cylindrical array pattern to be the product of a ring array pattern and a linear array pattern. The patterns do not include the effects of mutual coupling.

75

Chapter 8 - Conformal Microstrip Antenna Arrays

Grating Lobes A cylinder can be covered with a regular lattice but the projection in any direction produces unequal spacing in azimuth. The element spacings in elevation direction are uniform and so conventional grating lobe theory can be used. Azimuth spacing does not produce high-amplitude grating lobes but the sidelobes may increase if the element spacing is too large. Element spacing and the cylinder radius are the two factors needed to calculate grating lobes. Amplitude tapering is typically used to produce moderately low sidelobes. For proper pattern calculation element spacing, cylinder radius, element pattern and amplitude taper must be included. Cylindrical arrays that are phased to produce narrow beams tend to be more susceptible to grating lobe problems than do comparable planar arrays. This is due to two factors: (i) the element patterns on the sides of the active part of the array do not point in the direction of the main beam; and (ii) it is necessary to have a large inter-element phase shift into the excitation to compensate for curvature of the cylinder. Hence inter-element spacing must be kept relatively small to prevent the formation of grating lobes. The book “Phased Array Antennas” by R.C. Hansen presents mathematical analysis of grating lobes. The grating lobe of the cylindrical staggered array is equal to the grating lobe of the linear array, with spacing d, times the grating lobe of a ring array with spacing 2s (s = elevation spacing). Figure 8.5 below shows the grating lobe height as a function of scan angle for regular and staggered configuration and the position in elevation and azimuth of the staggered array lobe.

Figure 8.5

Grating lobe position and height vs. scan angle [18].

76

Conformal Antenna Arrays for 3G Cellular Base Stations

The figures below show the pattern for variousθ 0 . The effect of staggering is easily seen. If the rings are spaced at d = 0.72λ, a beam at θ 0 = 30 deg gives a grating lobe at about –60 deg. A regular array gives the grating lobe height at –11dB, which is the difference between beams at θ 0 = 30 deg and θ 0 = 60 deg. For a staggered array, the grating lobe height is the difference between the beam at θ 0 = 30 deg and the grating lobe at θ 0 = 60 deg (about –28dB).

(a) 30 dB Chebyshev patterns for θ 0 = 0

(b) 30 dB Chebyshev patterns for θ 0 = 30

(c) 30 dB Chebyshev patterns for θ 0 = 60 Figure 8.6

30 dB patterns for (a) θ 0 = 0, (b) θ 0 = 30 and (c) θ 0 = 60 degrees [18].

Figure 8.7 below present the patterns for regular and staggered arrays at various θ 0 . The same single ring parameters are assumed as for previous Figure 8.6 in addition, 32 rings spaced at 0.72λ are used with a 30dB Chebyshev distribution. The grating lobe can be reduced and elevation scan extended by reducing the azimuth or elevation spacing of the staggered array. For example, reducing the azimuth spacing 77

Chapter 8 - Conformal Microstrip Antenna Arrays

from 0.65λ to 0.5λ (with d = 0.72λ) increases the scan angle limit from 30 deg to about 40 deg to maintain a grating lobe of 30 dB and further reduction to 0.4λ allows scanning to above 75 deg with the grating love below 40 dB. Reduction of elevation spacing (with s = 0.65λ) from 0.72 to 0.6λ allows scanning to above 50 deg for a grating lobe below 40 dB.

Figure 8.7

30 dB Chebyshev patterns [18].

78

Conformal Antenna Arrays for 3G Cellular Base Stations

Scan Element Pattern Scan element patterns of elements disposed around a cylinder can be obtained by solving an equation for each azimuthal mode separately. Figure 8.8 shows scan element pattern for circumferential dipoles around a cylinder of diameter (120/π)λ, with axial dipole spacing of 0.72λ, and circumferential spacings of 0.5, 0.6 and 0.72λ. As expected from the analogous H-plane planar array shows scan element pattern, the wider spacing show oscillations at broadside, leading to a drop at a grating lobe angle. The drops occur at angles smaller than those for the planar case, but are less steep.

Figure 8.8

Scan element patterns for several spacings [18].

Figure 8.9 shows scan element pattern for an array of rectangular waveguide radiators around a cylinder of diameter (185/π)λ, with axial spacing of 0.8λ and circumferential spacing of 0.6λ. E-field was circumferential, with guide dimensions of 0.32 by 0.75λ. This larger cylinder shows a steeper drop, more like the planar array results.

79

Chapter 8 - Conformal Microstrip Antenna Arrays

Figure 8.9

Scan element pattern [18].

Sector Arrays On Cylinders Patterns and Directivity In many applications such as missiles and aircraft, full azimuth scanning is not needed. Sector arrays, where the elements occupy a sector of some angle are appropriate. When sector angle is small the array can be designed as a planar array with minor adjustments. However, large sectors require the examinations of all curvature effects. Figure 8.10 shows relative directivity versus sector included angle for several element pattern variations. The directivity is the projected area times the element directivity.

80

Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 8.10 Arc array directivity relative to flat array across diameter [19].

An example of an array with 0.65λ spacing on a 27.4λ diameter cylinder, for sector angles of 60, 90 and 120 deg is shown in Figure 8.11. Larger projected element spacings at large angles allow the grating lobe to increase with larger sector angle. This lobe could be suppressed with closer element spacing of course. However the directivity penalty cannot. From a 60 deg sector to one of 120 deg, the projected area has doubled but directivity has increased only 1.0 dB.

Figure 8.11

Scan element patterns of arc arrays [18].

81

Chapter 8 - Conformal Microstrip Antenna Arrays

Comparison of Planar and Sector Arrays The question often arises whether the cylindrical array makes efficient use of aperture and hardware, in particular when compared with standard planar array approach. For 360 deg azimuth coverage, four planar arrays each scanning 90 deg, are generally used, so the cylinder is compared with the four-sided planar configuration (assuming identical elements). For elevation scanning and elevation pattern, the two configurations give nearly identical results. The planar array elevation pattern is the array factor multiplied by the elevation element pattern, while the cylindrical array elevation pattern is the array factor multiplied by the “ring array” elevation pattern. The advantage of the cylinder is that the ring array elevation pattern tend to suppress sidelobes more than does the element pattern alone. For smaller arcs that are excited in the ring the main beam and first few sidelobes are almost identical to the linear array results because for small angles the curvature has a negligible effect on the phased distribution from each element. Also, effects of element spacing become apparent only at larger angles. Chebyshev distribution method could be used to form the desired beamwidth and constrain the inner sidelobes of an arc array. If grating lobes are controlled, all sidelobes will be below the inner sidelobe. It has been calculated [R.C. Hansen] that about 92% to 100% of the elements required for a four-sided linear array are required to obtain the equivalent ring array. There are some disadvantages of the planar array which the cylindrical array avoids. The ring array beam is identical for all beam positions, while the planar array beam is broader in scanning off broadside. As the ring array is scanned by commutating the distribution, it is always formed by a distribution which is symmetrical in phase and amplitude. This results in superior beam pointing accuracy independent of frequency change. Cylindrical array gives 360 deg coverage in azimuth with none of the handover problems associated with the use of several planar arrays. In some applications these advantages can be very important. The cylindrical array however has some disadvantages. For scanning, the amplitude as well as the phase must be switched in azimuth and the feeding system that results will be more complex than that of a planar array system. The greatest disadvantage would be 82

Conformal Antenna Arrays for 3G Cellular Base Stations

that the cylindrical array cannot be physically separated as can the four planar array. This means that the cylinder must be in position to look 360 deg, while each planar array needs to see only 90 deg sector. More important, it means that the cylinder cannot be tilted back to increase elevation coverage, as is common practice with the planar arrays.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 9 Single Element Design The design of a single layer element involves selecting the material to be used, i.e. the substrate and conductor, calculating the approximate patch size and the width of the feedline (dependent on desired input impedance) and simulating and optimising the design using the Ensemble microstrip and HFSS computer-aided drawing (CAD) software.

Overview of Ensemble CAD Software Ensemble is a CAD package for microstrip antenna design using the full-wave moment method technique, designed for the Windows operating systems and is produced by Boulder Microwave Technologies. It is used to model elements and small arrays with a high degree of accuracy and has the ability to determine all the relevant electrical parameters for various antenna shapes, layers and array feed networks. The graphical user interface allows an easy on-screen antenna design according to the number of layers and material parameters specified. It is able to estimate transmission line, quarter wave transformer and patch dimensions by specifying the resonant frequency or impedance required. The design can then be simulated with various simulation options provided such as sparameters, 2D and 3D far fields, as well as the frequency range of interest. Results are then available in different graphical forms. The design can then be adjusted and optimised to provide desired results and characteristics.

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Chapter 9 - Single Element Design

Overview of High Frequency Structure Simulator (HFSS) Software HFSS is a 3D EM simulation software for RF & wireless design which is produced by Agilent Technologies. It was first introduced in 1990 as the first commercial software tool to simulate complex 3D geometries. The software gained instant popularity because it brought the power of finite element method FEM to design engineers. Since 1990, numerous improvements have allowed open regions for antenna design, fast frequency sweeps for wideband simulation, ferrite materials for nonreciprocal devices, and new features for antenna design. HFSS is an interactive software package that computes s-parameters and full-wave fields for arbitrarily-shaped 3D passive structures. Structures are simulated in HFSS using the finite element method (FEM) together with advanced techniques such as automatic adaptive mesh generation and refinement, tangential vector finite elements, and Adaptive Lanczos Pade Sweep (ALPS). An initial mesh - or subdivision of the geometry into tetrahedral elements - is created based on the structure drawn in the CAD package. This initial mesh is solved quickly to provide field solution information identifying regions of high field intensity or rapid field gradients. The mesh is then refined only where needed, saving computational resources while maximizing accuracy. HFSS automatically computes multiple adaptive solutions until a user-defined convergence criterion is met. Field solutions calculated from first principles accurately predict all high-frequency behavior such as dispersion, mode conversion, and losses due to materials and radiation. Analyzing antennas, waveguide components, RF filters and many other structures is as simple as drawing the structure, specifying material characteristics, and identifying ports and special surface characteristics. HFSS automatically generates field solutions, port characteristics, and s-parameters. It is quickly able to calculate antenna metrics such as gain, directivity, far-field pattern cuts, far-field 3D plots, and 3dB beamwidth.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Materials The most sensitive parameter in the estimation of antenna performance is the dielectric constant of the substrate material. Propagation constant of an electromagnetic wave travelling in the microstrip substrate must be accurately known as well. Small variations in the substrate dielectric constant or dimensional changes due to temperature fluctuations can result in frequency shift. Therefore substrates used in the design of microstrip antennas need to be of a high quality in terms of stability in their mechanical and electrical properties. From chapter 8 we have seen that materials with lower dielectric constant will provide greater bandwidth, more directive and more efficient antennas however with thinner substrates, as is the case in this design, the bandwidth will be small. This design will use substrate parameters from a very common substrate known as RT Duroid 5880. Antennas for WCDMA applications will need to have substantially large bandwidth. From Pozar [16], the requirements to increase the impedance bandwidth are thick and low permittivity substrates. This also has the desirable qualities of high radiation efficiency and low surface radiation. However this design will only concern single layer microstrip design as the focus of the thesis primarily lies in effects of curvature on conformal antenna arrays. Hence material chosen are: •

Microstrip Substrate: 62 mils RT Duroid 5880, permittivity εr = 2.22, and thickness h = 1.5875 mm, ½ ounce copper cladding.

•

Patch Substrate: Metal ½ ounce copper cladding, thickness = 0.017 mm and conductivity 5.800 x 107.

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Chapter 9 - Single Element Design

Patch Size Calculation This process provides a reasonably accurate starting point although it does not provide the final patch dimensions. The equations have been obtained from Balanis [17] and the values calculated refer to the dimensions illustrated in Figure 9.1.

Figure 9.1 Dimensions of a single layer element [17].

Specified parameters:

•

Resonant Frequency:

fr = 2.15 GHz

Substrate Permittivity:

εr = 2.22

Substrate Thickness:

h = 1.5875 mm

Calculate width of patch:

W=

1 2 f r µ 0ε 0

2 c = ε r +1 2 fr

2 εr +1

where W is the width in metres and c is the free space velocity of light. The calculated width is:

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Conformal Antenna Arrays for 3G Cellular Base Stations

W=

•

1 2 f r µ 0ε 0

2 0.3 2 = = 55.16 mm. ε r + 1 2(2.15) 2.2 + 1

Calculate effective dielectric constant:

ε reff =

ε rr + 1 ε r − 1  h + + 1 12 W  2 2 

−1 / 2

Thus ε reff = 2.0742 •

Calculate element extension length due to fringing effects:

(ε

W  +0.3) + 0.264  h  ∆L = h(0.412) W (ε reff −0.258) + 0.8  h  reff

Thus ∆L = 0.8421 mm

So finally the actual length of patch:

L=

1 2 f r ε r µ 0ε 0

− 2∆L

therefore the length of the patch is L = 46.758 mm.

Ensemble Simulation and Optimisation The single element design was put into Ensemble with the previous calculated dimensions and the following initial dimensions: 88

Chapter 9 - Single Element Design

Width of 50 Ohm input transmission line:

w = 4.92 mm

Length of 50 Ohm input transmission live: l = 26.9 mm The transmission line was located at the edge of the patch in the centre of the width side. The design was then simulated for range of frequencies (typically 2.0 to 2.3 GHz) and s-parameters were inspected. This scattering parameter is also known as return loss and it specifies the ratio of the reflected signal to the input signal. It is usually used to determine how well the feedline is matched relative to the antenna patch (in terms of impedances). From this graph the impedance bandwidth can also be measured. It is defined as being the range of frequencies for which the return loss response is below – 10dB. Figure 9.2 shows the return loss that was obtained:

Figure 9.2

Return loss for a single element.

The low return loss is due to mismatching of the feedline and the microstrip patch. A quarter wave transformer was then used to match the feedline to the patch. This was estimated using Ensemble. Again the estimate was not completely perfect but it gave a

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Conformal Antenna Arrays for 3G Cellular Base Stations

good starting point for further trial and error simulations. Through trial and error process it was clear which parameters affect the antenna characteristics. The length L of the patch determines the resonant frequency where as the patch width W and transmission line length determine the coupling between the transmission line and the patch element and therefore the return loss and bandwidth. Eventually the best return loss obtained (-44 dB) is shown in Figure 9.3 and dimensions for this antenna element are shown in Figure 9.4.

Figure 9.3

Figure 9.4

Return loss for a single element.

Single patch dimensions for the return loss of -44 dB.

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Chapter 9 - Single Element Design

Now that the desirable return loss was obtained using the Ensemble software the next step was to simulate the single element using HFSS, using same patch dimensions and parameters. When the exact same patch was simulated using HFSS the following return loss was obtained:

Figure 9.5

Return loss obtained for the same patch using HFSS.

One logical answer as to why such a low return loss was obtained using HFSS is that this software takes into consideration the substrate and ground plane area that need to be specified before hand. On the other hand the Ensemble software assumes infinite ground plane and therefore the effects of surface waves are not taken into considerations when performing simulation calculations. After a lengthy trial and error simulation period an acceptable return loss was obtained. The width of the quarter wave transformer and the width of the patch were increased. The right resonance frequency was obtained by varying the length of the patch. The size of the substrate affected the return loss as well and it too was varied until reasonable results were obtained. Return loss of –23 dB was obtained at exactly 2.15 GHZ as shown in Figure 9.6.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Figure 9.6 Return loss for the optimised element in HFSS.

Using HFSS, the following dimensions gave the best return loss (-23 dB): •

Microstrip Patch Length:

L = 42.41 mm

•

Microstrip Patch Width:

W = 56.7 mm

•

Quarter wave transformer:

width = 3.142 mm, length = 26.96 mm

•

Feedline:

width = 4.9 mm, length = 26 mm

The next chapter will focus on the design of a conformal array using microstrip antenna elements with dimensions obtained in this chapter.

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Chapter 10 Conformal Array Design and Simulation The three-element conformal array was designed and simulated using the High Frequency Structure Simulator (HFSS). Once the desired results were obtained for a single element, each element in the array was then designed using the same dimensions and parameters previously obtained in Chapter 9. Due to the nature of the HFSS software, the full conformability of the array could not be designed. Instead, in order to approximate the curvature on which elements would be situated, elements were tilted by number of degrees from each other. Each element is flat on it’s own, however once each element was tilted away from the other, the array would form a structure that resembles a conformal array.

PC Hardware Requirements for HFSS Simulations Due to the very complex nature in which HFSS performs its simulations and calculations the software requires following hardware requirements: •

At least 2GB of RAM

•

8 hours for each simulation on a computer using Pentium III 500 MHz or faster

Due to these hardware requirement the final design could only be limited to a single three-element array instead of the fully cylindrical array with rows and columns of antenna elements.

Design and Simulation Results The conformal array was approximated in the following matter as shown in figure 10.1. The middle patch of the three-element array was left horizontal while the adjacent two 93

Chapter 10 - Conformal Array Design and Simulations

elements were tilted away from the centre patch. To approximate different curvature radii the adjacent elements were tilted by 5, 15, 25 and 35 degrees away from the centre patch. Also to simulate the effects of element spacing on the array performance following element spacing were used in conjunction with the patch tilting; 0.52λ, 0.62λ, 0.72λ, 0.82λ and 0.92λ. This design process is illustrated in Figure 10.1. Table 10.1 to 10.5 indicate conformal array parameters such as radius and number of elements that would be required to cover full cylinder along the circumference for 0.52λ, 0.62λ, 0.72λ, 0.82λ and 0.92λ element spacing respectively.

Figure 10.1

Angle

3-Element conformal array geometry.

Number of Elements

Circumference

Radius

covering full cylinder

Radius in wavelengths (λ)

5o

53

3710 mm

590 mm

4.23

15o

18

1260 mm

200 mm

1.43

25o

11

770 mm

122 mm

0.87

8

560 mm

89 mm

0.64

35

o

Table 10.1

Angle

Conformal array parameters for 0.52λ element spacing.

Number of Elements

Circumference

Radius

covering full cylinder 5

o

Radius in wavelengths (λ)

53

4346 mm

692 mm

4.96

15

o

18

1476 mm

235 mm

1.68

25

o

11

902 mm

144 mm

1.03

35

o

8

656 mm

105 mm

0.75

Table 10.2

Conformal array parameters for 0.62λ element spacing.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Angle

Number of Elements

Circumference

Radius

covering full cylinder

Radius in wavelengths (λ)

5o

53

4982 mm

793 mm

5.68

15o

18

1692 mm

269 mm

1.93

25o

11

1034 mm

165 mm

1.18

35o

8

752 mm

120 mm

0.86

Table 10.3

Angle

Conformal array parameters for 0.72λ element spacing.

Number of Elements

Circumference

Radius

covering full cylinder 5o

Radius in wavelengths (λ)

53

5830 mm

928 mm

6.65

15

o

18

1980 mm

315 mm

2.26

25

o

11

1210 mm

193 mm

1.38

35

o

8

880 mm

140 mm

1.00

Table 10.4

Angle

Conformal array parameters for 0.82λ element spacing.

Number of Elements

Circumference

Radius

covering full cylinder

Radius in wavelengths (λ)

5o

53

6890 mm

1097 mm

7.86

15o

18

2340 mm

372 mm

2.67

25o

11

1430 mm

228 mm

1.63

35o

8

1040 mm

166 mm

1.19

Table 10.5

Conformal array parameters for 0.92λ element spacing.

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Chapter 10 - Conformal Array Design and Simulations

Conformal Array with 0.52λ Element Spacing Planar Array:

Figure 10.2

Planar array 0.52λ element spacing.

Figure 10.3

Horizontal radiation pattern.

Figure 10.4

Vertical radiation pattern.

Figure 10.5

Return Loss.

Figure 10.6

Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

5 Degree Element Shift:

Figure 10.7

Conformal array (Radius 4.23λ and 0.52λ element spacing).

Figure 10.8

Horizontal radiation pattern.

Figure 10.9

Figure 10.10

Return Loss.

Figure 10.11 Insertion Loss.

97

Vertical radiation pattern.

Chapter 10 - Conformal Array Design and Simulations

15 Degree Element Shift:

Figure 10.12

Conformal array (Radius 1.43λ and 0.52λ element spacing).

Figure 10.13

Horizontal radiation pattern.

Figure 10.14

Vertical radiation pattern.

Figure 10.15

Return Loss.

Figure 10.16

Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

25 Degree Element Shift:

Figure 10.17

Figure 10.18

Conformal array (Radius 0.87λ and 0.52λ element spacing).

Figure 10.19 Vertical radiation pattern.

Horizontal radiation pattern.

Figure 10.20 Return Loss.

Figure 10.21 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

35 Degree Element Shift:

Figure 10.22

Figure 10.23

Conformal array (Radius 0.64λ and 0.52λ element spacing).

Horizontal radiation pattern.

Figure 10.25 Return Loss.

Figure 10.24 Vertical radiation pattern.

Figure 10.26 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Conformal Array with 0.62λ Element Spacing Planar Array:

Figure 10.27

Planar array 0.62λ element spacing.

Figure 10.28

Horizontal radiation pattern.

Figure 10.29 Vertical radiation pattern.

Figure 10.30

Return Loss.

Figure 10.31 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

5 Degree Element Shift:

Figure 10.32

Conformal array (Radius 4.96λ and 0.62λ element spacing).

Figure 10.33

Horizontal radiation pattern.

Figure 10.34 Vertical radiation pattern.

Figure 10.35

Return Loss.

Figure 10.36 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

15 Degree Element Shift:

Figure 10.37

Conformal array (Radius 1.68λ and 0.62λ element spacing).

Figure 10.38

Horizontal radiation pattern.

Figure 10.39 Vertical radiation pattern.

Figure 10.40

Return Loss.

Figure 10.41

103

Insertion Loss.

Chapter 10 - Conformal Array Design and Simulations

25 Degree Element Shift:

Figure 10.42

Conformal array (Radius 1.03λ and 0.62λ element spacing).

Figure 10.43

Horizontal radiation pattern.

Figure 10.44 Vertical radiation pattern.

Figure 10.45

Return Loss

Figure 10.46 Insertion Loss

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Conformal Antenna Arrays for 3G Cellular Base Stations

35 Degree Element Shift:

Figure 10.47

Conformal array (Radius 0.75λ and 0.62λ element spacing).

Figure 10.48

Horizontal radiation pattern

Figure 10.49 Vertical radiation pattern

Figure 10.50

Return Loss.

Figure 10.51 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

Conformal Array with 0.72λ Element Spacing Planar Array:

Figure 10.52

Planar array 0.72λ element spacing.

Figure 10.53

Horizontal radiation pattern.

Figure 10.54 Vertical radiation pattern.

Figure 10.55

Return Loss.

Figure 10.56 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

5 Degree Element Shift:

Figure 10.57

Conformal array (Radius 5.68 λ and 0.72λ element spacing).

Figure 10.58

Horizontal radiation pattern.

Figure 10.59 Vertical radiation pattern.

Figure 10.60

Return Loss.

Figure 10.61 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

15 Degree Element Shift:

Figure 10.62

Conformal array (Radius 1.93λ and 0.72λ element spacing).

Figure 10.63

Horizontal radiation pattern.

Figure 10.64 Vertical radiation pattern.

Figure 10.65

Return Loss.

Figure 10.66

108

Insertion Loss.

Conformal Antenna Arrays for 3G Cellular Base Stations

25 Degree Element Shift:

Figure 10.67

Conformal array (Radius 1.18λ and 0.72λ element spacing).

Figure 10.68

Horizontal radiation pattern.

Figure 10.69 Vertical radiation pattern.

Figure 10.70

Return Loss.

Figure 10.71 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

35 Degree Element Shift:

Figure 10.72

Conformal array (Radius 0.86λ and 0.72λ element spacing).

Figure 10.73

Horizontal radiation pattern.

Figure 10.74 Vertical radiation pattern.

Figure 10.75

Return Loss.

Figure 10.76 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Conformal Array with 0.82λ Element Spacing Planar Array:

Figure 10.77

Planar array 0.82λ element spacing.

Figure 10.78

Horizontal radiation pattern.

Figure 10.79 Vertical radiation pattern.

Figure 10.80

Return Loss.

Figure 10.81

111

Insertion Loss.

Chapter 10 - Conformal Array Design and Simulations

5 Degree Element Shift:

Figure 10.82

Conformal array (Radius 6.65λ and 0.82λ element spacing).

Figure 10.83

Horizontal radiation pattern.

Figure 10.84 Vertical radiation pattern.

Figure 10.85

Return Loss.

Figure 10.86 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

15 Degree Element Shift:

Figure 10.87

Conformal array (Radius 2.26λ and 0.82λ element spacing).

Figure 10.88

Horizontal radiation pattern.

Figure 10.89 Vertical radiation pattern.

Figure 10.90

Return Loss.

Figure 10.91 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

25 Degree Element Shift:

Figure 10.92

Conformal array (Radius 1.38λ and 0.82λ element spacing).

Figure 10.93

Horizontal radiation pattern.

Figure 10.94 Vertical radiation pattern.

Figure 10.95

Return Loss.

Figure 10.96 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

35 Degree Element Shift:

Figure 10.97

Figure 10.98

Conformal array (Radius 1.00λ and 0.82λ element spacing).

Horizontal radiation pattern.

Figure 10.100 Return Loss.

Figure 10.99

Vertical radiation pattern.

Figure 10.101 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

Conformal Array with 0.92λ Element Spacing Planar Array :

Figure 10.102 Planar array 0.92λ element spacing.

Figure 10.103 Horizontal radiation pattern.

Figure 10.104 Vertical radiation pattern.

Figure 10.105 Return Loss.

Figure 10.106 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

5 Degree Element Shift:

Figure 10.107 Conformal array (Radius 7.86λ and 0.92λ element spacing).

Figure 10.108 Horizontal radiation pattern.

Figure 10.109 Vertical radiation pattern.

Figure 10.110 Return Loss.

Figure 10.111 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

15 Degree Element Shift:

Figure 10.112 Conformal array (Radius 2.67λ and 0.92λ element spacing).

Figure 10.113 Horizontal radiation pattern.

Figure 10.114 Vertical radiation pattern.

Figure 10.115 Return Loss.

Figure 10.116 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

25 Degree Element Shift:

Figure 10.117 Conformal array (Radius 1.63λ and 0.92 λ element spacing).

Figure 10.118 Horizontal radiation pattern.

Figure 10.119 Vertical radiation pattern.

Figure 10.120 Return Loss.

Figure 10.121 Insertion Loss.

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Chapter 10 - Conformal Array Design and Simulations

35 Degree Element Shift:

Figure 10.122 Conformal array (Radius 1.19λ and 0.92λ element spacing).

Figure 10.123 Horizontal radiation pattern.

Figure 10.124 Vertical radiation pattern.

Figure 10.125 Return Loss.

Figure 10.126 Insertion Loss.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Tabulated Results of Simulations Array Configuration Planar 5o 15o 25o 35o Table 10.6

Array Configuration Planar 5o 15o 25o 35o Table 10.7

Array Configuration Planar 5o 15o 25o 35o Table 10.8

Array Configuration Planar 5o 15o 25o 35o Table 10.9

Return Loss (dB) -21 -13 -12 -12 and -8 -12 and -8

Insertion Loss Insertion Loss (Patch 2 - Patch 1) dB (Patch 2 - Patch 3) dB -28 -55 -25 -50 -28 -55 -28 -50 -28 -50

Simulation results of an array with 0.52λ element spacing.

Return Loss (dB) -21 -14 -12 and -8 -10 and -7 -10 and -7

Insertion Loss Insertion Loss (Patch 2 - Patch 1) dB (Patch 2 - Patch 3) dB -38 -65 -35 -63 -33 -60 -31 -55 -30 -50

Simulation results of an array with 0.62λ element spacing.

Return Loss (dB) -21 -14 -13 and -8 -12 and -6 -11 and -6

Insertion Loss Insertion Loss (Patch 2 - Patch 1) dB (Patch 2 - Patch 3) dB -50 -80 -45 -75 -42 -62 -40 -60 -40 -58

Simulation results of an array with 0.72λ element spacing.

Return Loss (dB) -21 -14 -12 and -8 -12 and -7 -11 and -6

Insertion Loss Insertion Loss (Patch 2 - Patch 1) dB (Patch 2 - Patch 3) dB -65 -80 -62 -66 -58 -58 -55 -55 -55 -55

Simulation results of an array with 0.82λ element spacing.

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Chapter 10 - Conformal Array Design and Simulations

Array Configuration Planar 5o 15o 25o 35o Table 10.10

Return Loss (dB) -20 -12 -12 and -9 -12 and -7 -12 and -6

Insertion Loss (Patch 2 - Patch 1) dB -70 -70 -60 -60 -56

Insertion Loss (Patch 2 - Patch 3) dB -90 -90 -70 -70 -66

Simulation results of an array with 0.92λ element spacing.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 11 Discussion of Results This chapter gives a detailed analysis of the results obtained from simulations carried out using HFSS. Comparisons will be made with reference to the theoretical aspects linked to the results and any discrepancies noted will be discussed and commented. As explained in earlier chapter, simulations were limited due to the extensive hardware requirements of the software. Due to these PC hardware requirements the size of the design had to be restricted to three-element array with minimal physical size as possible. As such, each simulation would take approximately 8 hours to complete and in total 25 final simulations were performed. Note that the PC in the thesis lab did not have the required hardware to support the full calculation of a simulation and therefore all simulations were performed on a Super Computer with the help of the postgraduate student Eddie Tsai who had access to the computer.

Radiation Pattern Results The conformal array and different curvature radii was approximated by adjusting the angle of tilting of the left and right element from the center element. Additionally the array was simulated under different element spacing (ie. 0.52λ, 0.62λ, 0.72λ, 0.82λ and 0.92λ). Firstly the effect of curvature then the effect of element spacing on the radiation pattern will be discussed. Firstly the planar array was simulated in order to obtain the basis for comparison to the conformal array. As expected the three-element array has a major lobe and two minor side lobes on each side. The radiation pattern in azimuth direction is affected mostly. As the curvature was increased, i.e. as the tilting angle was increased, the radiation pattern suffered from increased level of side lobes. When the tilting angle is increased by 10

123

Chapter 11 - Discussion of Results

degrees, which is equivalent in reducing the curvature radius by few wavelengths total deformation in the radiation pattern was observed. In cases for large angles such as 15o, 25o and 35o representing curvature radius of only 1-3 wavelengths the radiation pattern suffered complete degradation when compared to the planar array radiation pattern. The sidelobes and the major lobe combine into a large single lobe representing an almost omni-directional radiation pattern. The radiation pattern in elevation direction does not change much with the change in curvature. It preserves its shape for all simulated curvature radii. The element spacing was varied between 0.52λ and 0.92λ in each simulation. As the element spacing is increased the radiation pattern suffered from larger sidelobes and in some cases major grating lobes. The best results were obtained for the array with 0.62λ and 0.72λ element spacing. Note that some radiation patterns are unclear in their shape and do not offer the full insight into the effects of various element spacing. It is also observed that back lobes are present for larger element spacing.

Return Loss Results Even tough theory suggests that the return loss of each microstrip patch should not be affected by the element spacing or curvature radii, it is discussed here because simulations performed show some discrepancies. It can be seen from the tables of results in chapter 10 that the return loss decreased as the curvature radius was decreased. However it does not change as the element spacing is increased. This may be due to an error in simulation or in design of the array. For the planar array the return loss obtained was –21 dB which is almost the value obtained for the single element microstrip patch. This return loss level is an acceptable level in most literatures. As the curvature is increased the return loss dropped to about -12 dB and in most cases the return loss for the other element dropped to about -8 dB. The reason for this sort of effect on the return loss is not yet clear. It is most likely due to some fault in the design of the array in HFSS however it is a single layer microstrip array which is one of the most simplest designs.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Insertion Loss Results The insertion loss is the measure of the effect of each element on each other in an array. Thus it can be used to indicate the level of mutual coupling in the array. As the theory suggest the smaller the element spacing in the antenna array the greater the level of mutual coupling. This is also confirmed from the simulations performed. From tables of results in chapter 10 the insertion loss between Patch 2 and Patch 3 and the insertion loss between Patch 2 and Patch 1 is measured. The results show that insertion loss between Patch 2 and Patch 1 is always greater than the insertion loss between Patch 2 and Patch 3. As elements are spaced exactly same distance from the center patch (Patch 1) this result was not expected. However the effect of the element spacing on mutual coupling is still evident. As shown in tables of results in chapter 10 the insertion loss decreased by approximately 10 dB for every wavelength increase in element spacing. At 0.52λ element spacing the insertion loss is about -28 dB and it decreased to about -70 dB for element spacing of 0.92λ. However, as the curvature radius was decreased the level of mutual coupling increased. This is clearly evident if the insertion loss of the planar array is compared to the conformal array. In most cases the level of insertion loss increased by maximum of 10 dB for maximum decrease in curvature radius. As the curvature radius was slowly decremented the level of insertion loss rose only by few dB.

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Conformal Antenna Arrays for 3G Cellular Base Stations

Chapter 12 Conclusion and Future Developments

Summary This section summarizes the work carried out in this thesis. Cylindrical conformal array with three elements is studied, designed and simulated using the HFSS simulation software. The effects of various curvature radii and different element spacing on the radiation pattern and mutual coupling are studied. Most optimal radiation patterns are obtained for element spacing of 0.62λ and 0.72λ. The best results were obtained for curvature radius of 4λ or greater however smaller radii were also simulated to show the effect of larger curvatures. The result shows that the resonant frequency is not affected by curvature however the radiation patterns are significantly affected. The radiation pattern in the elevation direction is strongly dependant on the cylinder radius but much less so in the azimuthal direction. The high level of side lobes is present for smaller curvature radii. It also shows that the array exhibits high sidelobes that can be reduced by making the element spacing smaller than is necessary with the planar array. To achieve smooth slopes in the main beam, unlike in planar arrays, the excitation and phase distribution should be kept non-uniform along the curvature. The level of mutual coupling increases slightly with decrease in curvature radius. However the level of insertion loss is minimum for larger element spacing and can be neglected. The return loss decreases as the curvature radius is decreased however this effect may be due to some array design issues in HFSS software.

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Chapter 12 - Conclusion and Future Developments

Future Work Topics that can be explored or expanded following this thesis are listed as follows: •

Design of a larger conformal array with more than three elements

•

Design of a complete cylindrical array with several arrays in both horizontal and vertical direction

•

Study of various curvatures bigger than 3λ in radius and several element spacing to find the optimal configuration

•

Use of stacked mictrostrip patch, aperture stacked, aperture coupled approach or any other approach to provide much wider bandwidth, as single layer microstrip patch does not provide enough bandwidth required for next generation mobile communication systems

•

Manufacture of such an array that includes both beam forming and beam steering electronics and possible smart antenna system as it is a vital feature for next generation mobile base station antennas

•

Design of cylindrical array with multiband operation capability as next generation cellular systems require antennas that work in both 2G and 3G spectral bands so in order to reduce the number of antennas on base stations

•

The system can then be put into tests in an actual cellular environment to determine the feasibility of such an employment in current and future cellular systems

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Conformal Antenna Arrays for 3G Cellular Base Stations

Conclusion This thesis presents background on current antennas used in mobile communication systems and fundamental concepts of conformal antenna arrays. As next generation of mobile communication systems migrate into a new spectral band they will require new types of mobile base station antennas that will operate in that spectrum and be able to replace current base station antennas. Cylindrical antenna arrays are prospective candidates for the next generation mobile communications systems and cellular base stations due to their full field of view advantage. However in order to design such new types of antennas there are few issues that should be taken into consideration when designing and manufacturing such antennas. Firstly the curvature of the cylindrical array affects the radiation pattern of the antenna and the optimal radius should be found depending on the application on hand. Secondly the spacing between elements both in horizontal and vertical direction (assuming full cylindrical array design) is very important to consider as it affects the level of mutual coupling in the array. Simulations were conducted to measure and study the effects of curvature of conformal arrays on the radiation pattern. Additionally the effects of element spacing hence mutual coupling are simultaneously studied. In conclusion this thesis has provided an insight into conformal antenna arrays and will form a platform for researchers working towards realizing the implementation of conformal arrays in current and future cellular systems.

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