-
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
dcd-wgC1.jpg
-
This Page Intentionally Left Blank
-
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.
This text is printed on acid-free paper.
Copyright © 1993 by John Wiley & Sons, Inc.
All rights reserved. Published simultaneously in Canada.
No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means,
electronic, mechanical, photocopying, recording, scanning or
otherwise, except as permitted under Sections 107 or 108 of the
1976 United States Copyright Act, without either the prior written
permission of the Publisher, or authorization through payment of
the appropriate pcr-copy fee to the Copyright Clearance Center, 222
Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978)
750-4470. Requests to the Publisher for permission should be
addressed to the Permissions Department, John Wiley & Sons,
Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax
(201) 748-6008, E-Mail: [email protected].
To order books or for customer service please, call
1(800)-CALL-WILEY (225-5945).
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 systems—Design. I. Title. II. Series.
TK6570.M6L36 1993 621.3845'6—dc20 92-21130
-
To My Parents
-
This Page Intentionally Left Blank
-
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 40Λ 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-
ditions—Shadow 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 ψΓ(ή 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 άΒμ 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 Hopping—Spread
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 XVÜ
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
-
This Page Intentionally Left Blank
-
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 me—and always will be.
William C. Y. Lee
XIX
-
This Page Intentionally Left Blank
-
Mobile Communications Design Fundamentals
-
This Page Intentionally Left Blank
-
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 given—approximately 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 αΒμΥ.
-
REPRESENTATION OF A MOBILE RADIO SIGNAL 3
BASE STATION ANTENNA
>}\iw»ni»TW>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)
Λ /-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. Í.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 [4π(ά/λ)]2 (1.2.1)
-
6 THE MOBILE RADIO ENVIRONMENT
where c is the speed of light, Λ 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. Í.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)