RF Engineering for Wireless Networks

Hardware, Antennas, and Propagation

Daniel M. Dobkin

• Éditeur: Newnes
• Année: 2011
• ISBN:
  9780080470474
• Sujet: Technik & Maschinenbau
• Sous-sujet: Mobile & drahtlose Kommunikation

Finally, here is a single volume containing all of the engineering information needed to successfully design and implement any type of wireless network! Author Dan Dobkin covers every aspect of RF engineering necessary for wireless networks. He begins with a review of essential math and electromagnetic theory followed by thorough discussions of multiplexing, modulation types, bandwidth, link budgets, network concepts, radio system architectures, RF amplifiers, mixers and frequency conversion, filters, single-chip radio systems, antenna theory and designs, signal propagation, as well as planning and implementing wireless networks for both indoor and outdoor environments.

The appendices contain such vital data as U.S., European, and Japanese technical and regulatory standards for wireless networks, measurements in wireless networks, reflection and matching of transmission lines, determining power density, and much more. No matter what type of wireless network you design—Bluetooth, UWB, or even metropolitan area network (MAN)—this book is the one reference you can't do without!

• The A-to-Z guide to wireless network engineering—covers everything from basic electromagnetic theory to modulation techniques to network planning and implementation!
• Engineering and design principles covered are applicable to any type of wireless network, including 802.11, 802.16, 802.20, and Bluetooth.
• Discusses state-of-the-art modulation techniques such as ultra wideband (UWB) and orthogonal frequency-division multiplexing (OFDM).

Chapter 1 Introduction

1. The Beauty of Wires, the Inevitability of Wireless

The first gem of wisdom I ever acquired about consulting, obtained many years ago from a former schoolmate, was to ensure that everything is plugged in: no continuity, no data. Wires carry voltages and currents from one place to another. Their behavior is reasonably simple and predictable—at least for sufficiently low data rates and short lengths—and they can be seen, grabbed, traced, and tugged.

Wires are also an irritating and sometimes intolerable umbilical cord to the mother network. Look behind a typical personal computer or engineering workstation: you’ll be hard put to find which plug goes with which cable. One person in a café with a laptop plugged into an Ethernet port on the wall is a curiosity; five, sitting at different tables with colorful category 5 cables snaking randomly along the floor between chairs and people, are an eyesore and a safety hazard. It’s great to return your rental car and receive an instant printed receipt; the folks who provide the service would have a tough time dragging wires behind them down the long lines of cars, to say nothing of the angry businessmen and -women falling flat on the asphalt as they hurried to their flights. Sometimes wires won’t do.

Accept that we have to cut the cords and break the old rules: no continuity, but lots of data. How is it done? What new complexities are introduced when the old metal conduit for current is removed? How are the resulting difficulties surmounted? What limits remain? It turns out that the way signals are modulated, transmitted, propagated, and received in a wireless link all change drastically from their wired counterparts. However, these changes can mostly be concealed from the data network: if things are done right, the network can’t tell that the wires have been left behind.

This book is about the measures that must be taken, the obstacles that are encountered, and the limitations that result when data are to be moved wirelessly from place to place. The book is focused on local and personal area networks—LANs and PANs—although we will show how technologies developed for local communications can in some cases be deployed over long distances. We will concentrate on Institute of Electrical and Electronic Engineers (IEEE) 802.11–based wireless LANs—Wi-Fi networks—though we will discuss related technologies, and many of the lessons we will learn are broadly applicable.

Though we will touch on recent standards developments and provide examples of commercial practice, this book is not an attempt to provide an up-to-date snapshot or exhaustive survey of the state of the art; even in an age of rapid publication, any such summary will rapidly be rendered obsolete by the continual advancement of technology and industry. Rather, the reader should be armed upon completion of this book to understand why things work the way they do and thus distinguish between pretense and progress.

Most importantly, this book is about signals, not bits. We will touch upon the digital side of the problem of wireless networking, but we are primarily concerned with what happens between the point where bits are converted to voltages here and where the reverse operation occurs at a distant there. Some readers may be familiar with the Open Systems Interconnect (OSI) reference model for digital communications, a standard for imposing some hierarchical order on the various tasks required to communicate between two end-users within a network (Figure 1-1). In OSI terms, this book is about the physical layer of a wireless data link, with a few digressions into the medium-access control sublayer, but rarely higher. The reader whose interest extends also up the stack and sideways into network integration is referred to the many excellent texts that already touch upon the digital side of wireless LANs; a few examples can be found in the suggested reading at the end of this chapter.

2. What You Need to Proceed

The book is intended to be nearly self-contained: no previous acquaintance with radio technology is necessary. The reader ought to have some background in the physical sciences and an acquaintance with the basic electrical engineering concepts of voltage, current, resistance, capacitance, and inductance, along with their conventional schematic representations. Familiarity with the importance of harmonic (sinusoidal) excitations is also assumed, though we provide a brief review of the use of complex exponentials to represent such signals. Though every effort is made to emphasize physical intuition before mathematical methodology, the reader ought to be comfortable with algebra and recognize the meaning of derivatives and integrals even if the mechanics of performing them is a bit rusty. Vectors and gradients will inevitably appear in the discussion of antennas and propagation. Certain aspects of Fourier transform theory, which play a vital role in wireless communications, will be briefly reviewed, though again a prior encounter with the notions of frequency and time representations of a signal is very helpful.

Figure 1-1: Location Relative to the OSI Reference Model

The more sophisticated reader may perhaps encounter a surprising dearth of certain expected vectorial complexities: I nowhere resort to Maxwell’s equations, and nary a cross product is to be found. I do sink to taking a divergence of a vector field here and there, but the reader can bypass those scattered indulgences with little cost. Furthermore, though diffraction is examined in some detail, the estimable Mr. Huyghens and his principle are not in evidence, and though near-field and far-field regions are defined, the distinction is found to be mathematical rather than physical. In fact, the treatment of electromagnetism presented herein is exclusively in terms of the vector and scalar potentials, A and φ, acting on and created by actual currents and charges. In this attempt I have followed in part the prescriptions of Carver Mead in his book Collective Electrodynamics; I hope that the resulting presentation will provide a simpler and more appealing method of approaching a complex topic, though the results are equivalent to those obtained by conventional means, and readers with the requisite expertise can always fall back on more traditional approaches.

3. An Overview of What Is to Come

Figure 1-2 gives an overview of the remainder of the text.

We first cover some of the basics of wireless communications. We begin with the idea of multiplexing and then examine what happens when a high-frequency carrier is modulated to convey information, and why a trade-off between bandwidth, data rate, and noise tolerance must inevitably arise. We survey the typical modulation schemes used in digital data transmission, including a pair of somewhat exotic beasts—orthogonal frequency-division multiplexing and pulsed ultrawideband—that play an increasing role in the modern wireless world. We introduce the idea of a wireless link and the specialized terminology used in the radio world to describe voltage, power, and noise.

In the unavoidable exception to prove the rule, we digress into the digital domain to cover some basics of wireless local area networks, emphasizing the IEEE 802.11 committee’s alphabet soup but not forgetting the unexpected radio legacy of King Harald “Bluetooth” Blätand of ancient Denmark, or some of the interesting activities of other task groups within the IEEE. While emphasizing the radio-related aspects of the standards, we also touch gingerly upon the domains of coding and encryption, by which the digital designers seek to compensate for the inadequacies of the uncertain wireless link.

Figure 1-2: Overview of the Remainder of the Text

We then plunge into the workings of digital radios: amplification, noise, and distortion; frequency conversion and spurious signals; frequency synthesis and phase noise; filters and filtering. We summarize by trying to design a radio chain to achieve a desired performance and examining how the requirements translate into the cost and complexity of the resulting chipset. The chapter ends with some examples of radio chips for wireless local area network and wireless personal area network applications and a quick look at the other pieces of the puzzle: the bit of PC board and external components that together with the integrated chips make a low-cost digital radio.

Having examined the creation of a signal, we examine its transmission and reception by means of antennas, exposing an antenna as nothing but a means to avoid cancellation of the effects of its currents. We show how antenna directivity allows us to estimate the amount of the transmitted power that arrives at the receiver given the antenna characteristics, at least for the ideal environment we know we will never encounter in this life. We briefly survey the operating principles behind a number of different types of commonly encountered antennas, with some emphasis on the tools for understanding how arrays of antennas can be used to produce interesting results not achievable from a single radiator.

We next abandon the pretense of ideality and treat propagation in some small fraction of the unmanageable complexity it deserves. Our poor transmitted wave finds itself reflected, absorbed, diffracted, and refracted by all manner of obstacles, on scales from the microscopic to the global, leading to the bugaboos of the radio world: attenuation, fading, and multipath. We examine how fading is surmounted with the aid of diversity antennas, appropriate modulations, and more sophisticated adaptive antenna arrays.

Our theoretical apparatus as complete as it is going to be, we plunge into the empirical realities of real indoor and outdoor networks. We review construction practices in sufficient detail to provide insight into existing buildings but not to obtain a general contractor’s license and combine them with published information on microwave absorption and reflection to obtain some idea of where waves ought to go and where they ought not. We provide a survey of software tools to automate the tasks of getting the network to work, and some samples of their output. Our roving eye then moves into the wild outdoors, where we examine how buildings confine signals through reflection and diffraction, and how trees play a key role in limiting outdoor microwave links. We examine area coverage networks, point-to-multipoint networks for providing Internet access, and point-to-point links between fixed locations. The curvature of the earth, both real and effective, reminds us that we’ve gone as far as short-range radios, and this book, are going to take us. A brief review of safety precautions for outdoor work brings us to the end of the journey, save for the unusually determined reader, for whom appendices on regulatory and measurement considerations, as well as a few derivations mercifully avoided in the text, are provided.

4. Acknowledgments

The book you’re about to read has benefited from the time, thought, and experience of many helpful people. Special thanks go to Jay Kruse of Tropos Networks, who reviewed many incarnations of the materials in detail and provided helpful insight and suggestions. Jay’s colleagues Cyrus Behroozi, Tom Blais, and Malik Audeh also provided support and allowed me to tag along on surveying expeditions. The folks at WJ Communications, including Ron Buswell, Don McLean, Mike O’Neal, Rich Woodburn, Mark Bringuel, John Tobias, Kevin Li, Ray Allan, and Steve Weigand, gave me access to test equipment and facilities for many of the measurements reported here as well as encouragement and microwave wisdom. Jim Mravca and Nathan Iyer were particularly helpful in providing insight on system and integrated circuit design. Bill McFarland and David Su of Atheros and Kevin Wang at Silicon Wave helped correct the descriptions of RF chipsets. Greg des Brisay and Bob Arasmith shared their practical knowledge of outdoor network installations; Pat McCaffrey of Hidden Villa kindly provided access to the network site described in Chapter 7. Rajeev Krishnamoorthy shared his experiences in the early development of wireless LAN technology. Miki Genossar provided guidance on the ultrawideband work of the IEEE standards bodies. Markus Moisio and Jussi Kiviniemi of Ekahau and Baris Dandar of ABP Systems provided demonstration software and guidance in surveying. Skip Crilly clarified the operation of the Vivato access point. Thanks also to Mark Andrews, Martin Chaplin, Rob Martin, Ana Bakas, William Stone, Franz Chen, W.R. Vincent, Richard Adler, David Freer, Simon Perras, Luc Bouchard, and Vinko Erceg. Harry Helms, my editor at Elsevier, was always entertaining as well as encouraging. Last but not by any means least, my long-suffering spouse, Nina, not only put up with innumerable weekend absences but also provided expert linguistic support on short notice to navigate the Chinese Ministry of Information Industries website.

Further Reading

General Introductions to Networking

Understanding Data Communications (6^th Edition), Gilbert Held, New Riders, 1999: Simple and accessible, though some coverage areas are already rather dated.

Communication Networks, Alberto Leon-Garcia and Indra Widjaja, McGraw-Hill, 2003: An academic text with modern emphasis on Internet Protocol.

Introductions to Wireless LANs

802.11 Networks: The Definitive Guide, Matthew Gast, O’Reilly, 2002: It is, at least on the digital side. A thorough review of standards and tools.

Jeff Duntemann’s Drive-By Wi-Fi Guide, Paraglyph Press, 2003: Accessible introduction and hands-on guide to setting up common equipment.

Wireless LANs, Jim Geier, Macmillan, 1999: The technical information here is already somewhat old, but this book has a nice discussion of how to plan and manage network installation projects.

Chapter 2 Basics of Wireless Communications

1. Harmonic Signals and Expon...