AIDC TECHNOLOGIES
Automatic identification and data capture refers to the methods of automatically identifying objects, collecting data about them through analysis of images, sounds or radio waves and entering that data directly into computer systems without human involvement (a transducer is employed to convert scanned data into a digital form).
Technologies typically considered as part of AIDC include barcodes, RFID, biometrics (acquisition and identification of characteristics such as finger image, palm image, facial image, iris print or voice print), magnetic stripes, OCR, smart cards and voice recognition.
In the next paragraphs the two main AIDC technologies will be discussed: barcode and RFID.
2.1 Barcode
A barcode is a machine-readable representation of information in a visual format on a surface: it is printed using dark ink on a white substrate to create high and low reflectance which is converted to 1s and 0s by an optical scanner called barcode reader.
Originally barcodes stored data in the widths and spacings of printed parallel lines, but today they also come in patterns of dots, concentric circles and text codes hidden within images.
FIGURE 2.1 – An example of barcode label
2.1.1 History
In 1932 a business student named Wallace Flint wrote a master’s thesis in which he envisioned a supermarket where customers would pierce cards to mark their selections: at the checkout counter they would insert them into a reader, which would activate machinery to bring the purchases to them on conveyor belts; store management would also have a record of what was being bought.
The problem was that the card reading equipment of the day was utterly unwieldy and hopelessly expensive.
Even if the country had not been in the middle of the Great Depression, Flint’s scheme would have been unrealistic for all but the most distant future.
The first step toward today’s bar codes came in 1948, when Bernard Silver, a graduate student, overheard a conversation in the halls of Philadelphia’s Drexel Institute of Technology.
The president of a food chain was pleading with one of the deans to undertake research on capturing product information automatically at checkout; the dean turned down the request, but Bernard Silver mentioned the conversation to his friend Norman Joseph Woodland, a twenty seven year old graduate student and teacher at Drexel.
His first idea was to use patterns of ink that would glow under ultraviolet light and the two men built a device to test the concept; it worked, but they encountered problems ranging from ink instability to printing costs.
Woodland was convinced he had a workable idea, so he quit Drexel and moved to his grandfather’s Florida apartment to seek solutions; after several months of work he came up with the linear bar code, using elements from two established technologies, movie soundtracks and Morse code.
After starting with Morse code, he just extended the dots and dashes downwards and made narrow lines and wide lines out of them.
To read the data, he made use of Lee De Forest’s movie sound system from the 1920’s: De Forest had printed a pattern with varying degrees of transparency on the edge of the film, then shined a light through it as the picture ran; a sensitive tube on the other side translated the shifts in brightness into electric waveforms, which were in turn converted to sound by loudspeakers.
Woodland planned to adapt this system by reflecting light off his wide and narrow lines and using a similar tube to interpret the results.
The symbology was made up of a pattern of four white lines on a dark background:
the first line was a datum line and the positions of the remaining three lines were fixed with respect to the first line; the information was coded by the presence or absence of one or more of the lines.
This allowed seven different classifications of articles, but the inventors noted that if more lines were added, more classifications could be coded: with 10 lines 1023 classifications could be coded.
Woodland decided to replace his wide and narrow vertical lines with concentric circles, so that they could be scanned from any direction: this became known as the
“bull’s eye code”.
The two filed a patent application on October 20, 1949; in the following years they set out to build the first actual bar code reader.
The device was the size of a desk and had to be wrapped in black oilcloth to keep out ambient light; it relied on two key elements: a five hundred watt incandescent bulb as the light source and an RCA 935 photo-multiplier tube, designed for movie sound systems, as the reader.
Woodland hooked the RCA 935 tube up to an oscilloscope, then he moved a piece of paper marked with lines across a thin beam emanating from the light source so that the reflected beam was aimed at the tube.
Woodland got what he wanted since as the paper moved, the signal on the oscilloscope jumped: he and Silver had created a device that could electronically read printed material.
Woodland’s and Silver’s idea was not immediately usable:
• primitive computers of the day had poor calculation power and were the size of a typical frozen food section
• hex codes were clearly a technology whose time had nowhere near come
• the five hundred watt bulb could cause eye damage and created an enormous amount of light, only a tiny fraction of which was read by the RCA 935 tube (the rest was released as expensive, uncomfortable waste heat)
• in 1952 lasers did not exist
So without a cheap and safe way to read and record data from barcodes, their idea would be no more than a curiosity, but Woodland and Silver, sensing the potential pressed on and in October 1952 their patent was granted; in 1962 they sold the patent to Philco that in turn sold it to RCA.
In the same years David J. Collins was working for Sylvania Corporation which was trying to find military applications for a computer it had built and he knew that the railroads needed a way to identify cars automatically and then to handle the information gathered.
Collins thought to utilize some sort of coded label as the easiest and cheapest approach to retrieve the former and Sylvania’s computer to do the latter.
Strictly speaking, the labels Collins came up with were not bar codes: instead of relying on black bars or rings they used groups of orange and blue stripes made of a reflective material which could be arranged to represent the digits 0 through 9.
Each car was given a four-digit number to identify the railroad that owned it and a six-digit number to identify the car itself; when cars went into a yard, readers would flash a beam of colored light onto the codes and interpret the reflections.
Collins foresaw applications for automatic coding far beyond the railroads and in 1967 he pitched the idea to his bosses at Sylvania: in a classic case of corporate short- sightedness, the company refused to fund him, so Collins quit and co-founded Computer Identics Corporation.
Sylvania never saw profits from serving the railroad industry since the system worked as expected, but it was simply too expensive: although computers had been getting a lot smaller, faster and cheaper they still cost too much to be economical in the quantities required.
Meanwhile Computer Identics prospered; its system used lasers, which in the late 1960s were just becoming affordable: a milliwatt helium-neon laser beam could easily match the job done by Woodland’s unwieldy five hundred watt bulb.
A thin light moving over a bar code would be absorbed by the black stripes and reflected by the white ones, giving scanner sensors a clear on/off signal; moreover lasers could read bar codes anywhere from three inches to several feet away and they could sweep back and forth like a searchlight hundreds of times a second, giving the reader many looks at a single code from many different angles (that would prove to be a great help in deciphering scratched or torn labels).
It was the grocery industry that would provide the impetus to push the technology forward: in the early 1970s the industry set out to propel to full commercial maturity the technology that Woodland and Silver had dreamed up and Computer Identics had proved feasible.
In mid-1970, an industry consortium established an ad hoc committee to look into bar codes; the committee set guidelines for barcode development and created a symbol selection subcommittee to help standardize the approach.
At the heart of the guidelines were a few basic principles: to make life easier for the cashier barcodes would have to be readable from almost any angle and at a wide range of distances and because they would be reproduced by the millions, the labels would have to be cheap and easy to print.
There were massive savings in labour and other areas and gigantic savings available in the use of the information and the ability to deal with it more easily than before.
In the spring of 1971 RCA demonstrated a bull’s eye bar code system at a grocery industry meeting; IBM executives at that meeting noticed the crowds RCA was drawing and worried that they were losing out on a huge potential market.
Soon Woodland, whose patent had expired in 1969, was transferred to IBM’s facilities in North Carolina, where he played a prominent role in developing the most popular and important version of the technology: the Universal Product Code (UPC).
Printing presses sometimes smear ink in the direction the paper is running; when this happened to bull’s-eye symbols, they did not scan properly, while with the UPC any extra ink simply flows out the top or bottom and no information is lost: after a while the technically elegant UPC won the battle to be chosen by the industry.
The adoption of the Universal Product Code, on April 3, 1973, transformed bar codes from a technological curiosity into a business juggernaut: no event in the history of modern logistics was more important.
Standardization made it worth the expense for manufacturers to put the symbol on their packages and for printers to develop the new types of ink, plates and other technology to reproduce the code with the exact tolerances it requires.
When the UPC took over, companies had to give up their individual methods and register with a new Uniform Code Council (UCC).
Cheap lasers and integrated circuits finally made scanners simple and affordable enough: when Woodland and Silver first came up with their idea, they would have needed a wall full of switches and relays to handle the information a scanner picked up by a microchip.
On June 26, 1974, all the tests were done, all the proposals were complete, all the standards were set and at a Marsh supermarket in Troy, Ohio, a 10 pack of Juicy Fruit chewing gum became the first retail product sold with the help of a scanner.
Woodland never got rich from bar codes, though he was awarded the 1992 National Medal of Technology by president Bush; all those involved in the early days speak of the rewards of having brought a new way of doing business to the world.
2.1.2 Symbologies
The mapping between messages and barcodes is called a symbology.
The specification of a symbology includes the encoding of the single characters of the message, the start and stop markers into bars and space, the size of the quiet zone required to be before and after, the barcode and the computation of a checksum.
2.1.2.1 Linear symbologies
Linear symbologies are a monodimensional way of representing information and can be classified by continuousness and width.
Characters in continuous symbologies usually abut, with one character ending with a space and the next beginning with a bar, or vice versa.
Characters in discrete symbologies begin and end with bars: the intercharacter space is ignored, as long as it is not wide enough to look like the code ends.
Bars and spaces in two-width symbologies are wide or narrow: how wide a wide bar is exactly has no significance as long as the symbology requirements for wide bars are adhered to (usually two to three times more wide than a narrow bar).
Bars and spaces in many-width symbologies are all multiples of a basic width called the module (most such codes use four widths of 1, 2, 3 and 4 modules).
Some symbologies use interleaving: the first character is encoded using black bars of varying width; the second character is then encoded by varying the width of the white spaces between these bars, thus characters are encoded in pairs over the same section of the barcode (interleaved 2 of 5 is an example of this).
Linear symbologies can be read with a laser scanner, that sweep a beam of light across the barcode in a straight line, reading a slice of the bar code, or a CCD imager, that does not require moving parts: in the last years linear imaging is surpassing laser scanning as the preferred scan engine for its performance, durability and reliability.
FIGURE 2.2 – Anatomy of an UPC barcode label
The original UPC code is split into two halves of six digits each:
• the first digit is always zero, except for products that have variable weight and a few other special items
• the next five are the manufacturer’s code
• the next five are the product code
• the last is a check digit used to verify that the preceding digits have been scanned properly
Hidden cues in the structure of the code tell the scanner which end is which, so it can be scanned in any direction.
Manufacturers register with the UCC to get an identifier code for their company and then register each of their products; thus each package that passes over a checkout stand has its own unique identification number.
Since their invention barcodes have slowly become an essential part of modern civilization.
Along the way refinements of the basic UPC have been developed, including the European Article Numbering system (EAN), developed by Joe Woodland, which has an extra pair of digits and is on its way to becoming the world’s most widely used system.
While traditionally barcode encoding schemes represented only numbers, newer symbologies added new characters such as the uppercase alphabet to the complete ASCII character set and beyond.
2.1.2.2 Stacked symbologies
Stacked symbologies consist of a given linear symbology repeated vertically in multiple rows and can be read by laser scanners, with the laser making multiple passes across the barcode, or CCD imagers.
2.1.2.3 2D symbologies
2D symbologies are a bidimensional way of representing information, similar to a linear (monodimensional) barcode, but with more data representation capability.
The most common 2D symbologies are matrix codes, which feature square or dot- shaped modules arranged on a grid pattern.
Other visual formats use circular patterns or employ steganography by hiding an array of different sized or shaped modules within a specified image (DataGlyph).
FIGURE 2.4 – Dolby digital film audio is but one use of 2D barcodes
2D symbologies cannot be read by a laser as there is typically no sweep pattern that can encompass the entire symbol: they must be scanned by a camera capture device.
2.1.3 Uses
In point-of-sale management, the use of barcodes can provide very detailed up to date information on key aspects of the business, enabling decisions to be made much more quickly and with more confidence:
• fast-selling items can be identified quickly and automatically reordered to meet consumer demand
• slow-selling items can be identified, preventing a build-up of unwanted stock
• the effects of repositioning a given product within a store can be monitored, allowing fast-moving more profitable items to occupy the best space
• historical data can be used to predict seasonal fluctuations very accurately
• items may be repriced on the shelf to reflect both sale prices and price increases
Some modern applications of barcodes are described in the following list.
− Practically every item purchased from a grocery store, department store and mass merchandiser has a barcode on it.
This greatly helps in keeping track of the large number of items in a store and also reduces instances of shoplifting (since shoplifters could no longer easily switch price tags from a lower-cost item to a higher-priced one).
− Document Management tools often allow for barcoded sheets to facilitate the separation and indexing of documents that have been imaged in batch scanning applications.
− Rental car companies keep track of their cars by means of barcodes on the car’s glass or bumper.
− Airlines track passenger luggage with barcodes, reducing the chance of loss.
− Researchers have placed tiny barcodes on individual bees to track the insects’
mating habits.
− The movement of nuclear waste can be tracked easily with a bar-code inventory system.
2.1.4 Benefits and limits
The barcode has transformed the way organizations in nearly every industry track and manage their inventory and assets, changing the way companies keep track of everything from blood donations to diesel engines.
Today, warehouse management and point of sale systems depend on barcodes to track or control inventory; in fact barcode use is still increasing and the UCC estimates that about five billion scans are read worldwide each day.
As effective as barcodes are today, the technology does have some limitations:
barcode technology requires a line of sight access to an optical scanner so reading a barcode often requires human interaction to position the scanner over the coded item.
Because today’s barcodes are limited in size, they can only carry a finite amount of information that is fixed once the barcode is printed and can never be changed.
As the requirements on supply chains increase, the need to track and transmit more detailed item information make the inadequacies of barcoding ever more obvious to supply chain professionals.
2.2 Radio frequency identification
RFID uses radio frequency based communications to allow contactless reading of entities’ identity.
FIGURE 2.5 – An RFID system
Radio frequency identification systems consists of readers and transponders, that is RFID tags or contactless cards, containing an electronic circuit attached to an antenna, that communicate data to the readers via electromagnetic radio waves using air interface and data protocol as well as many other protocols.
FIGURE 2.6 – RFID tags
2.2.1 History
During the first few seconds or so of the universe, protons, neutrons and electrons began formation when photons, the quantum element of electromagnetic energy, collided converting energy into mass.
The electromagnetic remnant of the Big Bang survives today as a background microwave hiss so the ancestry of RFID can be traced back to the beginning of time.
In fact the development of this technology is due to the discovery of how to harness electromagnetic energy in the radio region.
The Chinese were probably the first to observe and use magnetic fields in the form of lodestones in the first century BC; scientific understanding progressed very slowly after that until about the 1600s.
Then there was an explosion of observational knowledge of electricity, magnetism and optics accompanied by a growing base of mathematically related observations.
The 1800s marked the beginning of the fundamental understanding of electromagnetic energy.
Michael Faraday, a noted English experimentalist, proposed in 1846 that both light and radio waves are part of electromagnetic energy.
In 1864, James Clerk Maxwell, a Scottish physicist, published his theory on electromagnetic fields and concluded that electric and magnetic energy travel in transverse waves that propagate at a speed equal to that of light.
Soon after in 1887, Heinrich Rudolf Hertz, a German physicist, confirmed Maxwell’s electromagnetic theory and produced and studied electromagnetic waves (radio waves), which he showed are long transverse waves that travel at the speed of light and can be reflected, refracted and polarized like light.
Hertz is credited as the first to transmit and receive radio waves and his demonstrations were followed quickly by Aleksandr Popov in Russia.
In 1896, Guglielmo Marconi demonstrated the successful transmission of radiotelegraphy across the Atlantic and the world would never be the same.
In 1906, Ernst F. W. Alexanderson demonstrated the first continuous wave (CW) radio generation and transmission of radio signals; this achievement signals the beginning of modern radio communication, where all aspects of radio waves are controlled.
The early 20th century, approximately 1922, was considered the birth of radar; the work in radio detecting and ranging during World War II was a significant technical development critical to the success of the allies.
FIGURE 2.7 – A radar antenna
Radar sends out radio waves for detecting and locating an object by the reflection of the radio waves; this reflection can determine the position and speed of an object.
Radar’s significance was quickly understood by the military, so many of the early developments were shrouded in secrecy.
FIGURE 2.8 – A radar screen
Since RFID is the combination of radio broadcast and radar technologies, the thoughts of RFID occurred on the heels of the development of these two disciplines.
An early, if not the first, work exploring RFID is the landmark paper by Harry Stockman, “Communication by means of reflected power” (October 1948), in which he stated that considerable research and development work had to be done before the remaining basic problems in reflected power communication could be solved and before the field of useful applications could be explored.
The 1950s were an era of exploration of RFID techniques following technical developments in radio and radar in the 1930s and 1940s: several technologies related to RFID were being explored such as the long-range transponder systems of
“identification, friend or foe” (IFF) for aircraft.
Developments of the 1950s include such works as F. L. Vernon’s “Application of the microwave homodyne” and D.B. Harris’ “Radio transmission systems with modulatable passive responder”: the wheels of RFID development were turning.
The 1960s were the prelude to the RFID explosion of the 1970s.
R. F. Harrington studied the electromagnetic theory related to RFID in his papers
“Field measurements using active scatterers” and “Theory of loaded scatterers” in 1963-1964.
Inventors were busy with RFID related inventions such as Robert Richardson’s
“Remotely activated radio frequency powered devices” in 1963, Otto Rittenback’s
“Communication by radar beams” in 1969, J. H. Vogelman’s “Passive data transmission techniques utilizing radar beams” in 1968 and J. P. Vinding’s
“Interrogator-responder identification system” in 1967.
Commercial activities were beginning in the 1960s and companies like Sensormatic and Checkpoint developed electronic article surveillance (EAS) equipment to counter theft; these types of systems used 1-bit microwave or inductive tags: only the presence or absence of a tag could be detected, but the tags could be made inexpensively and provided effective anti-theft measures.
EAS is arguably the first and most widespread commercial use of RFID.
In the 1970s developers, inventors, companies, academic institutions and government laboratories were actively working on RFID and notable advances were being realized at research laboratories and academic institutions such as Los Alamos Scientific Laboratory, Northwestern University and the Microwave Institute Foundation in Sweden among others.
An early and important development was the Los Alamos work that was presented by Alfred Koelle, Steven Depp and Robert Freyman “Short-range radio-telemetry for electronic identification using modulated backscatter” in 1975.
Large companies were also developing RFID technology, such as Raytheon’s
“Raytag” in 1973, RCA’s “Electronic identification system” in 1975 and “Electronic license plate for motor vehicles” in 1977 and Fairchild’s “Passive encoding microwave transponder” in 1978.
The Port Authority of New York and New Jersey were also testing systems built by General Electric, Westinghouse, Philips and Glenayre: results were favorable, but the first commercially successful transportation application of RFID, electronic toll collection, was not yet ready for prime time.
The 1970’s were characterized primarily by developmental work; intended applications were for animal tracking, vehicle tracking and factory automation.
The number of companies, individuals and institutions working on RFID began to multiply: the potential for RFID was becoming obvious.
The 1980s became the decade for full implementation of RFID technology, though interests developed somewhat differently in various parts of the world.
The greatest interests in the United States were for transportation, personnel access and to a lesser extent, for animals.
In Europe, the greatest interests were for short-range systems for animals, industrial and business applications, though toll roads in Italy, France, Spain, Portugal and Norway were equipped with RFID.
In the Americas, the Association of American Railroads and the Container Handling Cooperative Program were active with RFID initiatives: tests of RFID for collecting tolls had been going on for many years and the first commercial application began in Europe in 1987 in Norway and was followed quickly in the United States by the Dallas North Turnpike in 1989.
Also during this time, the Port Authority of New York and New Jersey began commercial operation of RFID for buses going through the Lincoln Tunnel.
RFID was finding a home with electronic toll collection and new players were arriving daily.
The 1990s were a significant decade for RFID since it saw the wide scale deployment of electronic toll collection in the United States
The world’s first open highway electronic tolling system opened in Oklahoma in 1991, where vehicles could pass toll collection points at highway speeds, unimpeded by a toll plaza or barriers and with video cameras for enforcement.
The world’s first combined toll collection and traffic management system was installed in the Houston area by the Harris County Toll Road Authority in 1992.
Also a first was the system installed on the Kansas turnpike using a system based on the Title 21 standard with readers that could also operate with the tags of their neighbor to the south, Oklahoma.
The Georgia 400 would follow, upgrading their equipment with readers that could communicate with the new Title 21 tags as well as the existing tags; in fact, these two installations were the first to implement a multi-protocol capability in electronic toll collection applications.
In the Northeastern United States, seven regional toll agencies formed the E-Z Pass Interagency Group (IAG) in 1990 to develop a regionally compatible electronic toll collection system: this system is the model for using a single tag and single billing account per vehicle to access highways of several toll authorities.
Interest was also keen for RFID applications in Europe during the 1990s.
Both microwave and inductive technologies were finding use for toll collection, access control and a wide variety of other applications in commerce.
A new effort underway was the development of the Texas Instruments TIRIS system, used in many automobiles for the vehicle engine’s starting control.
The Tiris system (and others such as from Mikron, now a part of Philips) developed new applications for dispensing fuel, gaming chips, ski passes, vehicle access and many other applications.
Additional companies in Europe were becoming active in the RFID race as well with developments including Microdesign, CGA, Alcatel, Bosch and the Philips spin-offs of Combitech, Baumer and Tagmaster.
A pan-European standard was needed for tolling applications in Europe and many companies were at work on the CEN standard for electronic tolling.
Tolling and rail applications were also appearing in many countries including Australia, China, Philippines, Argentina, Brazil, Mexico, Canada, Japan, Malaysia, Singapore, Thailand, South Korea and South Africa.
With the success of electronic toll collection, other advancements followed, such as the first multiple use of tags across different business segments: now, a single tag (with dual or single billing accounts) could be used for electronic toll collection, parking lot access and fare collection, gated community access and campus access.
Research and development did not slow down during the 1990s since new technological developments would expand the functionality of RFID.
For the first time, useful microwave Schottky diodes were fabricated on a regular CMOS integrated circuit; this development permitted the construction of microwave
RFID tags that contained only a single integrated circuit, a capability previously limited to inductively-coupled RFID transponders.
With the growing interest of RFID into the item management work and the opportunity for RFID to work along side bar code, it becomes difficult in the later part of this decade to count the number of companies who enter the marketplace.
In the last years much advancement was done making RFID one of the most promising solutions to the inadequacies of the barcode and expanding AIDC technologies potential.
2.2.2 RFID readers
Readers, also called interrogators or controllers, are devices that allow retrieving information from tags; they typically contain a transceiver and a control unit and are connected to a coupling element (antenna).
There are two types of readers: fixed readers and integrated readers
Fixed readers are standalone devices that can include an antenna or control one or more external antennas: in this way readers and antennas can be in different locations.
A fixed reader can provide network connection so that it can be managed and interact with the company’s information systems.
Usually fixed readers are mounted on a vehicle like a forklift (vehicle mounted readers), a tunnel, a portal or a gate:
FIGURE 2.9 – Vehicle mounted RFID reader
FIGURE 2.10 – An RFID tunnel
FIGURE 2.12 – An RFID gate
Integrated readers include the antenna and can be embedded into a mobile, handheld or tablet pc in the form of an integrated circuit, a PC Card, a CF card or a SD card:
FIGURE 2.13 – An integrated circuit RFID reader
FIGURE 2.14 – PC Card, CF card and SD card RFID readers
2.2.3 RFID tags
An RFID tag is a radio frequency transponder and contains at least two parts:
1. an integrated circuit for storing and processing information, modulating and demodulating a radio frequency signal and other specialized functions
2. an antenna for receiving and transmitting the signal
The word transponder, derived from transmitter/responder, indicates how the device functions and suggests the essential components of RFID systems.
FIGURE 2.15 – An RFID tag
2.2.3.1 Modulation
To efficiently transfer data in the air space separating the tag and the reader, the data is superimposed on a sinusoidal carrier wave within a designated frequency band.
This superimposition is typically called modulation and is performed by changing the value of one of three primary features of the alternating sinusoidal carrier wave: its amplitude, its frequency or its phase.
As a result, the three primary modulation schemes are: amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK).
Other methods of modulating include pulse position modulation (PPM), phase jitter modulation (PJM) and pulse duration modulation (PDM).
Each modulation scheme has attributes that favor its use and in some cases different modulating techniques are used in each direction (to and from the tags).
FIGURE 2.16 – Modulations used with RFID
2.2.3.2 Channel encoding
Noise, interference and distortion can all corrupt transmitted data, making error free data recovery difficult to achieve; this is compounded by the fact that data communication processes are asynchronous or unsynchronized, so care must be taken with the form in which the data is communicated.
Using channel encoding schemes to structure the bit stream, it is possible to create the desired communication performance and solve many data corruption problems.
FIGURE 2.17 – Channel encoding schemes
• NRZ – A binary 1 is represented by a high signal and a binary zero by a low signal. This coding scheme is often used with FSK or PSK modulation.
• Manchester (or bi-phase) – A binary 1 is represented by a negative transition half-way through the clock cycle and binary 0 is represented by a positive transition. This coding scheme is often used in RFID systems employing load modulation using a sub-carrier.
• FM – A binary 1 is coded by a transition of any type and a binary 0 is coded by lack of transition
• Miller (or Modified FM or MFM) – A binary 1 is represented by a transition of any type at half-bit period and a binary 0 is represented by the continued level of the previous 1 over the next bit period. A series of zeros causes a transition at the start of the next bit period.
• Modified Miller – Each transition is replaced by a negative pulse. This coding scheme is very useful for inductively coupled RFID systems due to the very short pulse durations. By having tpulse < Tbit, a continuous power supply can be provided to the transponder from the field of the reader even during data transfer.
According to which coding scheme is used the spread of the signal will change and the RFID system will have a different susceptibility to interference (transmission errors) and power supply interruption.
2.2.3.3 Anti-collision and multiplexing
Reading data requires a finite period of time, hence when a large volume of tags must be read together in the same RF field, the application needs anti-collision and multiplexing features that enable the reader to receive data from each tag.
An RFID collision involves multiple tags crashing into each other within a reader’s field because they are sending a response to the reader at the same time, such that the reader cannot differentiate between them.
The likelihood that more than one tag will at times be within a reader’s field is great, especially in applications where a large number of tags are packed tightly together.
There are many different anti-collision protocols; most are proprietary, since there are no standards for how this function is to be accomplished, but all are related to making sure that only one tag is “talking” with the reader at any one time.
Anti-collision protocols are built upon these multiplexing technologies:
• SDMA – Space Division Multiple Access
• FDMA – Frequency Division Multiple Access
• FHSS – Frequency Hopping Spread Spectrum
• TDMA – Time Division Multiple Access
SDMA technologies are limited to frequencies over 850 MHz and are characterized by high cost and complicated circuitry.
FDMA or FHSS transponders utilize adjustable frequency techniques to separate frequencies to/from the transponders.
TDMA technology is the most popular multiplexing method for RFID applications and can be either reader-driven (polling and binary search method) or transponder driven (ALOHA).
2.2.3.4 Write capabilities
According to the type of memory used a tag has different writing capabilities.
A read-only tag uses a ROM so minimizing space occupation on the chip and costs;
data are written during manufacturing and the user can only read them: it is much like a barcode.
A WORM (write once read many) tag uses a PROM: the only advantage over read only tags is the possibility to write the tag after it has been manufactured.
A read/write tag uses an EEPROM or an FRAM (Ferroelectric RAM) so that users can write and read information.
EEPROMs require a high operating voltage and longer read/write times, can accept a maximum of 100 000 write cycles and have a data retention time of up to 10 years.
FRAMs are characterized by a lower operating voltage and a higher read/write speed, can be written an infinite number of times and retain data for more than 10 years.
2.2.3.5 Operating frequencies
With regard to operating frequencies, it is necessary to make some preliminary remarks (see figure 2.18).
The electromagnetic spectrum is shared by civil, government and military users of all nations according to International Telecommunications Union (ITU) radio regulations; for communications purposes the usable frequency spectrum extends from 3Hz to 300GHz and this range has been split into regions, each one corresponding to a different frequency band.
RFID technology’s operating frequencies are:
• LF in the 120 ÷ 145 kHz subband
• HF in the subband centered on 13.56 MHz
• low-UHF in the 433 ÷ 435 MHz subband
• mid-UHF in the 865 ÷ 870 MHz subband in Europe, in the 902 ÷ 928 MHz subband in the USA and in the subband centered on 950 MHz in Asia
• high-UHF in the subband centered on 2.4 GHz
• SHF in the subband centered on 5,8 GHz
Designation Frequency Wavelength ELF extremely low frequency 3 Hz to 30 Hz 100’000 km to 10’000 km SLF superlow frequency 30 Hz to 300 Hz 10’000 km to 1’000 km ULF ultralow frequency 300 Hz to 3 kHz 1’000 km to 100 km VLF very low frequency 3 kHz to 30 kHz 100 km to 10 km LF low frequency 30 kHz to 300 kHz 10 km to 1 km MF medium frequency 300 kHz to 3 MHz 1 km to 100 m HF high frequency 3 MHz to 30 MHz 100 m to 10 m VHF very high frequency 30 MHz to 300 MHz 10 m to 1 m UHF ultrahigh frequency 300 MHz to 3 GHz 1 m to 10 cm SHF superhigh frequency 3 GHz to 30 GHz 10 cm to 1 cm EHF extremely high frequency 30 GHz to 300 GHz 1 cm to 1 mm
FIGURE 2.18 – Electromagnetic spectrum and frequency bands for communication purposes
LF tags are very common since they were the first tags used for automatic identification.
HF tags are the most widespread ones since they can be used all over the world without problems.
As for the other kinds of tag, the more the frequency increase the less they are common, also because regulations and spectrum allocation vary from one country to another.
2.2.3.6 Power source
According to the power source used by a tag, it can be classified as passive, semi- passive and active.
Passive tags do not require a battery to operate because they can extract energy from the electromagnetic radiation generated by the reader’s antenna with which they come in contact.
The power that can be obtained from the reader’s signal is low, decreases with the square of the distance and is limited by national regulations on readers’ power emissions: as a result operating range is at most 8 meters.
The most important technological goals of passive tags are the absence of power consumption and the possibility of managing noisy RF signals.
In terms of computational power they contain only a basic logic and state machines capable of decoding simple instructions.
Passive tags are very widespread and used in massive applications since they are cheap and available for any frequency band used in RFID applications.
Semi-passive tags work in a way similar to passive tags: they extract energy from the reader’s field to activate their RF section, but they use a battery to power their internal circuits.
In this way the RFID chip can implement more complex functions and operate also when an electromagnetic field is not active, but, as in the passive case, there is no integrated transmitter; this limits operating range to a maximum of 100 meters.
Greater functionalities lead to a higher cost so the price of this kind of tag is around some Euros.
Some semi-passive tags “sleep” until they are “awakened” by a particular signal send
Active tags are equipped with a battery and an RF transceiver so they can communicate at distances of over several kilometers.
Normally active tags are produced only for UHF and SHF frequency bands and their price is around some dozen Euros (they are intended for special applications in which the same tag is used more than once).
Battery’s cost, duration and pollution problems are critical issues for semi-passive and active tags: as a solution some tags use small solar cells or inertial systems.
On not passive tags, technological evolution has allowed the use of a bigger rewritable memory and the implementation of some interesting features like item’s remote localization, tag’s visual or sound localization (with a led or a buzzer) and the monitoring of environmental parameters with the use of embedded sensors.
In this way a tag can monitor temperature, humidity, pressure, motion, shock, container’s door opening, light entering near container’s doors and more.
Thanks to battery’s power source, sensors can do measurements and store detected values with temporal information into the tag’s memory so that they can be read later on interrogator’s request.
2.2.3.7 Coupling
Passive and semi-passive RFID tags use the RF energy they receive as a power source and they do not generate the carrier used for data transmission (no need for a local oscillator and less power required): they modulate a part of the energy transmitted by the interrogator’s continuous wave signal.
This modulation is accomplished via an internal circuit that alternately open-circuits and short-circuits the antenna, varying its impedance match between two or more states, according to the data contained in tag’s memory; as a result there is a corresponding change in the amplitude or phase of the reflected signal depending on whether the real or reactive part of the impedance is changed.
Typical handshake of a tag and reader is as follows:
1. The reader continuously generates an RF carrier sine wave, watching always for modulation to occur: detected modulation of the field would indicate the presence of a tag
2. A tag enters the RF field generated by the reader
3. The received power is rectified and stored on a capacitor to supply power to the tag’s control logic
4. Once the tag has received sufficient energy to operate correctly it reads data from memory
5. The tag modulates the received carrier using its output transistor that shunts the antenna sequentially corresponding to the data which is being clocked out of the memory array
6. This causes a momentary fluctuation (dampening) of the antenna’s impedance match, which is seen as a slight change in the amplitude or phase of the carrier wave
7. The reader detects modulated data and processes the resulting bitstream according to the encoding and data modulation methods used
There is a huge variety of different operating principles for RFID systems, but the most important ones are inductive coupling and electromagnetic backscatter coupling.
Electromagnetic theory developed by Maxwell in the 19th century shows that any conductor (e.g. an antenna) supplied with an alternating current produces a varying magnetic field (H-field) which in turn, produces electric field lines (E-field) in space.
FIGURE 2.19 – An electromagnetic wave
In the near field both the E and H fields are relatively static with no propagation:
they only vary in strength as the current varies, with the magnetic flux of the H-field coming out from the antenna and going back in and the E-field emanating out- wards.
Maxwell also proved that beyond this quasi-static near field, at a certain distance, both the E-fields and H-fields detached themselves from the conductor and
constant ratio of E/H = 120π or 377 Ω (ohms are used because the E-field is measured in volts per meter V/m and the H-field in amps per meter A/m).
The point at which this happens is called the far field.
FIGURE 2.20 – Transition from near to far field
The theoretical boundary between near and far field is directly proportional to wavelength, but actually many factors reduce it to the following values:
Frequency band Near field region Far field region
LF < 120 m > 12 km
HF < 1 m > 110 m
UHF < 1.65 cm > 1.65 m
SHF < 0.25 cm > 0.25 cm
Inductive coupling is used in near field conditions, that is when the wavelength of the frequency range used is several times greater than the distance between the reader’s antenna and the transponder; in this case the electromagnetic field may be treated as a simple magnetic alternating field.
An inductively coupled transponder comprises an electronic data-carrying device, usually a single microchip and a large area coil that functions as an antenna.
Inductively coupled transponders are almost always LF or HF passive tags, so all the energy needed for the operation of the microchip has to be provided by the reader.
For this purpose, the reader’s antenna coil generates a strong, high frequency electromagnetic field, a small part of which penetrates the antenna coil of the transponder generating an induced voltage.
This voltage is rectified and serves as the power supply for the data-carrying device.
FIGURE 2.21 – Inductive coupling
In the reader a capacitor Cr is connected in parallel with the antenna coil; its capacitance is selected such that it works with the inductance of the antenna coil to form a parallel resonant circuit with a resonant frequency that corresponds with the transmission frequency of the interrogator.
Very high currents are generated in the reader’s antenna coil by resonance step-up in the parallel resonant circuit, which can be used to generate the required field strengths for the operation of the remote transponder.
The transponder’s antenna coil and the capacitor C1 connected in parallel with it, form a resonant circuit tuned to the transmission frequency of the reader; the voltage at the transponder coil reaches a maximum due to resonance step-up in the parallel resonant circuit.
If a resonant transponder (i.e. a transponder with a self-resonant frequency corresponding with the transmission frequency of the reader) is placed within the magnetic alternating field of the reader’s antenna, it draws energy from the magnetic field and the resulting feedback on the reader’s antenna can be represented as transformed impedance ZT in the antenna coil of the reader.
Switching on and off a load resistor (in the figure a MOSFET) at the tag’s antenna brings changes in the impedance ZT and in the voltage at the reader’s antenna.
This has the effect of a modulation of the voltage at the reader’s antenna coil by the remote transponder: if the timing with which the load resistor is switched on and off
is controlled by data, they can be transferred from the transponder to the reader and this data transfer is called load modulation.
Inductive coupling works much like a transformer does, except that it occurs in free space: the reader and tag play the parts of two coils in the transformer, so, as the secondary winding (tag coil) is momentarily shunted, the primary winding (reader coil) experiences a momentary voltage drop.
Electromagnetic backscatter coupling is used in far field conditions, that is when the wavelength of the frequency range used is several times less than the distance between the reader’s antenna and the transponder as with passive and semi- passive UHF and SHF tags.
In this case the effects of a time varying electromagnetic field are seen: inductive coupling is no more the prevalent effect hence dipole antennas are used like in traditional radiocommunication systems.
FIGURE 2.22 – Electromagnetic backscatter coupling
Because of free space attenuation only a small proportion P1’ of the power P1 emitted from the reader’s antenna reaches the transponder’s antenna.
This power is supplied to the antenna as HF voltage and after rectification by two low barrier Schottky diodes (low threshold voltage) it can be used as a power supply for short ranges or as turn-on voltage for semi-passive tags’ “power down” mode.
A proportion of the incoming power P1’ is reflected by the antenna and returned as power P2: tags use their antenna to re-radiate the signal received by the reader after having backscatter modulated it.
Radar principles tell that electromagnetic waves are reflected by objects with dimensions greater than around half the wavelength of the wave; the efficiency with which an object reflects electromagnetic waves is described by its radar cross section.
Objects that are in resonance with the wave front that hits them, as is the case for antennas at the appropriate frequency, have a particularly large radar cross section.
The reflection characteristics (radar cross section) of the antenna can be influenced by altering the load connected to it: in order to transmit data from the transponder to the reader, a load resistor RL connected in parallel with the antenna is switched on and off in time with the data stream to be transmitted.
The power P2 reflected from the transponder can thus be modulated (backscatter modulation) before radiating it into free space.
Because of free space attenuation only a small proportion P2’ of this power is picked up by the reader’s antenna; the reflected signal therefore travels into the antenna connection of the reader in the backwards direction, is decoupled using a directional coupler and then transferred to the receiver input of the reader.
The forward signal of the transmitter, which is stronger by powers of ten, is to a large degree suppressed by the directional coupler.
2.2.3.8 Tag standards
A great deal of work has been going on over the past decade to develop standards for different RFID frequencies and applications: there are existing and proposed RFID standards that deal with the air interface protocol (the way tags and readers communicate), data content (the way data is organized or formatted), conformance (ways to test that products meet the standard) and applications (how standards are used on shipping labels, for example).
The International Organization for Standardization (ISO) has created many standards related to different applications:
• Cattle tracking – ISO 11784 that defines how data is structured on the tag, ISO 11785 that defines the air interface protocol and ISO 14223, an extension of the other two standards that specifies the structure of the radio frequency code for advanced transponders and describes the air interface between transceiver and advanced transponder
• Contactless cards – ISO 14443 and ISO 15693 for proximity/vicinity cards
• Automatic identification and item management – the ISO 18000 series that covers the air interface protocol for systems likely to be used to track goods in the supply chain, employing the major frequencies used in RFID systems around the world
18000–1 generic parameters for the air interface for globally accepted frequencies 18000–2 parameters for air interface communications below 135 kHz
18000–3 parameters for air interface communications at 13.56 MHz 18000–4 parameters for air interface communications at 2.45 GHz 18000–5 parameters for air interface communications at 5.8 GHz 18000–6 parameters for air interface communications at 860 to 960 MHz 18000–7 parameters for air interface communications at 433 MHz
In 1999 was set up a not-for-profit consortium called Auto-ID Center to develop an RFID system with the following requirements:
− identify each item manufactured using something called Electronic Product Code or EPC
− low-cost since the tags needed to be disposable (a manufacturer putting tags on products shipped to a retailer was never going to get those tags back to reuse them)
− use of UHF band, because only this band delivered the read range needed for supply chain applications, such as reading pallets coming through a dock door
− use of standardized air interface protocol
− use of a network architecture, integrated with the Internet and based on open standards used on the Internet, so companies could easily and cheaply share information associated with serial numbers stored on tags
This RFID system needed to be global, because the aim was to use it to track goods as they flowed from a manufacturer in one country or region to companies in other regions and eventually to store shelves.
FIGURE 2.24 – The Electronic Product Code
Like a bar code, the Electronic Product Code is divided into numbers that identify the manufacturer, product, version and serial number, but the EPC uses an extra set of digits to identify unique items.
There is a header identifying the encoding scheme that has been used and three sets of data partitions: the first partition identifies the manufacturer, the second identifies the product and the third is the serial number unique to the item.
EPC encoding schemes dictate the length, type and structure of the EPC and frequently contain a serial number which can be used to uniquely identify one object.
The following coding schemes are supported:
• GID – General Identifier
• SGTIN – a serialized version of the GS1 Global Trade Item Number
• SSCC – GS1 Serial Shipping Container Code
• GLN – GS1 Global Location Number
• GRAI – GS1 Global Returnable Asset Identifier
• GIAI – GS1 Global Individual Asset Identifier
• DoD construct
By separating the data into partitions, readers can search for items with a particular manufacturer’s code, or a particular product code or with the same manufacturer and product code but which have unique numbers in a certain sequence.
This allows, for example, to quickly find products that might be nearing their expiration date or that need to be recalled.
