Mobile Communications

Design Fundamentals Second Edition

William C. Y. Lee Vice President and Chief Scientist

Applied Research and Science PacTel Corporation

A Wiley-lnterscience Publication

JOHN WILEY & SONS, INC.

New York Chichester Brisbane Toronto Singapore

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Mobile Communications Design Fundamentals

WILEY SERIES IN TELECOMMUNICATIONS

Worldwide Telecommunications Guide for the Business Manager Walter L. Vignault

Expert System Applications to Telecommunications Jay Liebowitz

Business Earth Stations for Telecommunications Walter L. Morgan and Denis Rouffet

Introduction to Communications Engineering, 2nd Edition Robert M. Gagliardi

Satellite Communications: The First Quarter Century of Service David W. E. Rees

Synchronization in Digital Communications, Volume 1 Heinrich Meyr et al.

Telecommunication System Engineering, 2nd Edition Roger L. Freeman

Digital Telephony, 2nd Edition John Bellamy

Elements of Information Theory Thomas M. Cover and Joy A. Thomas

Telecommunication Transmission Handbook, 3rd Edition Roger L. Freeman

Computational Methods of Signal Recovery and Recognition Richard J. Mammone

Telecommunication Circuit Design Patrick D. van der Puije

Mobile Communications Design Fundamentals, Second Edition William C.Y. Lee

Mobile Communications

Design Fundamentals Second Edition

William C. Y. Lee Vice President and Chief Scientist

Applied Research and Science PacTel Corporation

A Wiley-lnterscience Publication

JOHN WILEY & SONS, INC.

New York Chichester Brisbane Toronto Singapore

A NOTE TO THE READER This book has been electronically reproduced from digital information stored at John Wiley & Sons, Inc. We are pleased that the use of this new technology will enable us to keep works of enduring scholarly value in print as long as there is a reasonable demand for them. The content of this book is identical to previous printings.

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Copyright 1993 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

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Library of Congress Cataloging in Publication Data:

Lee, William C. Y. Mobile communications design fundamentals / William C.Y. Lee.

2nd ed. p. cm. (Wiley series in telecommunications)

"A Wiley-Interscience publication." Includes bibliographical references and index. ISBN 0-471-57446-5 (alk. paper) 1. Mobile communication systemsDesign. I. Title. II. Series.

TK6570.M6L36 1993 621.3845'6dc20 92-21130

To My Parents

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CONTENTS

Preface xv

Acknowledgments xix

Chapter 1 The Mobile Radio Environment 1

1.1 Representation of a Mobile Radio Signal 1 1.1.1 Description of a Mobile Radio Environment 1 1.1.2 Field-Strength Representation 2 1.1.3 Mobile Radio Signal Representation 5

1.2 Causes of Propagation Path Loss 5 1.3 Causes of Fading 6

1.3.1 Long-Term Fading, m(t) or m(x) 7 1.3.2 Short-Term Fading, r0(t) or r0(x) 10 1.3.3 Classification of Channels 16 1.3.4 Effects of Weather 18

1.4 Reciprocity Principle 20 1.5 Definitions of Necessary Terms and Their

Applications 20 1.5.1 Averages 20 1.5.2 Probability Density Function (pdf) 23 1.5.3 Cumulative Probability Distribution (CPD) 27 1.5.4 Level-Crossing Rate (1er) and Average Duration

of Fades (adf) 31 1.5.5 Correlation and Power Spectrum 33 1.5.6 Delay Spread, Coherence Bandwidth,

Intersymbol Interference 38 1.5.7 Confidence Interval 41 1.5.8 False-Alarm Rate and Word-Error Rate 42

References 44 Problems 44

vii

Vi i i CONTENTS

Chapter 2 Prediction of Propagation Loss 47

2.1 The Philosophy behind the Prediction of Propagation Loss 47

2.2 Obtaining Meaningful Propagation-Loss Data from Measurements 47 2.2.1 Determining the Length L 47 2.2.2 Determining the Number of Sample Points

Required over 40 49 2.2.3 Mobile Path and Radio Path 51

2.3 Prediction over Flat Terrain 52 2.3.1 Finding the Reflection Point on a Terrain 52 2.3.2 Classification of Terrain Roughness 53 2.3.3 The Reflection Coefficient of the Ground

Wave 57 2.3.4 Models for Predicting Propagation Path

Loss 58 2.3.5 A Theoretical Model for Path Loss 59 2.3.6 An Area-to-Area Path-Loss Prediction

Model 61 2.3.7 The Model of Okumura et al. 68 2.3.8 A General Path-Loss Formula over Different

Environments 69

2.4 Point-to-Point Prediction (Path-Loss Prediction over Hilly Terrain) 72 2.4.1 Point-to-Point Prediction under Nonobstructive

Conditions 72 2.4.2 Point-to-Point Prediction under Obstructive Con-

ditionsShadow Loss 79 2.5 Other Factors 83

2.5.1 Foliage Effects 84 2.5.2 Street Orientation Channel Effect 85 2.5.3 The Tunnel and Underpass Effects 86

2.6 The Merit of Point-to-Point Prediction 87 2.7 Microcell Prediction Model 88

References 94 Problems 96

Chapter 3 Calculation of Fades and Methods of Reducing Fades 101

3.1 Amplitude Fades 101 3.1.1 Level-Crossing Rates 101 3.1.2 Average Duration of Fades 106 3.1.3 Distribution of Duration of Fades 106

CONTENTS IX

3.1.4 Envelope Correlation between Two Closely Spaced Antennas at the Mobile Unit 108

3.1.5 Power Spectrum 110 3.2 Random PM and Random FM 112

3.2.1 Random Phase ( 113 3.2.2 Random FM ifir{t) 113

3.3 Selective Fading and Selective Random FM 115 3.3.1 Selective Fading 115 3.3.2 Selective Random FM 116

3.4 Diversity Schemes 116 3.4.1 Macroscopic Diversity (Apply on Separated

Antenna Sites) 117 3.4.2 Microscopic Diversity (Apply on Co-located

Antenna Site) 117 3.5 Combining Techniques 119

3.5.1 Combining Techniques on Diversity Schemes 119

3.5.2 Combining Techniques for Reducing Random Phase 123

3.6 Bit-Error Rate and Word-Error Rate in Fading Environment 125 3.6.1 In the Gaussian Noise Environment 125 3.6.2 In a Rayleigh Fading Environment 128 3.6.3 Diversity Transmission for Error

Reduction 129 3.6.4 Irreducible Bit-Error Rate 129 3.6.5 Overall Bit-Error Rate 131

3.7 Calculation of Signal Strength above a Level in a Cell (for a Stationary Mobile Unit) 132

3.8 Single-Sideband (SSB) Modulation 136 References 138 Problems 139

Chapter 4 Mobile Radio Interference 141

4.1 Noise-Limited and Interference-Limited Environment 141 4.1.1 Noise-Limited Environment 141 4.1.2 Interference-Limited Environment 141

4.2 Co-channel and Adjacent-Channel Interference 141 4.2.1 Co-channel Interference 141 4.2.2 Adjacent-Channel Interference 144

X CONTENTS

4.3 Intermodulation (IM) 147 4.3.1 Through a Power Amplifier 147 4.3.2 Through a Hard Limiter 150

4.4 Near-End-to-Far-End Ratio 152 4.5 Intersymbol Interference 154 4.6 Simulcast Interference 155 4.7 Radius of Local Scatterers 157

References 159 Problems 159

Chapter 5 Frequency Plans and Their Associated Schemes 161

5.1 Channelized Schemes and Frequency Reuse 161

5.1.1 Channelized Schemes 161 5.1.2 Frequency Reuse 162

5.2 Frequency-Division Multiplexing (FDM) 164 5.2.1 FDM Signal Suppression 164 5.2.2 FDM Signal Distortion 166

5.3 Time-Division Multiplexing (TDM) 169 5.3.1 TDM Buffers 170 5.3.2 TDM Guard Time 170 5.3.3 The Bit Rate and the Frame Rate 172 5.3.4 TDM System Efficiency 172

5.4 Spread Spectrum and Frequency Hopping 173 5.4.1 Spread Spectrum 174 5.4.2 Frequency Hopped (FH) Systems 176

5.5 Cellular Concept 181 5.5.1 Frequency Reuse and Cell Separation 182 5.5.2 Hand-off (HO) 183 5.5.3 Cell Splitting and Power Reducing 184 5.5.4 Reduction of Near-End-to-Far-End Ratio

Interference 184

5.6 Spectral Efficiency and Cellular Schemes 186 5.6.1 Multiple-Channel Bandwidth Systems 186 5.6.2 One-third Channel Offset Scheme 191 5.6.3 An Application of a Hybrid System 195

References 196 Problems 197

Chapter 6 Design Parameters at the Base Station 199

6.1 Antenna Locations 199 6.2 Antenna Spacing and Antenna Heights 200

6.2.1 Antenna Orientation Dependency 202

CONTENTS X

6.2.2 Antenna Height/Separation Dependency 202 6.2.3 Frequency Dependency 206 Antenna Configurations 207 6.3.1 Directional Antennas 207 6.3.2 Tilting Antenna Configuration 207 6.3.3 Diversity Antenna Configuration 210 6.3.4 Comments on Vertical Separation 210 6.3.5 Physical Considerations in Horizontal

Separation 213 Noise Environment 216 6.4.1 Automotive Noise 218 6.4.2 Power-Line Noise and Industrial Noise 218 Power and Field Strength Conversions 219 6.5.1 Conversion between and dBm in Power

Delivery 220 6.5.2 Relationship between Field Strength and

Received Power 222 6.5.3 A Simple Conversion Formula 222

References 224 Problems 224

Chapter 7 Design Parameters at the Mobile Unit 227

7.1 Antenna Spacing and Antenna Heights 227 7.2 Mobile Unit Standing Still and in Motion 229 7.3 Independent Samples and Sampling Rate 230 7.4 Directional Antennas and Diversity Schemes 231

7.4.1 Directional Antennas 231 7.4.2 A Diversity Scheme for Mobile Units 233 7.4.3 Difference between Directional Antenna

Arrays and Space-Diversity Schemes 235

7.5 Frequency Dependency and Independency 236 7.5.1 Operating Frequency Dependency on Space

Diversity 236 7.5.2 Operating Frequency Independence of

Frequency Diversity 236

7.6 Noise Environment 238 7.7 Antenna Connections and Locations on the Mobile

Unit 241 7.7.1 The Impedance Matching at the Antenna

Connection 242 7.7.2 Antenna Location on the Car Body 244 7.7.3 Vertical Mounting 244

6.4

6.5

XU CONTENTS

7.8 Field Component Diversity Antennas 244

7.8.1 The Energy Density Antenna 245 7.8.2 Uncorrelated Signal Diversity Antenna 247

References 248 Problems 249

Chapter 8 Signaling and Channel Access 251

8.1 Criteria of Signaling Design 251 8.2 False-Alarm Rate 251 8.3 Word-Error Rate 252

8.3.1 In a Gaussian Environment 253 8.3.2 In a Rayleigh Environment 256 8.3.3 A Fast-Fading Case in a Rayleigh Fading

Environment 256 8.3.4 A Slow-Fading Case in a Rayleigh Fading

Environment 260 8.3.5 A Comparison between a Slow-Fading Case and

a Fast-Fading Case 263 8.4 Channel Assignment 263

8.4.1 Co-channel Assignment 263 8.4.2 Channel Assignment within a Cell 265 8.4.3 Channel Sharing 266 8.4.4 Channel Borrowing 282

8.5 Switching Capacity Consideration 283 References 284 Problems 285

Chapter 9 Cellular CDMA 287

9.1 Why CDMA? 287 9.2 Narrowband (NB) Wave Propagation 287

9.2.1 Excessive Path Loss of a CW (Narrowband) Propagation in a Mobile Radio Environment 289

9.2.2 Multipath Fading Characteristics 290 9.2.3 Time Delay Spread 291

9.3 Wideband (WB) Signal Propagation 292 9.3.1 Wideband Signal Path Loss in a Mobile Radio

Environment 293 9.3.2 Wideband Signal Fading 296

9.4 Key Elements in Designing Cellular 297 9.5 Spread Techniques in Modulation 298

9.5.1 Spread Spectrum Techniques 298 9.5.2 Time HoppingSpread Time Technique 299

CONTENTS XU!

9.6 Description of DS Modulation 300 9.6.1 Basic DS Technique 300 9.6.2 Pseudonoise (PN) Code Generator 301 9.6.3 Reduction of Interference by a DS Signal 303

9.7 Capacities of Multiple-Access Schemes 303 9.7.1 Capacity of Cellular FDMA and TDMA 305 9.7.2 Radio Capacity of Cellular CDMA 306 9.7.3 Power Control Scheme in CDMA 309 9.7.4 Comparison of Different CDMA Cases 312

9.8 Reduction of Near-Far Ratio Interference in CDMA 313

9.9 Natural Attributes of CDMA 313 References 318 Problems 319

Chapter 10 Microcell Systems 321

10.1 Design of a Conventional Cellular System 321 10.2 Description of New Microcell System Design 324

10.2.1 Signal Coming from Mobile Unit 324 10.2.2 Signal Coming from Base Site 324

10.3 Analysis of Capacity and Voice Quality 327 10.3.1 Selective Omni-Zone Approach 327 10.3.2 Selective Edge-Excited Zone Approach 330 10.3.3 Nonselective Edge-Excited Zone Approach 331 10.3.4 Summary 332

10.4 Reduction of Hand-offs 332 10.5 System Capacity 333 10.6 Attributes of Microcell 334

References 335 Problems 335

Chapter 11 Miscellaneous Related Systems

11.1 PCS (Personal Communications Service) 337 11.1.1 Requirements of PCS 337 11.1.2 PCS Environment 340 11.1.3 Some Concerns 341

11.2 Portable Telephone Systems 342 11.2.1 Propagation Path Loss 343 11.2.2 Body Effects 344 11.2.3 Radio Phenomenon of Portable Units 11.2.4 System-Control Considerations 349

11.3 Air-to-Ground Communications 350 11.3.1 Propagation Path Loss 350

337

345

11.3.2 Co-channel Separation 351 11.3.3 Altitude Zoning Considerations 354 11.3.4 Frequency Allocation Plan and Power

Control 355

4 Land-Mobile/Satellite Communications System 357 11.4.1 Propagation Path Loss 357 11.4.2 Noise 360 11.4.3 Fading 360 11.4.4 Applications 361

References 364 Problems 364

PREFACE

The first edition of this book was published in 1986 by Howard W. Sams Co., then a subsidiary of ITT. When Sams was sold by ITT, it changed its direction of interest to computers and terminated its list of radio communications books. Since that time, many readers have requested that I reissue this book. I am beholden to John Wiley & Sons, Inc. for its willingness to publish this second edition.

Cellular systems have proven to be both high capacity and high quality systems. However, in a realistic situation, due to the frequency re-use scheme for increasing capacity, cellular operators always struggle between quality and capacity, putting all co-channel cells closer together for capacity or putting all co-channel cells farther apart for quality. In August 1985, I was invited by the FCC to give a public presentation on "Spectrum Efficiency Comparison of FM and SSB." My analysis concluded that because of frequency re-use in cellular systems, there was no advantage in splitting the FM channels for capacity. The spectrum efficiencies of FM and SSB are the same. In 1987, I was the first co-chairperson of a newly formed CTIA subcommittee of Ad-vanced Radio Technology (ARTS) and led the cellular industry in setting the requirements for the first North American Digital Cellular System. TIA, then formed a new group TR45.3 to develop cellular digital standards for North America. I personally preferred FDMA because it was a low-risk approach due to the fact that the cellular analog system is also FDMA and the equalizer is not needed. Furthermore, with ARTS' suggestions of having dual-mode subscriber sets and sharing setup channels, the availability of digital FDMA systems by 1990, as purposely stated in the ARTS UPR (User's Performance Requirement), could be easily met. The major vendors such as AT&T and Motorola were voting for FDMA also.

In September 1987,1 was invited by the FCC to speak publicly about how to develop digital cellular for capacity from the cellular operator's point of view and introduced a new radio capacity formula (appearing in the new Chapter 9) to measure spectrum efficiency for digital FDMA and TDMA systems. In February 1989, when Qualcomm came to PacTel to present their first version of CDMA, I emphasized the implementation of power control

xv

XVi PREFACE

for reducing near-far interference. Although PacTel supported TDMA, but due to the relative high-risk of developing TDMA and for the sake of safety, PacTel helped Qualcomm develop an alternative digital cellular CDMA sys-tem. CDMA has been theoretically proven to have twenty times more capacity over the current analog cellular system. PacTel was unselfish, following MFJ restrictions, in contributing its technical and financial resources to help Qualcomm, a small but technically strong U.S. company, develop world-leading cellular CDMA technology for capacity. A trial CDMA system was built in six months starting from scratch, and a cellular CDMA dem-onstration was held in San Diego on Two PacTel sites. Cellular CDMA is introduced in the new Chapter 9.

I also felt the need to develop a microcell technology to further increase capacity in analog and digital systems for future PCS (Personal Communi-cation Service). The difference between conventional microcells and the PacTel patented microcell is that the former are dumb cells and the latter are intel-ligent cells. The new microcell system is introduced in the new Chapter 10.

Since publication of the first edition in 1986, there has been a tremendous increase in the use of mobile communications. In the United States there were 650 thousand cellular units in operation in 1986 and revenues of $46.2 million, and now in 1992 there are 8 million units and revenues close to $4 billion. In the year 2000 the predicted number of cellular units in operation will be 20 million. In the European community, there were 815 thousand cellular units in 1987 and there are 5 million units now. This rapid growth in wireless communication shows the need for technology that will increase capacity and improve system performance. Also, narrowband and wideband radio access technologies are needed. Furthermore in June 1990, the FCC encouraged the wireless communication industry to look into "Personal Com-munication Service (PCS)" systems. PCS is a generic name for a future per-sonal wireless communication system. In Europe, current systems such as cellular communications (analog and digital), cordless telephone-2 (CT-2) system, and the personal communication networking (PCN) system are all mobile radio communications. In Japan, digital cellular and PCS are already in the development stage. Therefore, this book, with its new added material, such as CDMA and microcell technologies can aid in understanding and developing all mobile radio systems including the future PCS.

Besides adding two new chapters, 9 and 10, I have also expanded the discussion in Chapter 2 on the microcell prediction model, in Chapter 5 on spectrum efficiency and cellular systems, in Chapter 6 on basestation design, and in Chapter 7 on field component diversity antennas. To make the book suitable for graduate course work, I have added problems to the end of each chapter.

I have written three books covering the why, what, and how of mobile radio system design. My other two volumes in this series of books deal with the why and what. This volume presents the theoretical framework for radio

PREfACE XV

communications and tells the reader how such present and future systems are designed.

Knowing why, how, and what is critical for developing confidence in any system design. I hope that this book will build your strength and knowledge in designing future mobile radio systems.

Walnut Creek, California William C. Y. Lee

November, 1992

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ACKNOWLEDGMENTS

H.W. Sams & Company is changing its business strategy by moving into computer science and was kind enough to release the copyrights to me for this book. This gave me the opportunity to have John Wiley & Sons, Inc. publish the Second Edition of this book.

I deeply appreciate the inspiration I have received from my professors, C. H. Walter and Leon Peters, who introduced me to the field of commu-nications and wave propagation. I also wish to thank C. C. Cutler and Frank Blecher who gave me valuable advice and encouragement in writing the first edition of this book.

During the revision stage of this second edition, I was encouraged by the engineers who took my courses at George Washington University and by my colleagues at PacTel. I am obliged to Mr. George Telecki and Ms. Cynthia Shaffer of John Wiley & Sons, Inc. who constantly watched my progress during revisions and to Ms. Susan Shaffer who was always patient in typing my untidy manuscript.

Last but certainly not least, I thank my lovely wife, Margaret, and my daughters, Betty and Lily, for their support and patience. I have promised them that I would make up for all the time they have spent alone while I was writing. It has not happened yet, which I have to blame on the rapid advances in mobile communication technologies which keep me on the hook. I hope they will kindly accept my good intentions.

This book is dedicated to the memory of my parents. The many books my father wrote himself were an inspiration to me, and I am proud to carry on in his spirit. Even though he passed away at the beginning of my writing of the first edition of this book, his spirit has been with meand always will be.

William C. Y. Lee

XIX

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Mobile Communications Design Fundamentals

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1

THE MOBILE RADIO ENVIRONMENT

1.1 REPRESENTATION OF A MOBILE RADIO SIGNAL

The mobile radio signals we describe in this book are mainly ground mobile signals. The ground mobile radio medium is unique and complicated. Much research is still being done in this field. But before we can consider the theoretical aspects of a mobile radio signal, we must try to understand the mobile radio environment.

1.1.1 Description of a Mobile Radio Environment

A wave propagation mechanism is closely affected by the wavelengths of the propagation frequencies. In the human-made environment houses and other buildings ranging from 18 to 30 m wide and from 12 to 30 m high (60 to 100 ft and 40 to 100 ft) are found in suburban areas, and there are even larger buildings and skyscrapers in urban areas. Whether suburban or urban, build-ings are natural wave scatterers. The sizes of buildings are equivalent over many wavelengths of a propagation frequency, creating reflected waves at that frequency. For the mobile radio environment treated in this book, we assume that all buildings are scatterers as long as the antenna height of a mobile unit is much lower than the height of an average house. Given these conditions, the propagation frequency has to be above 30 MHz in order to form a multipath propagation medium. The frequency range for a mobile radio multipath environment would be 30 MHz and higher. The base-to-mobile link length is usually less than 24 km (15 miles), so no radio horizon (no radio-path loss attributable to the curvature of the earth) needs to be considered. When the interfering signal comes from more than 24 km (15 miles) away, the radio horizon usually contributes an additional radio-path loss, and the effective interference becomes even weaker. The earth's natural curvature helps to reduce interference and makes it easier for a system design to deal with long-distance interference.

1

2 THE MOBILE RADIO ENVIRONMENT

In designing a mobile radio environment with a large cell radius of 6.5-13 km (or 4-8 miles), we would consider the height of the base-station antenna which is usually 30 to 50 m (100 to 150 ft) in small suburban towns, and over 50-91 m (150 ft) in large cities. The height of a mobile-unit antenna is about 2-3 m (6-10 ft). The base-station antenna is usually clear of its surroundings, whereas the mobile-unit antenna is embedded in them. The terrain config-uration, as well as the human-made environment in which a communication link between a base station and a mobile unit lies, determines the overall propagation path loss.

From this description of the environment, we might imagine that the mobile site will receive many reflected waves and one direct wave. The reflected waves received at the mobile site would come from different angles equally spaced throughout 360, as shown in fig. 1.1. Often a direct wave is presented and relatively strong as comparing with the reflected waves. The described situation is called Rician statistical model. However, a mobile communication system design cannot be based upon this optimistic situation; it is based on the case of weak or nondirect waves which normally occur at the fringe area. All the reflected waves received at the mobile unit combine to produce a multipath fading signal. This described situation is called Rayleigh statistical model. Both Rician and Rayleigh statistics are appeared in Sections 1.5.2 and 1.5.3.

1.1.2 Field-Strength Representation1

The field strength of a signal can be represented as a function of distance in space (the spatial domain) or as a function of time (the time domain). As soon as the height of a base-station transmitting antenna at a site is fixed (Fig. 1.2,4), the field strength* (the envelope r(x) of a received signal s(x) along X-axis in the space) is then defined as illustrated in Fig. 1.2B. The field strength at every point along the x-axis is measured by a mobile receiver whose antenna height is givenapproximately 3 m (10 ft) above the ground. The received field strengths along the x-axis show severe fluctuation when the mobile unit is away from the base station. Field strengths r(x) can be studied either by associating them with geographical locations (areas) or by averaging a length of field strength data to obtain a so-called local mean (see Section 1.3.1) at each corresponding point. The speed of the mobile unit V must remain constant while the data are measured. Since the speed is kept constant, the time axis (i = xlV) can be converted to the spatial axis. The field strengths rx(i) and r2{t), with speeds of 48 and 24 km/h (30 mph and 15 mph), can be seen in Figs. 1.2C and 1.2D, respectively. It is clear that rx(t) in Fig. 1.2C fluctuates much faster than r2{t) in Fig. 1.2>. However, both speeds can be scaled to the same spatial axis, as shown in the two figures. If

*Field strength expressing in a ratio refers to usually one microvolt/meter as .

REPRESENTATION OF A MOBILE RADIO SIGNAL 3

BASE STATION ANTENNA

>}\iwniTW>iii)iw

BASE STATION ANTENNA

PROPAGATION PATH LOSS REGION (UP TO 24 km)

MULTIPATH FADING REGION (100-400 WAVELENGTHS)

TOP VIEW

Figure 1.1. Description of a mobile radio environment.

the mobile unit does not maintain a constant speed while receiving the signal, information of changing speed vs time has to be recorded. The field strength with various speeds is shown in Figure 1.2E. The signal field strength r(t) of Fig. 1.2E has to be converted to Figure 1.25 before processing the data. This process is called the velocity-weighted conversion. The technique is shown in

4 THE MOBILE RADIO ENVIRONMENT

SEA LEVEL

(A) Terrain contour with a base-station antenna site.

rW lo r r ( t ) )

DISTANCE ALONG THE MOBILE PATH x (=Vt)

(B) r(x) along x-axis in the space.

g +5 x 0 5 -5 z 3

- 1 0 S - 1 5 S - 2 0 = -25

r(t) /

r/ 1 \ A/\

3 6 I 1

9 1

30 MPH (850 MHz)

B

12 '15 18 21 1 1 1 1

24 C H -

24 DISTANCE (WAVELENGTH)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 TIME (SEC)

(C) V is large.

+5 z 0 a 5 z J

a-io S - 1 5

ri-20 = -25

^/ \ / \ ; V I

1

1.5 1 1

r(t)

\ f \ s^\.l V \

3 4.5 6 1 1 1

15 MPH (850 MHz)

/-A / \ /^ \ / \ / \ I \ / \ / \ V V 7.5 9 10.5 1 1 1

^

12 Dl r-

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 TIME (SEC)

(D) V is small.

y> no

-3-V h"

vy

TIME t

(E) V varies.

Figure 1.2. The character is t ics of f ield s t rength.

CAUSES OF PROPAGATION PATH LOSS 5

V (VELOCITY)

THE MOBILE UNIT IS STANDING STILL

DISTANCE

(A) Curve of velocity. (B) Converted to spatial domain.

Figure 1.3. Velocity weight conversion.

Figure 1.3. The data are digitized in the time domain with equal intervals. The curve of velocity shown in Fig. .3A is then used to convert all the data points from the time domain to the spatial domain (Fig. 1.3B).

Another method of converting field strengths from time domain to spatial is to synchronize the turning speed of the vehicle wheels with the speed of the field-strength recording device. This method does not need a velocity-weighted conversion process. Both field-strength representations are useful. The representation r(i) in the time domain is used to study the signal-fading phenomenon. The representation r{x) in the spatial domain is used to generate the propagation path loss curves.

1.1.3 Mobile Radio Signal Representation

The mobile radio signal is received while the mobile unit is in motion. In this situation the field strength (also called the fading signal) of a received signal with respect to time /, or space x, is observed, as shown in Fig. 1.2. When the operating frequency becomes higher, the fading signal becomes more severe. The average signal level of the fading signal r(x) or 7(i) decreases as the mobile unit moves away from the base-station transmitter. The average signal level of a fading signal (field strength) will be defined later. This average signal level dropping is called propagation path loss.

1.2 CAUSES OF PROPAGATION PATH LOSS

In free space the causes of propagation path loss are merely frequency / and d, as shown in Eq. (1.2.1):

Por

P, (iTtdf/c)2 [4(/)]2 (1.2.1)

6 THE MOBILE RADIO ENVIRONMENT

where c is the speed of light, is the wavelength, P, is the transmitting power, and Por is the received power in free space.

As seen in Eq. (1.2.1), the difference between two received signal powers in free space, Ap, received from two different distances becomes

Ap = 10 l o g i o ^ ) = 20 l o g i o ( | ) (dB) (1.2.2)

If the distance d2 is twice distance dx, then the difference in the two received powers is

Ap = 20 1og10(0.5) = - 6 dB

Therefore the free-space propagation path loss is 6 dB/oct (octave), or 20 dB/dec (decade). An octave means doubling in distance, and a decade means a period of 10. Twenty dB/dec means a propagation path loss of 20 dB will be observed from a distance of 3 to 30 km (2 to 20 miles).

Example 1.1. What will y dB/oct be when converted to x dB/dec?

y = x log102 (1.2.3)

If y = 6 dB/oct, then x = 20 dB/dec. As described previously, in a mobile radio environment the propagation

path loss not only involves frequency and distance but also the antenna heights at the base station and the mobile unit, the terrain configuration, and the human-made environment. These additional factors make the prediction of propagation path loss of mobile radio signals more difficult. The prediction of propagation loss will be presented in Chapter 2.

1.3 CAUSES OF FADING

The signal strength r(t) or r(x), shown in Fig. .2B, is the actual received signal level in dB. Based on what we know about the cause of signal fading in past studies, the received r{i) can be artificially separated into two parts by cause: long-term fading m(t), and short-term fading r0(t) as

r(t) = /n(i) r0(t) (1.3.1)

or

r(x) = m(x) r0(x) (1.3.2)