Read an Excerpt
The RF in RFID
UHF RFID in Practice
By Daniel M. Dobkin
Newnes
Copyright © 2013 Elsevier Inc.
All right reserved. ISBN: 978-0-12-394830-4
Chapter One
Introduction
1.1 What, When, and Where, Wirelessly
To a quantum mechanic the whole universe is one godawful big interacting wavefunction – but to the rest of us, it's a world full of separate and distinguishable objects that hurt us when we kick them. At a few months of age, human children recognize objects, expect them to be permanent and move continuously, and display surprise when they aren't or don't. We associate visual, tactile, and in some cases audible and olfactory sensations with identifiable physical things. We're hardwired to understand our environment as being composed of separable things with specific properties and locations. We understand the world in terms of what was where when. So one can forgive us for being disappointed that the computers and networks that form so large a part of our lives, and often seem so intelligent in other respects (at least on a good day), are clueless when it comes to perceiving and recognizing all these discrete physical objects we so easily detect and categorize. Why do we have to laboriously inform a computer database, by typing or mousing or tapping a screen, that a perfectly recognizable object has arrived at our doorstep? Why is so much human intervention needed for such a simple task?
It is to correct this deficiency of networked sensibilities that the field of automated identification (auto-ID) has arisen. Auto-ID includes any means of automating the task of identifying a physical object. To date, by far the most common means of doing so is to print a special machine-decipherable bar code on an object, and then image or scan the code using an optical transducer to extract an identifying number. One-dimensional bar codes (so named because information is obtained in traversing the pattern in a single direction, not because such patterns are in fact absent width and height) are easily deciphered and, in the form of the Universal Product Code (UPC) and its more modern descendents, nearly ubiquitous in the commercial world. Two-dimensional bar codes are also available, and pack more information into the same space. Optical character recognition (OCR) can be used to acquire information from conventional human-readable text, at the cost of an increase in computing requirements and decreased reliability. However, all optical methods of identifying an object have some deficiencies. Most fundamentally, the sensing device must be able to see the identifying mark: optical techniques require a clear line of sight. Not only objects, but dirt, paint, ink, and other objectionably opaque but relentlessly commonplace substances can distort or deface bar codes and other optical marks, obscuring the information optical auto-ID techniques require. Mechanical damage to the marks or labels degrades their readability. To store more data requires more space, or the use of finer markings visible from a shorter distance. Finally, data stored in printed marks on a surface is not readily modified or extended, save perhaps by wholesale replacement. While optical techniques for object identification are versatile and inexpensive, it is clear that in many cases another approach may be helpful.
To remedy some of the deficiencies of optical ID, we can turn to an alternative technique, radio-frequency identification (RFID). RFID is the use of radio communications to identify a physical object. RFID is really not one but a suite of identification technologies, because of the differing characteristics of the radio waves of varying frequency employed, and the differing approaches to operating the sensors that serve to identify individual objects. RFID has existed for more than half a century, but its widespread application has had to wait for inexpensive integrated circuits to enable small, low-cost transponders (the parts of the system that get attached to an object to be identified, more commonly known as RFID tags) to be fabricated. Over the last three decades, as the capability of integrated circuits has doubled and the cost per function halved about every two years, religiously attending to Gordon Moore's famous law, new RFID applications have become economically feasible. In particular, since the mid-90's, a great deal of effort has been focused on the application of RFID in the manufacture and distribution of goods: supply chain management, where until recently the bar code reigned supreme. To serve the needs of manufacturing, distribution, and shipment functions, RFID tags must be very inexpensive, compact, mechanically robust, and readable from at least a meter or two away. As we will examine in more detail in chapter 2, this combination of requirements has led to the choice of ultrahigh-frequency (UHF) radio waves and passive RFID tags as the approach of choice for many supply chain applications, and it is UHF RFID technology that is the main topic of this book.
1.2 Why Would You Read This Book?
The purpose of The RF in RFID is to provide users of UHF RFID with an understanding of how identification information gets from a tag to a reader and in some cases back to the tag. We will use that understanding to see how the system of tags, readers, and antennas goes together, and analyze the capabilities and limitations resulting from the choices of tag, reader, antenna, and protocol. This book is for people who want to know why things RFID are the way they are and what (if anything) can be done about it, and perhaps be entertained upon occasion along the way.
As we will see as we proceed, the replacement or supplement of bar codes with RFID tags may give rise to a substantial increase in the amount of information available about objects being made, shipped, or sold, and thus create a need for improved software solutions to enable useful integration of this new knowledge into the existing infrastructure for managing such transactions. The reader must, alas, turn elsewhere for advice and insight on software integration issues: this book is focused on tags, readers, and their interactions. For folks familiar with the OSI reference model for communications systems, The RF in RFID is a book about the physical layer of a UHF RFID system, with some digressions into the data link layer, but no higher.
You don't need a prior acquaintance with radio technology to read the book (although it doesn't hurt), but familiarity with basic electrical engineering concepts of current, voltage, power, frequency, capacitance, and inductance is very helpful. A general familiarity with algebraic manipulation, and the concepts of an integral and derivative, will be needed to follow the derivations of key formulas; a brief review of a few more specialized mathematical tools that are widely used in electrical engineering is provided within the Appendices in the interests of completeness.
1.3 What Comes Next?
The structure of the remainder of this book is depicted in Figure 1.1.
Chapter 2 is a general introduction to RFID, including a bit of history, some terminology, and an examination of the various flavors of RFID and their characteristics and uses. Chapter 3 introduces the reader to the basics of radio technology: transmission, modulation, bandwidth, signal voltage and power. Chapter 4 describes how the specific radios used in UHF RFID readers work. Chapter 5 delves into the operation of passive UHF RFID tags. Chapter 6 examines reader antennas: how they work and how they are characterized and described. Chapter 7 extends this discussion to the peculiar requirements of passive tag antennas. Chapter 8 reviews the tag-reader protocols employed in UHF RFID. Chapter 9 returns to some old applications in the light of issues discussed in the book and introduces current and future applications of RFID. A brief Afterword rounds out the main text of the book. Appendices cover some supplemental information, including the radio regulatory world in which manufacturers and users of RFID systems must operate, and some electrical engineering background useful for those from other fields.
In the interests of clarity, detailed citations are not provided within the text. However, each chapter contains a Further Reading section directing the still-curious reader to additional materials related to the topics covered therein. Each chapter (except this one) also contains exercises to provide an opportunity to exercise concepts introduced in the text; answers may be found at the author's web site, www.enigmatic-consulting.com.
Readers familiar with the first edition of this book will find fewer minor errors, often courtesy of folks like themselves who brought said mistakes to my attention. New materials include a discussion of link budgets and wake-up challenges for active and battery-assisted tags, some recent progress in nonradiating antennas, an improved discussion of the T-match for tag antennas and some examples of near-metal antenna design, and coverage of battery-assisted and sensor provisions of ISO 18000-6, all in addition to the whole chapter on Applications. References to ETSI EN 302 208 have also been updated.
Chapter Two
History and Practice of RFID
2.1 It All Started with IFF
By the 1930's, the primitive biplanes of fabric and wood that had populated the skies above the battlefields of World War I had become all-metal monoplanes capable of carrying thousands of kilograms of explosives and traveling at hundreds of km per hour: by the time observers could visually identify an incoming flight, it was too late to respond. Detection of airplanes beyond visual range was the task of microwave radar, also under rapid development in the 30's, but mere detection of the presence of aircraft begged the key question: whose side were they on? It was exactly this inability to identify aircraft that enabled the mistaken assignment of incoming Japanese aircraft to an unrelated US bomber flight and so ensured surprise at Pearl Harbor in 1941. The problem of identifying as well as detecting potentially hostile aircraft challenged all combatants during World War II.
The Luftwaffe, the German air force, solved this problem initially using an ingeniously simple maneuver. During engagements with German pilots at the beginning of the war, the British noted that squadrons of fighters would suddenly and simultaneously execute a roll for no apparent reason. This curious behavior was eventually correlated with the interception of radio signals from the ground. It became apparent that the Luftwaffe pilots, when they received indication that they were being illuminated by their radar, would roll in order to change the backscattered signal reflected from their airplanes (Figure 2.1). The consequent modulation of the blips on the radar screen allowed the German radar operators to identify these blips as friendly targets. This is the first known example (at least to the author) of the use of a passive backscatter radio link for identification, a major topic of the remainder of this book. Passive refers to the lack of a radio transmitter on the object being identified; the signal used to communicate is a radio signal transmitted by the radar station and scattered back to it by the object to be identified (in this case an airplane).
As a means of separating friend from foe, rolling an airplane was of limited utility: any aircraft can be rolled, and no specific identifying information is provided. That is, the system has problems with security and the size of the ID space (1 bit in this case). More capable means of establishing the identity of radar targets were the subject of active investigation during the 1930's. The United States and Britain tested simple IFF systems using an active beacon on the airplane (the XAE and Mark I, respectively) in 1937/1938. The Mark III system, widely used by Britain, the US, and the Soviet Union during the war, employed a mechanically tunable receiver and transmitter with six possible identifying codes (that is, the ID space had grown to 2.5 bits). By the mid-1950's, the radar transponder still in general use in aviation today had arisen. Modern transponders are interrogated by a pair of pulses at 1030 MHz, in the ultra-high frequency (UHF) band about which we will have a lot more to say shortly. The transponder replies at 1090 MHz with 12 pulses each containing one bit of information, providing an ID space of 4096 possible codes. A mode C transponder is connected to the aircraft altimeter and also returns the current altitude of the aircraft. A mode-S transponder also allows messages to be sent to the transponder and displayed for the pilot. Finally, the typical distance between the aircraft and the radar is on the order of 1 to a few kilometers. Since it takes light about 3 s to travel 1 kilometer, the radar reflection from a target is substantially delayed relative to the transmitted pulse, and that delay can be used to estimate the distance of the object.
An aircraft transponder thus provides a number of functions of considerable relevance to all our discussions in this book:
• identification of an object using a radio signal without visual contact or clear line of sight: radio-frequency identification
(Continues...)
Excerpted from The RF in RFID by Daniel M. Dobkin Copyright © 2013 by Elsevier Inc. . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
