John wiley sons wireless information networks sep 2005 ling

These developments were all part of a major paradigm shift in the world of munication, a shift away from nearly exclusive reliance on wired networks to an era telecom-of “tetherless” com

WIRELESS INFORMATION

NETWORKS

TEAM LinG

WIRELESS INFORMATION NETWORKS

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To those from whom we learned,

to those we taught, and

to those we love

1.1 Introduction, 3

1.2 Network Architecture and Design Issues, 6

1.3 Key Trends in Wireless Networking, 20

1.4 Outline of the Book, 21

Questions, 22

2.1 Introduction, 23

2.2 Three Views of the Wireless Industry, 29

2.3 Three Generations of Cellular Networks, 32

2.4 Trends in Wireless Technologies, 43

Questions, 49

3.1 Introduction, 53

3.2 Multipath Fading and the Distance–Power Relationship, 55

3.3 Local Movements and Doppler Shift, 64

3.4 Multipath for Wideband Signals, 66

3.5 Classical Uncorrelated Scattering Model, 72

3.6 Indoor and Urban Radio Propagation Modeling, 81

4.2 Modeling Path Loss and Slow Shadow Fading, 96

4.3 Doppler Spectrum of Fast Envelope Fading, 110

4.4 Statistical Behavior of Fast Envelope Fading, 122

4.5 Simulation of Fast Envelope Fading, 126

5.2 Time-Domain Measurement Techniques, 151

5.3 Frequency-Domain Measurement Techniques, 171

5.4 Advances in Frequency-Domain Channel Measurement, 180

6.2 Wideband Time-Domain Statistical Modeling, 208

6.3 Wideband Frequency-Domain Channel Modeling, 234

6.4 Comparison Between Statistical Models, 243

6.5 Ray-Tracing Algorithms, 245

6.6 Direct Solution of Radio Propagation Equations, 261

6.7 Comparison of Deterministic and Statistical Modeling, 263

6.8 Site-Specific Statistical Model, 265

Appendix 6A: GSM-Recommended Multipath Propagation

7.1 Introduction, 282

7.2 Basic Modulation Techniques, 284

7.3 Theoretical Limits and Practical Impairments, 307

7.4 Traditional Modems for Wide-Area Wireless Networks, 312

7.5 Other Aspects of Modem Implementation, 328

9.2 Effects of Frequency-Selective Multipath Fading, 380

9.3 Discrete Multipath Fading Channel Model, 384

9.4 Adaptive Discrete Matched Filter, 389

9.5 Adaptive Equalization, 393

9.6 Sectored Antennas, 405

9.7 Multicarrier, OFDM, and Frequency Diversity, 411

9.8 Comparison of Traditional Broadband Modems, 421

9.9 MIMO in Frequency-Selective Fading, 423

Appendix 9A: Analysis of the Equalizers, 425

10.2 Principles of Frequency-Hopping Spread Spectrum, 439

10.3 Principles of Direct-Sequence Spread Spectrum, 444

10.4 Interference in Spread-Spectrum Systems, 464

10.5 Performance of CDMA Systems, 476

Questions, 494

Problems, 495

11.1 Introduction, 501

11.2 Topologies for Local Networks, 503

11.3 Cellular Topology for Wide-Area Networks, 506

11.4 Centrally Controlled Assigned Access Methods, 521

11.5 Distributed Contention-Based Access Control, 537

12.2 UWB Channel Characteristics, 584

12.3 Impulse Radio and Time-Hopping Access, 589

13.3 Modeling The Behavior of RF Sensors, 619

13.4 Wireless Positioning Algorithms, 626

14.4 IR Channel Characterization and Data-Rate Limitations, 644

14.5 Modulation Techniques for Optical Communications, 653

14.6 Multiple Access and Data Rate, 659

The first edition of this book, published in 1995, was the first textbook to provide acomprehensive introduction to the field of wireless information networks That bookpresented wireless networking as the enabling communications technology of the 1990sand beyond Now, only a decade later, mobile and portable telephones and wirelessdata services are a familiar part of our daily lives, as the twenty-first century witnesseswidespread deployment of wireless networks, which has revolutionized the concept

of communication and information processing for business, professional, and privateapplications The field of wireless communications continues to experience unprece-dented market growth, as evidenced by over 1.5 billion cellular telephone subscribersworldwide and the rapid increase in the size of the wireless local area network mar-ket for office, home, and public access applications The initial growth in the marketfor second-generation cellular products and services spurred important new initiativestoward the development and deployment of third-generation cellular networks Morerecently, attention has been focused on location-aware broadband ad hoc wirelessnetworks as the foundation for the next generation of wireless networking technology,which is expected to enable systems of geographically dispersed sensors The emergingwireless sensor and ad hoc networks are expected to interconnect numerous terminalswith a variety of data-rate requirements with traditional multimedia Internet networks

to create a worldwide communication medium among RFID tags, a variety of sensors,home appliances, and small robotic devices

These developments were all part of a major paradigm shift in the world of munication, a shift away from nearly exclusive reliance on wired networks to an era

telecom-of “tetherless” communications based largely on wireless technology, and a shift inthe computer industry toward integration of high-performance distributed computingand portable devices in a pervasive mobile computing environment We adopted the

title Wireless Information Networks in 1995 as an encompassing name intended to

include all applications related to evolving wireless networks in the tion and computer industries, and that book provided comprehensive coverage of thesignal processing and system engineering aspects of this field Given the tremendousgrowth of the industry in both its signal processing and systems engineering aspects,

telecommunica-it becomes increasingly difficult to treat all the important topics in a single volume

With that in mind, in 2002 the lead author published a new book, Principles of Wireless Networks—A Unified Approach, coauthored by Prashant Krishnamurthy, which is more

focused on systems engineering aspects, and began preparing this second edition ofthe original book, with more emphasis on signal processing topics The objective of

xi

Principles of Wireless Networks is to provide a systems engineering treatment that can

be taught to both electrical and computer engineering (ECE) and computer science

students in undergraduate or first-year graduate courses This second edition of less Information Networks places greater emphasis on signal processing and is more

Wire-suitable for ECE graduate students and possibly senior undergraduate students

At the emergence of the wireless industry in late 1980s and early 1990s, all munication enterprises that were involved in traditional wired communications servicesand product development made major investments in wireless technology Computercompanies invested in wireless communications to add mobile computing and ad hocnetworking features to the laptop, handheld, notepad, and other portable computingdevices that are coming into increasingly pervasive use Later, large corporations, asend users, included wireless components in their network infrastructures to extend theaccessibility of their networks to their traveling personnel Military agencies devel-oped location-aware ad hoc sensor networks for use in tactical environments, as well

telecom-as portable devices that place a large amount of computational power in the hands

of the foot soldier operating in urban fighting scenarios Today, almost all nies in engineering disciplines other than telecommunications are entering the wirelesscommunications business, for applications such as in-vehicle networking or home net-working, and are now a part of this wireless revolution All of this means that thereare a great many engineers, computer science specialists, and managers with a variety

compa-of interests who are faced with having to educate themselves in this area This majornew emphasis on wireless communications has also spurred a renewed emphasis onthe teaching of principles of wireless communications in colleges and universities.This second edition is designed to provide students, engineers, and scientists with anintroduction to the major signal processing aspects of wireless networks

The book is written from a systems engineering perspective, by which we mean thatthe various technical topics are presented in the context of ongoing development ofspecific new systems and services, as well as key recent developments in national andinternational spectrum allocations and standards Our method of presentation is to orga-nize the myriad of emerging wireless technologies into logical categories that reflectthe variety of perspectives that users have toward different networks and services.The book addresses the major segments of wireless technology: first-, second-, andthird-generation wide-area cellular networks, wireless local area networks (WLANs),and wireless personal area networks (WPANs), with special attention to the emerginglocation-aware broadband wireless sensor and ad hoc networks Although the bookcovers technology applicable to a wide range of wireless systems, as in the first edi-tion, particular attention is given to indoor wireless communications, an area that isnot treated in great depth in most other books

In writing the book, we have endeavored to bring together treatments of all themajor topics to be considered in the design of wireless information networks, but haveavoided the presentation of detailed mathematical derivations that are available in othertexts In each instance, we have tried to provide the motivation for various wirelesssystem design choices in the context of overall system considerations We believe thatthis is an effective approach to training systems engineers, who should have an overallperspective of an entire system as well as a working knowledge of how to apply theresults of specific research to an engineering problem

The first edition of the book has been used as a graduate-level textbook in versities throughout the world It has also been used as a reference book for indoor

uni-PREFACE xiii

radio communication research programs at DARPA and the National Science dation and for indoor radio channel modeling for WLAN and WPAN standardizationactivities such as IEEE 802.11n and IEEE 802.15.3 Being the first comprehensivetextbook on wireless networks, the first edition has served many of today’s leadingresearchers as a key resource in gaining a comprehensive understanding of the impor-tant issues related to wireless communications As a result, in this second edition wehave tried to maintain the comprehensive treatment and have included major tech-nical developments that have emerged since publication of the first edition, such asultrawideband communications, wireless positioning, space-time coding, multiple-inputmultiple-output antennas, orthogonal frequency-domain multiplexing, interference can-cellation, and multiuser detection Therefore, we have increased the number of chaptersfrom 12 to 15 to include ultrawideband communications and wireless positioning intwo new chapters and to expand the modem design chapters from three to four Somereaders of the first edition indicated that certain problem sets were overly difficult.Thus, in this edition we have added a number of simpler problems and have turnedsome of the more difficult ones into projects with expanded explanations, to make themeasier to understand As in the first edition, the questionnaire format is used to empha-size the importance of having a general understanding of the overall system at hand and

Foun-of the rationale behind key engineering design choices The traditional problem setsare exercises for derivation and understanding of the detailed mathematical analysis

of various concepts Projects provide more detailed exercises, usually involving puter simulations or extensive analysis of data We have directed these problems andprojects toward application-oriented issues This approach provides students with anunderstanding of the issues, motivates them to use the computer as a tool in the learningprocess, and shifts their viewpoint toward real-world engineering problems rather thanmathematical drills We believe that this approach is essential for the proper training

com-of engineers for productive careers in the market-driven telecommunication industry,where simple ideas and added features will often generate greater revenues than willthe latest technical inventions

This edition of the book covers four categories of topics, organized into four parts

In Part I of the book, Chapters 1 and 2, we provide an overview of major categories ofwireless communications and outline the user and market perspectives toward variouswireless systems and services Then we review briefly the current state of development

of wireless and mobile communications systems, including the important issues ofspectrum administration and standards

In Part II, Chapters 3 through 6, we describe the characteristics of radio gation, as well as measurement and simulation methods used in evaluating existingthird-generation cellular, WLAN, and WPAN systems and emerging location-awarebroadband wireless ad hoc networks We provide a detailed description of time- andfrequency-domain statistical channel modeling techniques and their application to pop-ular standards such as GSM, IEEE 802.11, and IEEE 802.15 We also describe theray-tracing algorithm and give a brief overview of direct solution of the radio propa-gation equations

propa-In Part III, Chapters 7 through 10, we discuss wireless modem design gies We begin in Chapter 7 with a description of traditional narrowband modemtechnologies and issues arising in their application to radio channels In Chapter 8

technolo-we address fading, diversity, and coding in relation to the analysis and performanceevaluation of wireless modems In this chapter we also introduce the concepts of

multiple-input multiple-output and space-time coding, which are gaining considerableattention as techniques to be used in modem design Chapter 9 is devoted to broadbandmodem technologies, including equalization techniques, smart antenna techniques, andorthogonal frequency-division multiplexing (OFDM) Chapter 10 is devoted to spread-spectrum techniques and code-division multiple-access techniques for direct-sequenceand frequency-hopping systems.

Part IV, Chapters 11 through 15, is devoted to network access and system aspects.Chapter 11 treats network access methods and provides a comprehensive description

of voice-oriented assigned access and data-oriented random access techniques anddiscusses performance evaluation methods as well Chapter 12 is devoted to ultra-wideband (UWB) communications In this chapter we discuss the detailed behavior

of UWB channels and describe impulse radio, multiband OFDM, and direct-sequenceUWB techniques being considered by IEEE 802.15 for the next generation of WPANapplications Chapter 13 is devoted to RF location-sensing techniques, the foundationfor wireless positioning and indoor geolocation science We discuss RF channel behav-ior in the context of positioning applications and describe the popular received signalstrength systems used in WLAN positioning as well as the time-of-arrival techniquesused for more accurate positioning These chapters provide a comprehensive under-standing of the emerging technologies for implementation of wireless sensor and adhoc networks Chapter 14 is the same as Chapter 10 of the first edition and provides theprinciples of infrared communications Chapter 15, devoted to systems and standards,

is a revision of the concluding chapter of the first edition

The book can be used in its entirety for a first- or second-year graduate course inwireless communications networks in electrical and computer engineering curricula Aspreparation for such a course, students should have an understanding of the elements

of probabilistic signal and system analysis and some background in the principles ofmodulation and coding This material is taught at the Worcester Polytechnic Institute(WPI) and the University of Oulu in Finland as a 14-week course meeting three hoursper week The first two chapters are taught in the first week, Chapters 3 to 6 in the nextfive weeks, Chapters 7 to 10 in four weeks, and Chapters 12 and 13 in two weeks.The remaining two weeks can be spent on other topics selected by the instructor

or on student presentations, followed by course exams Weekly assignments includeanswering selected questions and solving a few selected problems at the end of thechapter In addition, students are asked to do three of the projects throughout the course.The material in some chapters is covered completely, whereas material in other chapters

is covered with more emphasis on the concepts and less emphasis on the details ofmathematical derivations To cover all chapters of the book in full detail, a two-semestercourse format is advisable, although most of the material might be covered in a fast-paced one-semester course with selective omission of the more specialized topics Withappropriate selection of topics, the book can also be used at the undergraduate level

An extensive list of references is included, which will be especially helpful to theindividual reader using the book for self-study or reference purposes

Much of the material in the first edition and part of the material in the secondedition was drawn from the published work of the lead author and his students in theCenter for Wireless Information Network Studies (CWINS) at WPI We are pleased toacknowledge students’ contributions to advancing the understanding of wireless chan-nels and networks In particular, we thank Dr Steven Howard, Dr Rajamani Ganesh,

Dr Ker Zhang, Dr Ganning Yang, Dr Thomas Sexton, Dr Mitch Chase, Timothy

PREFACE xv

Holt, Dr Aram Falsafi, Glen Bronson, Joseph Meditz, Dr Mudhafar Hassan Ali, and

Dr Sheping Li, whose contributions were helpful in the preparation of the first edition

of the book In addition, we would like to express our appreciation to Dr PrashantKrishnamurthy, Dr Xinrong Li, Dr Ali Zahedi, Jeff Feigin, Dr Aram Falsafi, Dr.Robert Tingley, Hamid Hatami, Bardia Alavi, Emad Zand, Muzafer Kannan, NayefAlsindi, Mohammad Heidari, Leon Teruo Metreaud, and other recent affiliates of theCWINS Laboratory, as well as Dr Mika Ylianttila and Juha-Pekka Makela of the Uni-versity of Oulu, who have directly or indirectly helped the lead author to extend hisknowledge in this field and shape his thoughts for preparation of the second edition

of the book We owe special thanks to the National Science Foundation, DARPA, andthe United States Department of Defense, to TEKES, Nokia, Sonera, and the FinnishAir Force in Finland, and to many other companies and research organizations whosesupport of the CWINS program at WPI enables graduate students and the staff ofCWINS to pursue continuing research in this important field A substantial part of thenew material in this second edition has flowed out of these sponsored research efforts.Much of the writing of the lead author in this second edition was accomplished whileteaching and carrying out research at the University of Oulu, Finland, as well as duringhis sabbatical at Olin College of Engineering, Needham, Massachusetts He would like

to express his deep appreciations to the University of Oulu, Olin College, and WorcesterPolytechnic Institute for providing him with these opportunities In particular, he thanksProfessor Pentti Leppanen of the University of Oulu for his continual encouragementand creative administrative support, and Professor Matti-Latva-aho of the University

of Oulu and Dr Sassan Iraji of Nokia Research Center for fruitful discussions oncurrent developments in spread-spectrum and CDMA technologies He also thanksDavid Kerns, provost of Olin College, for providing him the opportunity to spend thefall 2004 semester at Olin Also, he thanks Professor Fred Looft, head of the WPI ECEDepartment, and WPI provost John Carney for their support of a sabbatical leave forwork on this second edition

The lead author would also like to express his deep appreciation to Dr Phillip Bello,Professor John Proakis, and Dr Jerry Holsinger, through whom he has increased thedepth of his understanding of the theory and practice of telecommunications, and toProfessor James Matthews for introducing him to the field of radio communications.His coauthor would like to express appreciation to his many colleagues at GTE(now Verizon) who during his industrial career helped in many ways in his work inmobile and cellular communications He would also like to thank Professors John Orrand Fred Looft and provost John Carney for providing him with the opportunity forgraduate teaching and participation in CWINS research activities at WPI

Most of all, the authors are indebted to their families for their patience and supportthroughout this long and challenging project

K P

A H L

tech-Chapter 1: Overview of Wireless Networks

In this chapter we provide an overview of wireless information networks We describethe basic elements of a wireless network and the key technical issues to be considered

in the design of these networks We also discuss the market sectors that constitute thewireless industry and the trends apparent in voice- and data-oriented networks In thefinal section of the chapter we outline the remaining chapters of the book

Chapter 2: Evolution of the Wireless Industry

In this chapter we consider the evolution of wireless networking technology, which hasbeen built upon developments occurring not only in the telecommunications industrybut also in the computer industry, as communications and computer technologies havedrawn closer together Many observers see the wireless industry as one that has inte-grated radio science with communications and computer technologies Thus, beforedelving into details of radio propagation and signal processing—the primary focus

of this book—it is useful to consider the wireless industry from the separate points of the telecommunications and computer industries, which is the approach wetake in this chapter We also discuss briefly the view of an important user commu-nity, military ground forces, which have had a long history of reliance on wirelesscommunications networks

view-Wireless Information Networks, Second Edition, by Kaveh Pahlavan and Allen H Levesque

Copyright  2005 John Wiley & Sons, Inc.

1

OVERVIEW OF WIRELESS NETWORKS

1.1 Introduction

1.1.1 Elements of a Wireless Network

1.1.2 Key Technical Issues for Wireless Networks

1.1.3 Four Market Sectors

1.2 Network Architecture and Design Issues

1.2.1 Network Architectures

1.2.2 Wireless Versus Wired Networks

1.2.3 Elements of a Wireless Network Architecture

1.2.4 Technical Aspects of a Wireless Infrastructure

1.2.5 Technical Aspects of the Air Interface

1.3 Key Trends in Wireless Networking

1.3.1 Voice-Oriented Networks

1.3.2 Data-Oriented Networks

1.3.3 Where Is the Complexity?

1.4 Outline of the Book

Questions

The second half of the twentieth century witnessed enormous transformations in tronic communications, including the development of data transmission over legacytelephone networks, the introduction of packet-data networks, the development ofhigh-speed local area networks (LANs), and the development of mobile wirelesscommunications networks, most notably cellular networks, paging systems, and evenmobile satellite systems By the start of the current century, cellular and paging serviceshad come into widespread use in support of business communications and personalcommunications as well The early analog cellular networks were rapidly supplanted

elec-by digital networks affording increased traffic capacity and capable of supporting anexpanding menu of data-oriented services In this first decade of the new century, we

Wireless Information Networks, Second Edition, by Kaveh Pahlavan and Allen H Levesque

Copyright  2005 John Wiley & Sons, Inc.

3

are seeing rapidly increasing interest in higher-rate forms of wireless data cation, including multimedia transmission to and from portable phones, and wirelessaccess to the Internet from laptop computers The technology underlying these wirelesscommunications developments is the specific focus of this book.

communi-The worldwide growth of the wireless communications industry has been trulyphenomenal At this writing, there are more than 1 billion cellular telephone usersthroughout the world, and the aggregate annual revenue of the wireless industry exceedsthe revenues of the wired-telephone service industry About 10 years ago, Internetaccess began expanding from the business environment to include the home envi-ronment, and this soon generated annual revenues comparable to those of traditional

telephone service and wireless service Currently, the information exchange industry,

defined to include both wired and wireless phone services as well as Internet access,enjoys annual revenues of several trillion dollars and is by far the largest industry inthe world

Underlying this rapid development of all communications services and networks hasbeen the ongoing evolution of digital technology, particularly large-scale integrationand microprocessor chip technology The digital revolution enabled transformation ofthe core of a traditional telephone network to a digital infrastructure providing greaterreliability, increased capacity, and an ever-widening array of services to customers.About 10 years ago, digital technology began to have an impact on mobile wirelessservices and networks, increasing network capacities and capabilities as well as low-ering the cost and increasing the battery life of mobile devices An interesting andimportant aspect of the burgeoning worldwide wireless communications industry hasbeen the rebirth of wireless LAN (WLAN) technology, driven by the steadily increas-ing popularity of laptop computers, the demand for wireless Internet access, and theexpanding deployment of wireless access points on campuses and, increasingly, inpublic commercial venues

Many of the wireless technology developments of the past decade have focused

on improved physical (PHY) layer and medium access control (MAC) layer designs.The technical core of these protocol layers comprises digital signal processing (DSP)techniques and technology, to which most of this book is directed

In this chapter we provide an overview of wireless information networks Wedescribe the basic elements of a wireless network and the key technical issues to

be considered in the design of these networks We also discuss the market sectorsconstituting the wireless industry and the trends apparent in voice- and data-orientednetworks In the final section of the chapter we outline the remaining chapters ofthe book

1.1.1 Elements of a Wireless Network

An information network is an infrastructure that interconnects telecommunicationdevices to provide them with means for exchanging information Telecommunicationdevices are terminals that allow users to run applications that communicate with otherterminals through the information network infrastructure The basic elements of aninformation network infrastructure are switches or routers that are connected by point-to-point links Switches include fixed- and variable-rate voice-oriented circuit switches,low-speed (X.25) and high-speed (frame relay) data-oriented packet switches (routers),and ATM switches The point-to-point links include a variety of fiber links, coaxialcables, twisted-wire pairs, and wireless connections

INTRODUCTION 5

To support transmission of voice, data, and video, several wired information networkinfrastructures have evolved throughout the past century Wireless networks allow amobile telecommunication terminal to access these wired information network infras-tructures At first glance it may appear that a wireless network is only an antenna siteconnected to one of the switches in the wired infrastructure, enabling a mobile terminal

to be connected to the backbone network In reality, in addition to the antenna site,

a wireless network may also need its own mobility-aware switches and base stationcontrol devices in order to support mobility, that is, enabling a mobile terminal tochange its point of connection to the network Thus, a wireless network has a fixedinfrastructure with mobility-aware switches and point-to-point connections, similar toother wired infrastructures, as well as antenna sites and mobile terminals

Important examples of wireless networks are cellular telephone networks and less Internet access networks, which we discuss in greater detail in Section 1.2 There,

wire-we show how these networks extend the structure and services of existing wired works to support either voice- or data-oriented wireless services

net-1.1.2 Key Technical Issues for Wireless Networks

As we can see in the two examples mentioned above, a wireless network includes notonly the wireless terminals and radio-frequency (RF) links to fixed antennas, but alsonetwork elements and functions needed to support both interoperation with the existingfixed-wired networks and mobility for the wireless user The set of characteristics of thewireless connection between the mobile terminal and a base station, including all thePHY- and MAC-layer details of access method, modulation, coding, and transmission

formats, is commonly referred to as the air interface Thus, we can say that the key

technical issues for wireless networks are networking issues and air-interface designissues Although these two sets of issues are not totally independent of each other, theyare largely independent and can be treated separately As we shall see in subsequentsections of the book, the networking issues relate primarily to interoperability betweenthe wireless and wired infrastructures and to support of user mobility On the otherhand, air-interface issues relate primarily to the quality of service provided to wirelessusers and to efficiency in the use of available RF bandwidth

1.1.3 Four Market Sectors

The market for wireless networks has evolved within four different segments that can

be divided logically into two classes: the voice-oriented market and the data-oriented

market The voice-oriented market has evolved around wireless connection to the public

switched telephone network These services evolved further into local and wide areamarkets The local voice-oriented market is based on low-power, low-mobility deviceswith a higher quality of voice, including cordless telephone, personal communicationservices (PCSs), wireless private branch exchanges, and wireless Telepoint The voice-oriented wide-area market evolved around cellular mobile telephone services usingterminals with higher power consumption, comprehensive coverage, and lower quality

of voice Figure 1.1a compares several features of these two sectors of the

voice-oriented market

The wireless data-oriented market evolved around the Internet and

computer-communication network infrastructure Data-oriented services can be divided into local

Tariff Intelligent network

Service quality Power consumption Coverage

Cellular Phone Cordless Phone and PCS

Mobile Data WLAN/WPAN

Users per network

Compatibility with LANs

Data rate Size/power

consumption

Mobility

Coverage Mobility

FIGURE 1.1 Wireless market sector comparisons: (a) voice-oriented networks; (b)

data-oriented networks.

broadband and ad hoc markets on the one hand, and wide-area mobile data markets onthe other The wide-area wireless data market provides Internet access for mobile users.Local broadband and ad hoc networks include wireless LANs and wireless personalarea networks (WPANs) that provide high-speed Internet access The local and adhoc networks will also support evolving ad hoc wireless consumer product markets

Figure 1.1b illustrates several differences among the local- and wide-area wireless

data networks

Next we describe the principal system architectures for wireless networks and outlinethe key design issues that must be addressed in the design of these networks Thesearchitectures and design issues are dealt with in detail in the remainder of the book

1.2.1 Network Architectures

Here we consider two prominent examples of wireless networks: cellular telephoneand wireless Internet

Cellular Telephone Figure 1.2 depicts wireless telephone service as an extension of

the familiar public-switched telephone network (PSTN) The PSTN, designed to provide

wired telephone services, is augmented with a wireless fixed infrastructure to supportcommunication with mobile terminals The mobile terminals communicate with thewireless fixed infrastructure via RF links to fixed antennas, each antenna connected to

or integral with a base station The PSTN infrastructure comprises switches,

point-to-point connections, and computers used for operation and maintenance of the network.The fixed infrastructure of the cellular telephone service has its own mobility-awareswitches, point-to-point connections, and other hardware and software elements that

NETWORK ARCHITECTURE AND DESIGN ISSUES 7

PSTN Infrastructure

Cellular Telephone Infrastructure

Switches with mobility support

FIGURE 1.2 Cellular telephone infrastructure as an extension of the PSTN.

are needed for operation and maintenance of the mobile network A wireless munication device (e.g., a cordless telephone) can connect to the PSTN infrastructure

telecom-by replacing the wire attachment with radio transceivers However, for the wirelessdevice to change its point of connection, switches in the PSTN must be able to supportmobility Switches in the PSTN infrastructure were not originally designed to supportmobility To solve this problem, a cellular telephone service provider adds its ownfixed infrastructure with mobility-aware switches The fixed infrastructure of the cellu-lar telephone service provider is an interface between the base stations and the PSTNinfrastructure that implements the functionality to support mobility Just as a wiredtelephone service network needs added infrastructure to allow a mobile telephone to

connect to the PSTN, a wireless data network needs its own added infrastructure to

support wireless Internet access Consider the next example

Wireless Internet Figure 1.3 shows the traditional wired data infrastructure together

with an additional wireless data infrastructure that allows wireless connection to theInternet The traditional data network consists of routers, point-to-point connections,and computers for operation and maintenance The elements of a wireless networkinclude mobile terminals, access points, mobility-aware routers, and point-to-pointconnections This new infrastructure has to implement all the functionalities needed tosupport mobility

The difference between the cellular telephone and wireless Internet examples isthat the wireless network in Fig 1.2 is a connection-based voice-oriented network,whereas the wireless network in Fig 1.3 is a connectionless data-oriented network A

connection-oriented operation needs a setup procedure to connect the communicating

terminals, and after the connection is established, a certain quality of service (QoS) is

guaranteed to the user throughout the communication session In connectionless tion there is no setup procedure and terminals are always connected to the network, in

opera-the sense that opera-the communication session remains intact, but opera-the QoS is not guaranteed.Instead, each protocol data unit (e.g., datagram or packet) is communicated betweennetwork access points on a best-effort basis Common examples of connectionless pro-

tocols are the Internet Protocol (IP) and the User Datagram Protocol (UDP), both of

Fixed components for a wireless data infrastructure Internet Infrastructure

FIGURE 1.3 Wireless connectivity to the Internet.

which are used to support the Transmission Control Protocol (TCP), a transport-layer

connection-oriented protocol, which in turn supports higher-layer protocols such as

Hypertext Transfer Protocol (HTTP) and Simple Mail Transfer Protocol (SMTP) TCP

is connection-oriented because a TCP session is set up and maintained for the fullduration of a higher-layer session such as a Web-access session

1.2.2 Wireless Versus Wired Networks

There are a number of fundamental differences between wired and wireless networks,essentially all stemming from the inherent characteristic of wireless communications(i.e., the replacement of fixed subscriber equipment connections by radio links) Thisfreedom from the wired “tether” provides enormous advantages for customers ofcommunications services but also introduces some new problems not encountered intraditional wired networks

Perhaps the most important characteristic of wireless networking is that a radiolink connecting the user’s device to a wired network infrastructure is inherently lessreliable than a fixed wired connection This characteristic should be obvious and will befamiliar to users of cellular phones who have experienced signal breakup and droppedconnections on cellular phone calls This inherent relative unreliability of radio linksleads to a need for considerably more complexity in the physical-layer design than is

required in traditional wired networks Also, there is a need for connection management

techniques as part of the solution to the radio-link reliability problem

Another important characteristic of wireless communications is the fundamental itation on the availability of frequency spectrum For systems that operate in licensedfrequency bands (cellular telephone service is the primary example), each serviceprovider operates its network within a fixed band of frequencies, and means must

lim-be provided to manage the sharing of allocated bandwidth among a large numlim-ber ofusers Furthermore, as the service provider’s subscriber volume grows, there must be

NETWORK ARCHITECTURE AND DESIGN ISSUES 9

means for expanding the overall capacity of the service network in an efficient manner

to accommodate the growth in demand for service

The bandwidth limitation problem also gives rise to the need for complexity in thedesign of source coding techniques (speech coding in the case of voice service, andother compression techniques in the case of multimedia transmission) so as to reducethe amount of bandwidth needed for each user channel signal while maintaining aprescribed level of signal quality as perceived by the user

A very practical issue for users of mobile wireless devices is the necessary reliance

on batteries, with the need for periodic recharging This issue has led to the clever

application of power management techniques in the design of mobile devices, so as to

extend talk time and recharging cycles

The inherent advantage of wireless networking—mobility for the user—adds plexity to the network design to manage changing the connection point to the fixednetwork infrastructure, including changes over both small and large geographical areas

com-This calls for greater complexity in registration and call routing techniques than are

needed in wired networks, and a need for the use of both permanent and temporaryaddressing to support mobility

Finally, the use of wireless transmission creates a vulnerability of the user’s munications to eavesdropping and fraudulent intrusion into the network Because of

com-these problems, considerable attention has been given to providing security and privacy

for wireless communications networks Security provisions include such techniques as

authentication to prevent unauthorized access to networks Privacy provisions include the encryption of transmitted digital streams to prevent eavesdropping.

1.2.3 Elements of a Wireless Network Architecture

It is useful to consider the elements of a wireless network in four categories: services,infrastructure, protocols, and network engineering

Services From the perspective of the user of the network, the principal aspect of

the network is the service or set of services the network is designed to support Infact, the various industry efforts that lead to interoperability standards invariably beginwith agreements among participants as to the array of services to be provided by theintended network standard, and considerable attention is given to the detailed features

of those services and the specific ways in which the user’s equipment will interact withthe network in the operation of the service Of course, the basic types of services arevoice and data services

In some networks, the voice services might comprise a menu of selectable digitaldata rates, the higher data rates providing a higher received voice quality at the cost

of higher bandwidth requirement, accompanied by an appropriate tariff differential

Data services may be provided in various forms, the simplest, called data-bearer service, being simple transport of data with minimal specification of data format at the

mobile data port Data service offerings might include a choice of transparent (T) ornontransparent (NT) data in either synchronous (clock-driven) or nonsynchronous(start/stop character-driven) formats Transparent data service will employ forward-error correction coding at a fixed transmission rate in the channel Nontransparentservice will employ error-detection coding and retransmission of faulty data blocks

so as to ensure greater accuracy in the delivered user data Other options might

include circuit-switched (connection-oriented) data versus packet (connectionless) vice Other, application-specific data services, such as Group-3 Facsimile service, willtypically be offered with a set of optional data rates.

ser-Short messaging service (SMS) is available in many cellular networks for mission and reception of short text messages displayed on a small screen The SMSmessages are embedded in the control channels of the cellular network, which enables

trans-rapid delivery A service that is growing trans-rapidly, called text messaging, is built on

SMS As the demand for wireless multimedia service grows, data services are beingprovided at increasingly high data rates

System Infrastructure Provisioning of the various services in a wireless network

in turn places requirements on hardware and software that must be included in theelements connecting the wireless service customer with the fixed networks We need

to consider two categories of system elements: the mobile terminal and the fixedwireless infrastructure that makes connection with the fixed network

The mobile terminal is the user’s device for sending and receiving signals over awireless link For a user requiring only basic voice service, the mobile terminal is thefamiliar cellular phone, nowadays probably a CDMA digital phone in the United States

or a GSM phone in Europe and many other regions of the world Many cellular phonesare designed with a standardized data port for connecting to a portable computer orother data terminal In supporting data connectivity, the cellular phone is function-ing as a wireless modem interfacing baseband data (e.g., ASCII-formatted) with thewireless network Currently, this wireless modem function is typically implemented

in a circuit card to be plugged into a socket on a laptop computer, or even a cardalready mounted inside the laptop As wireless networks evolve to support increasingcapability for multimedia transmission, a variety of new mobile devices are appearing

in the marketplace to support sending and receiving multimedia images

Signals from the mobile user terminal arrive at an antenna that provides an RFinterface to the fixed wireless infrastructure, and that infrastructure in turn provides

an interface to the fixed wired infrastructure In the case of cellular systems, the fixedwired infrastructure will typically be a public-switched telephone network (PSTN) or

a public-switched data network (PSDN) In the case of WLAN systems, the fixednetwork will typically be a wired Ethernet LAN in an office building, office complex,

or university campus

In the case of cellular systems, the fixed wireless infrastructure includes antennas,radio base stations (BSs), mobile switching centers (MSCs), and terrestrial lines (typi-cally, coaxial cable or optical fiber) to make connections among BSs and MSCs as well

as between MSCs and the PSTN The fixed wireless infrastructure will also includecomputers and a variety of instrumentation needed for operation and maintenance ofthe cellular network All of the equipment and software in place, from the antennas tothe PSTN connections, will be owned and operated by the cellular service provider.Currently, a cellular service company might have to deploy 50 to 100 BSs to providesatisfactory signal coverage over a major metropolitan area

Functional partitioning between network equipment elements may vary from onemanufacturer’s equipment to another’s, but in current cellular networks, the BS willtypically include not only RF transmission and reception equipment but also speech

coder/decoders (codecs) In such a configuration, all transmissions between the BS

and the PSTN are in digital form In such a configuration, the BS will also typically

NETWORK ARCHITECTURE AND DESIGN ISSUES 11

include interworking functions (IWFs), also called modem emulators, to modulate and

demodulate the data streams in support of wireless data services The MSCs includemobile-aware switches that provide for the setup and routing of call connections toand from mobile terminals and also handle the hand-off of call connections from one

BS to another as mobile users move about the cellular service area The MSCs alsoinclude the other hardware and software elements that are needed for mobile networkoperation, maintenance, and troubleshooting

Wired Backbones for Wireless Networks Since wireless networks depend heavily

on the wired infrastructures to which they connect, in this section we provide a briefoverview of the important wired infrastructures The most commonly used wired infras-tructures for wireless networks are PSTN, Internet, and hybrid fiber coax (HFC),originally designed for voice, data, and cable TV distributions applications, respec-tively Figure 1.4 provides an overall picture of these three networks and how theyrelate to other wired and wireless networks (A more detailed discussion of this topiccan be found in [Pah02a].)

The main sources of information transmitted through telecommunication devicesare voice, data, and video Voice and video are analog in nature, whereas data traffic

is digital The dominant voice application is telephony, that is, a bidirectional metric real-time conversation To support telephony, telephone service providers havedeveloped a network infrastructure that establishes a connection for a telephone call

Cellular Infrastructure PBX

FIGURE 1.4 Interconnection of PSTN, Internet, and HFC.

during the dialing process and disconnects it after completion of the conversation Thisnetwork is referred to as the public switched telephone network (PSTN) As shown atthe top of Fig 1.4, the cellular telephone infrastructure provides wireless access to the

PSTN Another network attached to the PSTN is the private branch exchange (PBX),

a local telephone switch owned privately by a business enterprise This private switchallows privacy and flexibility in implementing additional services in an office environ-ment The PSTN physical connection to homes is twisted-pair wiring that is also usedfor broadband xDSL services The core of the PSTN is a huge digital transmissionsystem that allocates a 64-kb/s channel for each direction of a telephone conversa-tion Other network providers often lease the PSTN transmission facilities needed tointerconnect their nodes

The infrastructure developed for video applications is cable television, shown in thelower part of Fig 1.4 This network broadcasts wideband video signals to residentialpremises A cable goes from an end office to a residential neighborhood, and allcustomers are fed from the same cable The set-top boxes leased by cable companiesprovide selectivity of channels, depending on the customer’s service subscription Theend offices, where groups of distribution cables arrive, are connected to one another

with fiber lines For this reason, the cable TV network is also called hybrid fiber coax

(HFC) Nowadays, cable distribution is also used for broadband residential access

to Internet

The data network infrastructure was developed for bursty data applications andevolved into the Internet, which supports Web access, e-mail, FTP, and Telnet appli-cations as well as multimedia (voice, video, and data) sessions with a wide variety ofsession characteristics The middle part of Fig 1.4 shows the Internet and its relation

to other data networks From a user point of view, data-oriented networks are alwaysconnected, but they use the transmission resources only when a burst of information

is to be transferred Sessions of popular data communications applications such asWeb browsing or FTP are often asymmetric, and a short upstream request burst results

in downstream transmission of a large amount of data Symmetric sessions such as

IP telephony over data networks (termed voice over IP, or VoIP) are also becoming

popular, providing an alternative to traditional telephony Residential Internet access

is a logical access that is physically implemented on other media, such as cable TVwiring or copper telephone lines Distribution of the Internet in office areas is usuallythrough Ethernet local area networks (LANs) Wireless LANs in offices are usuallyconnected to the Internet through the wired LANs Nowadays all other private datanetworks (PDNs), such as those used by banks or airline reservation agencies, arealso connected to the Internet The Internet also serves as the backbone for wirelessdata services

Protocol Layering Wireless communications networks, and cellular networks in

par-ticular, must accomplish many complex functions in order to establish call connections

to and from mobile users, to implement the services to which each user has subscribed,

to manage user authentication, and to provide mobility for wireless user terminals As

we have noted above, these tasks are performed in a number of mobile and fixed ments At the same time, these networks must provide smooth interoperability amonghardware and software elements supplied by a variety of manufacturers In complexsystems such as these, designers have found it beneficial to organize systems designs

ele-according to the concepts of protocol layering Perhaps the best known model for

NETWORK ARCHITECTURE AND DESIGN ISSUES 13

protocol layering is the Open System Interconnect (OSI) seven-layer reference model,adopted as an international standard in 1978 In the OSI model, the lowest layer,layer 1, the physical layer, provides a physical medium for the flow of informationacross a link The highest layer in the model, layer 7, the application layer, providesservices to users of the network In the intermediate five layers, the services providedmove progressively away from the physical medium toward network- and application-related functions

The basic concept of protocol layering is to manage the complexity of a network

design by segmenting the system functions into a set of layers, each layer built on the ones below it Each protocol layer can be described as performing specific services

for the higher layers while isolating the higher layers from the details of how theservices are actually implemented The set of rules by which information is processed

and formatted in any give layer constitutes a protocol for that layer This assures, for

example, that two pieces of equipment performing functions in the same layer caninteroperate properly at that layer A set of layers and their protocols is commonly

referred to as a network architecture A list of protocols used in a chosen system, one protocol per layer or sublayer, is referred to as a protocol stack [Gar00].

In all of the wireless networks we consider in this book, the system functions areorganized according to some version of protocol layering From one network standard

to another, the functional segmentation into layers may be somewhat different ever, the functional segmentation will be the same for all hardware and softwareelements manufactured to each particular standard For example, the GSM networkarchitecture consists of five layers: transmission, radio resource management, mobilitymanagement, communication management, and operation, administration, and main-tenance [Mou92, Meh97, Hei99] As a second example, the IEEE 802.11 family ofstandards encompasses two layers: MAC and PHY [O’Ha99]

How-Traffic Engineering and Deployment The cost of equipping and deploying a wireless

communications network can vary widely depending on the type of network and theapplication for which it is intended A WLAN might be installed in a business office

or in a university campus building for a few thousand dollars On the other hand, acellular telephone network built to serve a metropolitan area might incur costs of tens

of millions of dollars However, regardless of the wireless technology employed orthe intended application, principles of sound network engineering apply: The networkshould be designed to provide good signal coverage to wireless terminals over theintended floor, campus, or geographic service area with a reasonable expenditure ofcapital for equipment and installation

In the case of a WLAN installation, access points (sometimes called base stations)

will typically be installed on ceilings or high on walls in locations chosen to provideunobstructed signal coverage for some set of wireless terminals, such as desktop orlaptop computers Multiple access points will be installed to cover the total popula-tion of wireless terminals, typically with overlapping coverage areas to avoid gaps

in coverage

In the deployment of a cellular telephone network, the general principle of goodnetwork engineering is the same as for a WLAN deployment—install a number of cellsites in such a way as to provide unbroken signal coverage for mobile users over thegeographic area in which the cellular company offers service The cost of equippingand installing a single cell site might well be on the order of $1 or 2 million, including

acquisition of real estate, and thus it is important that the cell site layout be designed

to make optimum use of capital investment

A key element in the planning of a wireless network is a specification of the trafficthe network will be designed to handle In the case of a WLAN design, we wouldwant to know the number of wireless terminals and some statistics for the amountand type of traffic to be generated by the terminals We would want to know theprofile of short-message traffic, long file transfers, and so on, and the frequency of

these transmissions In other words, we would like to have a traffic model as a starting

point for planning the network With a traffic model in hand, and a specification of thetraffic capacity of an access point, we can determine the number of access points to

be provided Then specifying the distribution of wireless user terminals will allow us

to position the access points appropriately

In the case of a cellular network deployment, the considerations are much the same,but with the important difference that users are highly mobile and that communicationtraffic patterns can change significantly from day to day and even from hour to hour.Commuters caught in a traffic jam or in a severe rainstorm will generate an unusuallyhigh volume of calls as they try to contact their co-workers or family members to revisetheir schedules Fans at a Sunday afternoon football game will generate high volumes

of communication traffic in the vicinity of the stadium Another traffic characteristicspecific to the cellular case is the relatively high frequency of call handoffs as usersmove about the service area This contrasts with the typical relatively less frequenthandoffs experienced in the WLAN environment Thus, the efficient engineering of acellular network must take account not only of average statistics of generated trafficbut also of the potentially high variability of the traffic Once again, a traffic model

is needed, and the traffic model in the cellular case is likely to be considerably morecomplex than in the WLAN case We shall have more to say about wireless networkdeployment in subsequent sections

1.2.4 Technical Aspects of a Wireless Infrastructure

Next, we consider important issues that must be addressed in the design and operation

of the wireless network infrastructure These issues are often considered under the

heading network engineering, as they are issues concerning the design and operation

of the network as a whole

Network Deployment Planning In the preceding section we briefly discussed traffic

engineering as a key element in network planning With a traffic model, both temporaland geographic, as a starting point, the network engineer can begin to plan the layout

of the access points or cell sites that will carry the wireless traffic to and from thefixed wired backbone network This aspect of network planning is typically performed

with the aid of signal coverage prediction models.

Signal coverage prediction models, usually based on a combination of radio-wavepropagation theory and experimental measurements, provide the designer with a means

of estimating the optimum placement of access points or cell sites for covering theintended area of user terminals with acceptable signal quality A tutorial description

of signal coverage in a wireless network with multiple access points or cell sites willtypically illustrate signal coverage with a diagram of abutting hexagons or perhapscircles with some overlapping coverage areas Those are both highly idealized descrip-tions that do not accurately represent the real world of wireless signal propagation and

NETWORK ARCHITECTURE AND DESIGN ISSUES 15

coverage Even in a relatively benign office layout, planning for a WLAN tion must take account of the types and locations of office furniture and equipment,office partitions, walls, doorways, and so on, all of which can affect signal coverage

installa-In a factory setting, even more complex situations might be encountered, with variousmetal surfaces, manufacturing machinery, and so on, all affecting signal propagationthroughout the building In the case of cellular telephone networks covering largeservice areas, signal coverage prediction models must take account of a wide range

of factors, including terrain type (flat, hilly, mountainous), land use (rural, suburbandevelopment, city high-rise, urban canyon), and special situations such as roadways onhigh bridges and over-water propagation paths

Some of the more sophisticated cellular network planning tools are elaborate ware packages that incorporate time-varying traffic models, population distributions,cellular antenna types, optional propagation models, and call-handoff models in order

soft-to make accurate estimates of received signal quality for cussoft-tomers situated in varioussectors of a service region Even very sophisticated network planning tools can pro-vide only an estimate of network performance, and a network engineer may well also

conduct drive tests in selected regions of the service area in order to verify or refine

computer-generated performance estimates

Mobility and Location Management An important requirement that users will place

on wireless networks is mobility, freedom for the wireless user to maintain a reliablewireless connection while moving about an area that is relevant to the application

In the case of a WLAN system, users may want the capability to move their wirelessterminals to different locations in an office building, factory, or campus without having

to reregister with the network Here, users are not likely to move about rapidly, andthe problem is a relatively simple one However, in the case of WANs such as cellular

telephone networks, mobility is the raison d’etre of the technology and is the principal

differentiator between traditional wired telephone networks and wireless networks.Users expect to be able to move about freely on foot, by automobile, or even traveling

on trains, while enjoying seamless connectivity for their wireless communication Theyalso expect to be able to migrate from one cellular company’s coverage region toanother’s, placing and receiving calls reliably in any region

In traditional wired telephone networks, the subscriber’s telephone is always wired

to the same central office (CO) switch, and the network directs every incoming call

to the subscriber’s line using his or her telephone number Outgoing calls are alwaysmade through the same local CO to which the subscriber is permanently connected.However, in a cellular network, the cell site to which the user connects when receiving

a call depends on the user’s physical location at that moment In order for a subscriber

to receive a call, the network must determine the cell in which the user is currently

located This is the essence of the location management problem, and this problem

has been solved in cellular networks by designing location awareness into both thewireless and wired portions of a wireless network infrastructure

An important facet of location management is call handoff, the process in which a

user’s call connection is transferred seamlessly from one serving cell to another as theuser moves about the service area This comes under the heading of what is known

as mobility management in cellular systems This is accomplished by a combination

of signal strength measurements made in the releasing and the receiving cells, andcoordination of frequency channels in the two cells, typically done under the control

of a mobile switching center (MSC) Once again, this calls for a considerable amount

of complexity in the design of the wired and wireless segments of the wireless networkinfrastructure

Related to call handoff is the process of roaming, by which a user who has subscribed

to particular services in his or her home area can travel to another service provider’sregion and use the same services This feature greatly enhances the value of a wirelessservice to a subscriber by lessening geographic restrictions on his or her access toservices Roaming capabilities in cellular networks have been achieved by cooperationamong service providers and among manufacturers, largely in the venue of standardsbodies Roaming requires the adoption of a standardized air interface, standardization ofphone-type identification, and cooperation among the operators for transfer of locationdata between home and visited networks Cooperation is also required in administrativeareas such as transfer of calling charges and subscription information

Radio Resource and Power Management A characteristic of any wireless network

is that it must operate within a strictly defined spectrum allocation Radio spectrum

is a limited resource, and regulatory agencies set specific spectrum allocations fordifferent services For example, a cellular network operator has a license for 25 MHz

of bandwidth, 12.5 MHz for each direction of full-duplex communication With atypical cellular reuse factor of 7, about 3.6 MHz of bandwidth is available for two-way traffic in each cell, and this bandwidth must be shared among active users in thecell The bandwidth available is far less than what would be required if all subscribers

in the network were to demand call connections simultaneously This is in markedcontrast to a wired telephone network, in which we may always add new subscribers

to the network by installing additional local loops (To be sure, there is an issue inequipping a wired telephone network in sizing the central office switches and the long-haul switches to ensure enough connections through the switches to meet expected calldemand.) Thus, to ensure efficient sharing of the allocated spectrum, RF channels must

be assigned and released dynamically, on a per-call basis

Furthermore, directing a call from the wired network to a specific mobile scriber is not a trivial procedure Cellular networks reserve a portion of the allocated

sub-bandwidth for control channels, which are utilized in establishing and managing call

connections Paging messages are transmitted from cell sites, and a paging/responseprotocol is used to let the network determine which cell is currently the best one bywhich to reach the called subscriber Only when this location determination has beenmade is an RF channel assigned for the call All of these functions of assigning andmanaging the limited number of available RF channels come under the heading of

radio resource management.

Another important element of radio resource management is power management A

cellular network is designed to operate under interference-limited conditions That is,the dominant source of signal degradation in the network is interference from otheractive users of the network With frequency reuse, the signal power radiated from agiven cell is held to a sufficiently low level that the same subset of frequencies can beused simultaneously in another cell a reuse-separation distance away In some cellularnetworks, power control is performed in both the base stations and the mobile phones.Power control at both ends of the wireless link helps to hold radiated power to alevel sufficient to maintain good-quality communication without unduly increasing theoverall interference level in the network

NETWORK ARCHITECTURE AND DESIGN ISSUES 17

Security Although the use of wireless communications relieves the user of the wired

tether to the public telephone network, with the enormous advantage of freedom ofmovement, the wireless medium also makes the user’s communications vulnerable toeavesdropping and even fraudulent intrusion In fact, when standards were being devel-oped for digital wireless networks, a major benefit recognized for digital transmissionwas the facility it provided for the implementation of authentication and encryptiontechniques All of the digital wireless interoperability standards have included pro-cedures for authentication of users entering the network With respect to the privacyproblem, WLANs utilize spread-spectrum transmission, which has an inherent resis-tance to casual eavesdropping Cellular networks based on CDMA are also using spreadspectrum, providing inherently private transmission In other cellular networks, such asGSM, encryption is provided in some operators’ networks as a selectable feature Forcommunications that are particularly sensitive, some users may employ application-layer end-to-end encryption and use a wireless data service to carry encrypted trafficacross the network In this case, the user does not rely on the wireless network toprovide security or privacy

1.2.5 Technical Aspects of the Air Interface

If we examine the evolution of successive generations of wireless networks of any giventype, say WLAN or WAN, the principal differences appearing from one generation toanother lie in the details of the air-interface design, encompassing physical (PHY)- andmedium access control (MAC)–layer designs There is good reason for this: Improve-ments and new developments in PHY- and MAC-layer designs have yielded the mostsignificant enhancements in communication quality and spectrum utilization in thesenetworks To assess the technical issues underlying the various alternatives for PHY-and MAC-layer designs and why certain choices have been made in the evolution ofair-interface standards, it is necessary to begin with a good understanding of the RFmedium for each type of wireless system The fundamental characteristics of an RFtransmission medium are summarized next

Path Loss As an RF signal radiates from a transmitting antenna to a receiving antenna,

there is propagation path loss that attenuates the signal strength, the strength generallydecreasing with distance along the path In an idealized configuration, the loss could

be calculated simply as free-space loss, which increases with the square of distance.However, in practical applications, the propagation path is rarely describable by thefree-space model, and many other factors must be taken into account Different factorswill be important in different types of systems For WLAN applications, where thespatial scale is on the order of room and building dimensions, one must account forstructural elements and their properties as reflectors and absorbers of signal energy.For WANs such as cellular telephone networks, an even wider array of factors must beconsidered, and path loss is often difficult to compute theoretically The path loss willtypically be influenced by terrain type, land use, and sometimes by unique topological

or architectural elements situated on or near a particular path In all wireless networks,path loss also depends on the types of antennas used at both the mobile and fixed ends

of the wireless link

Multipath For wideband signals as used in WLANs and some cellular networks, time

dispersion, called multipath, must also be accounted for in modeling the propagation

medium As with path loss, multipath characteristics can vary widely from one type ofnetwork to another Specific characteristics will vary with operating frequency, appli-cation setting (indoor versus outdoor), antenna coverage patterns, and structural ortopological details of the coverage area Furthermore, the ways in which multipathcharacteristics are described and measured will vary depending on the bandwidths ofsignals employed in each type of network Given the wide variance among multipathcharacterizations for different types of networks, designers have tended to developmultipath modeling methods and databases that are specific to the individual types ofnetworks under consideration In some instances, standards-setting bodies have formal-ized sets of multipath models that manufacturers and service providers have agreed

to use in development, testing, and evaluation of new wireless products The chiefexample here is the Joint Technical Committee (JTC) of GSM, which has evolved

standardized sets of multipath models, commonly termed JTC models, applicable to a

variety of cellular propagation environments

Fading The third fundamental characteristic of an RF transmission medium is the

presence of time variations in the strength of received signals, an effect usually referred

to as fading Signal fading can arise from a wide variety of causes, all of them related

to dynamics of the propagation medium Movements of WLAN terminals, or evenmovements of people, chairs, or doors, in the vicinity of WLAN signal paths, cancause variations in received signal strength In a busy application environment such as afactory floor, there may be almost-constant movement of workers, vehicles, equipment,and materials, and all of these movements are potential sources of signal fading onWLAN paths

In a cellular network, an obvious source of signal fading is the mobility of wirelessusers When a cell phone is being operated in a moving automobile, or even by apedestrian walking on a city street, the characteristics of the propagation path to theserving base station are changing constantly Even if the wireless user is temporarilymotionless, the movement of nearly vehicular traffic can result in signal fading.Signal fading effects are closely related to multipath, discussed above, and in fact,most fading effects are due to the time-varying interaction of multipath signal compo-nents That is, the time-dispersed signal components are typically affected individually

by amplitude and phase fluctuations, and when these components are received together,

there is an observed variation in the composite signal, commonly termed multipath ing The multipath-fading phenomenon had been well understood for decades prior to

fad-the design of modern wireless networks, since it is a fundamental characteristic oflong-distance radio propagation in HF, VHF, and UHF frequency bands (3 MHz to

3 GHz) The long-standing interest in utilizing digital transmission techniques in thosehigh-frequency bands led, beginning in the 1950s, to development and refinement

of signal-processing techniques such as diversity combining, spread-spectrum RAKE receivers, and adaptive equalization as means of ameliorating the effects of multipath

fading in digital communication [Pri58] Thus, those earlier developments were brought

to bear in the PHY-layer designs of modern wireless networks, and there continues to

be active research on new and better techniques for dealing with the multipath fadingcharacteristics of wireless channels Current examples of the fruits of this research

include orthogonal frequency-division multiplexing (OFDM), turbo equalization, and space-time coding [Han02].

NETWORK ARCHITECTURE AND DESIGN ISSUES 19

PHY- and MAC-Layer Alternatives Most of the research in wireless networking

tech-nology in the past two decades has been directed toward advancements in PHY- andMAC-layer designs Research on these aspects of wireless air interfaces has yieldedsteady improvements in performance quality and reliability as well as in the efficiency

of utilization of spectrum High quality of service builds customer satisfaction andhealthy growth of the industry Improved spectrum utilization translates directly intoincreased traffic capacity in the network, enabling greater per-user cost efficiency in theinstallation and operation of the network The progression of successive generations

of WLAN and WAN standards and products has in fact been characterized largely byadvancements in PHY- and MAC-layer techniques

A key element of PHY-layer design is the modulation technique, and the spectralefficiency afforded by each technique is critical Thus, we have seen a steady pro-gression of increasingly spectrum-efficient modulations in wireless networks Access

methods used in WANs have evolved from simple frequency-division multiplexing (FDM), through embedded digital control channels, to code-division multiple access (CDMA) The legacy analog cellular systems had utilized simple frequency modu- lation (FM) with frequency-division multiplexing (FDM) of user channels, a design much the same as had been used for decades in land-mobile radio (LMR) systems

for taxicab fleets and public safety departments The earliest digital cellular designs(second-generation cellular) saw the introduction of digital modulation techniques such

as GMSK and π/4-QPSK together with time-division multiplexing of multiple

dig-ital traffic channels into TDMA frames, with various forms of control informationembedded on a per-frame basis TDMA transmissions were frequency-multiplexedinto a FDM-TDMA signal design The development of IS-95 for U.S CDMA intro-duced spread-spectrum PSK modulation with code division as the access method, thecombined techniques providing significant capacity enhancements relative to TDMA.Third-generation WAN standards will continue to build on CDMA designs, with higher-bandwidth signals and higher-data-rate services

Since the 1997 adoption of IEEE 802.11 as the first international standard forWLANs, this standard, together with its subsequent modifications, had dominated theWLAN products industry The initial 802.11 standard specifies a MAC layer and threePHY-layer options: (1) direct-sequence spread spectrum (DSSS) with both differen-tial binary PSK (DBPSK) and differential quaternary PSK supported, (2) frequency-hopping spread spectrum (FHSS) with Gaussian FSK (GFSK) in either two- orfour-level formats, and (3) infrared transmission using pulse position modulation(PPM), with two data rates supported The 802.11–1997 MAC-layer protocol com-prises two sublayers, the lower sublayer providing three access options The basic

access mechanism here is carrier-sense multiple access with collision avoidance

simultane-The IEEE 802.11b enhancement extends the 802.11 PHY layer using high-rateDSSS (HRDSSS) The 802.11b extension provides a modulation mode at 11 Mb/s

using complementary code keying (CCK) The IEEE 802.11g extension operates at

radio frequencies between 2.400 and 2.4835 GHz, the same band as that used by802.11b However, the 802.11g specification employs OFDM, the modulation schemeused in 802.11a to obtain higher data speed Computers or terminals set up for 802.11gcan fall back to speeds of 11 Mb/s

The early mobile data networks ARDIS, Mobitex, and CDPD used PHY-layer ulation techniques that were much like those used in the digital cellular standardsthen under development ARDIS used 4-ary FSK, whereas Mobitex and CDPD usedGMSK Since a high demand for those mobile data services never developed, therewas little motivation to develop new PHY-layer designs for increased capacity TheMAC-layer protocols used in those original mobile data networks were relatively sim-ple contention-based protocols, including data-sense multiple access (DSMA), slottedALOHA, and digital-sense multiple access, which is closely related to CSMA withcollision detection (CSMA/CD) In time, these mobile data services were subsumedunder the cellular data services offered by the major cellular operators

Now that we have outlined the key characteristics of wireless information networks, itwill be useful to summarize the trends to be observed in the continuing evolution ofwireless technology Let us look separately at voice- and data-oriented networks

1.3.1 Voice-Oriented Networks

From the first-generation starting point of legacy analog cellular networks, generation voice-oriented networks introduced a digital air interface, in part to facilitatethe introduction of digital data services and other new features and services, but also

second-to expand network capacity over that provided by the legacy analog networks Theintroduction of CDMA and third-generation wideband CDMA then provided improvedvoice quality and system capacity and also met the growing demand for higher datarates As fourth-generation designs develop and evolve, we will probably see introduc-tion of the use of space-time diversity techniques and multiple-input multiple-output(MIMO) antennas in the air interface, paving the way for further quality and capacityimprovements beyond the third-generation systems

1.3.2 Data-Oriented Networks

The 1997 ratification of the IEEE 802.11 standard was a major milestone in the WLANindustry The 10-year standardization effort not only produced an international standardassuring product compatibility among WLAN manufacturers, but the project also pro-vided good solutions to some difficult problems that had to be faced in creating wirelessextensions to the then-ubiquitous wired LANs The 802.11 standard dealt with mobil-ity, link reliability management, power management, interference minimization, andsecurity While the initial standard did not provide data rates as high as then-standard10-Mb/s Ethernet over wired LANs, the 1- to 2-Mb/s wireless rates met the needs of

OUTLINE OF THE BOOK 21

many users, who welcomed the mobility afforded by WLAN technology despite thedata-rate limitations The demand for higher data rates was inevitable, of course, andsubsequently, the IEEE was able to issue the 802.11a (rates up to 54 Mb/s), 802.11b(rates up to 11 Mb/s), and 802.11g (rates up to 54 Mb/s) enhancements, based onthe solid foundation of 802.11, the higher data rates achieved through use of the

OFDM and CCK modulation schemes Now appearing on the horizon is band (UWB) pulse transmission, which offers promise of further increases in data rates

ultrawide-and coexistence of even larger numbers of simultaneous users

1.3.3 Where Is the Complexity?

As we indicated earlier, the complexity of radio propagation and its variability from onedeployment situation to another pose major challenges in the design of efficient wirelessinformation networks The fundamental design objective will be to provide good signalcoverage and reliable, high-quality service on each link Thus, the designer must focus

on air-interface performance, which in turn places emphasis on PHY- and MAC-layerdesigns Because of this, research must continue to address a variety of complex signal-processing techniques, including new time, space and frequency diversity techniques,with the objective of achieving steadily higher data rates, better service quality, andincreased spectrum utilization

Related technologies are being pursued as well, including wireless positioning andoptical communications Wireless positioning is a particular example of new technologythat is affected critically by the complexity of the radio propagation environment, and

we can expect that increasingly sophisticated propagation modeling techniques will berequired as this area of application becomes more important

The book is organized into four parts, each focused on a different aspect of wirelessinformation networks

Part I, Chapters 1 and 2, provides an introduction to wireless information networks

In Chapter 1 we provide an overview of wireless networks, principal design issues,and current trends in the evolution of these networks, and in Chapter 2 we outline theevolution of the wireless industry

Part II, Chapters 3 to 6, deals with characterization of radio propagation, channelmeasurement and modeling for narrowband signaling, measurement of wideband chan-nel characteristics, and computer simulation of radio channels Both indoor and outdoorwireless channels are considered

Part III, Chapters 7 to 10, deals with modem design, addressing many details ofthe underlying signal-processing functions employed in wireless networks Chapter 7deals with narrowband modem technology, Chapter 8 deals with diversity and codingtechniques used to improve communication reliability in wireless channels, Chapter 9deals with broadband modem technologies, and Chapter 10 deals with spread-spectrumand UWB systems

Part IV, Chapters 11 to 15, deals with MAC-layer design, ultrawideband cations, geolocation technology, and optical wireless networks, and provides a currentsummary of important standards and systems

(a) Consider a digital cellular network giving customers wireless access to the PSTN

using their cell phones What essential functions must be performed by the cellularequipment in carrying a voice signal arriving from the PSTN to a customer’scell phone?

(b) Why do all digital cellular systems employ methods of speech compression? (c) Describe the phenomenon of multipath fading, and describe a few scenarios in

which this effect can be expected in a wireless call connection in a wide-areacellular network Next, describe a few scenarios applicable to indoor communica-tion over a wireless LAN

(d) How would you distinguish the traffic characteristics observed in a voice-oriented

wireless network from those of a data-oriented network?

(e) Why is accurate modeling of radio-wave propagation important in the planning of

a cellular service network?

(f) What technical and economic factors are most likely to cause a shift in dominance

among competing digital cellular standards?

(g) Digital cellular standards include specifications of interfaces between major

elements of each system Why are these interface specifications important

to (1) cellular network operators, (2) cellular equipment manufacturers, and(3) cellular service customers?

(h) What advantage do you see in the fact that all versions of the IEEE 802.11 standards

for WLANs share the same MAC-layer protocol?

(i) What in your view is the major factor accounting for the resurgence in popularity

of WLAN networking?

(j) Discuss and compare the issues involved in expanding capacity in the PSTN versus

expanding capacity of a cellular network

EVOLUTION OF THE WIRELESS

INDUSTRY

2.1 Introduction

2.1.1 Evolution of Voice-Oriented Networks

2.1.2 Evolution of Data-Oriented Networks

2.2 Three Views of the Wireless Industry

2.2.1 Telecommunications Industry View

2.2.2 Computer Industry View

2.2.3 Military Sector View

2.3 Three Generations of Cellular Networks

2.3.1 1G Systems and Networks

2.3.2 2G Systems and Networks

2.3.3 3G: W-CDMA for IMT-2000

2.3.4 Beyond 3G and Toward 4G Networks

2.4 Trends in Wireless Technologies

2.4.1 Reemergence of the WLAN Industry

2.4.2 Wireless Home Networking

2.4.3 Home Access Networks

2.4.4 WPANs and Ad Hoc Networking

2.4.5 IEEE 802.15 Working Group on WPAN

military use of walkie-talkies in World War II, and AT&T’s early mobile telephone

Wireless Information Networks, Second Edition, by Kaveh Pahlavan and Allen H Levesque

Copyright  2005 John Wiley & Sons, Inc.

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