Conventional and Chipless Rfid Systems Design and Applications

Chapter 1

Introduction Of late, Automatic Identification has found ubiquitous application in service industry, supply chain management, purchasing and distribution logistics, manufacturing companies, material flow systems and so on. Automatic identification procedures also exist to provide information about people, animals, goods and products in transit. The omnipresent barcode labels triggered a revolution in identification systems some considerable time ago. But they are found to be inadequate in an increasing number of cases. Barcodes may be extremely cheap, but their stumbling block is their low storage capacity, and the fact that they cannot be reprogrammed. The technically optimal solution would be the storage of data in a silicon chip. The most common form of electronic data-carrying devices in use in everyday life is the smart card based upon a contact field (telephone smart card, bank cards). However, the mechanical contact used in the smart card is often impractical. A contactless transfer of data between the data-carrying device and its reader is far more flexible. In the ideal case, the power required to operate the electronic datacarrying device would also be transferred from the reader using contactless technology. Because of the procedures used for the transfer of power and data, contactless ID systems are called RFID systems (radio frequency identification). In recent years contactless identification has been developing into an independent interdisciplinary field. It brings together elements from extremely varied fields: RF technology and EMC, semiconductor technology, data protection and cryptography, telecommunications, manufacturing technology and many related areas. The number of companies actively involved in the development and sale of RFID systems indicates that this is a market that should be taken seriously. The value of the radio frequency identification (RFID) market will almost triple in 2020, growing to $23.4 billion from a $7.88 billion arena in 2014. The RFID market

therefore belongs to the fastest growing sector of the radio technology industry (Figure 1.1).

Figure 1.1: Value of passive tags by application in US$ millions RFID systems exist in countless variants, depending on the design and application, and they are produced by almost equally high number of manufacturers. Very recently a new RFID technology called Chipless RFID has emerged, which seems to revolutionize the market as it can eventually make the RFID tag cost only a fraction of a cent! This literature will be mainly concentrated on the Chipless RFID technology after a discussion on the conventional chipped variant of RFID in the next chapter. It will also briefly deal with the paper based RF design methodology, which has the potential to make RFID systems "green" and far cheaper as well.

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Chapter 2

Basics of RFID Section 2.1: Automatic Identification Systems The technologies used in the world of automatic identification and data capture (AIDC) are varied (Figure 2.1). A comparative study of them is depicted in Table 2.1.

Figure 2.1: Overview of the most important auto-ID procedures The comparison between the identification systems highlights the strengths and weakness of RFID in relation to other systems.

Section 2.2: History of RFID In 1945 Léon Theremin invented an espionage tool for the Soviet Union which retransmitted incident radio waves with audio information. Similar technology, 3

such as the IFF transponder, was routinely used by the allies and Germany in World War II to identify aircraft as friend or foe. However, Mario Cardullo's device, patented on January 23, 1973, was the first true ancestor of modern RFID, as it was a passive radio transponder with memory. An early demonstration of reflected power (modulated backscatter) RFID tags, both passive and semipassive, was performed by Steven Depp, Alfred Koelle, and Robert Freyman at the Los Alamos National Laboratory in 1973. The portable system operated at 915 MHz and used 12-bit tags. The first patent to be associated with the abbreviation RFID was granted to Charles Walton in 1983.

Table 2.1: Comparison of different RFID systems showing their advantages and disadvantages

Section 2.3: Components of RFID Systems At this juncture, let us formally define Radio-frequency identification (RFID). It is a wireless data capturing technique that utilizes radio frequency (RF) waves for automatic identification of objects. RFID relies on RF waves for data transmission between the data carrying device, called the RFID tag, and the interrogator. Thus an RFID system is always made up of two components (Figure 2.2): 4

 the Tag or Transponder, which is located on the object to be identified;  the Interrogator or Reader, which, depending upon the design and the technology used, may be a read or write/read device. A reader typically contains a radio frequency module (transmitter and receiver), a control unit and a coupling element to the transponder. In addition, many readers are fitted with an additional interface (RS 232, RS 485, etc.) to enable them to forward the data received to another system (PC, robot control system, etc.). The transponder, which represents the actual data-carrying device of an RFID system, normally consists of a coupling element and an electronic microchip (Figure 2.3). Figure 2.4 shows a practical contact-less RFID tag-reader in use.

Figure 2.2: The reader and transponder of RFID system

Figure 2.3: The Basic layout of the RFID data-carrying device, the transponder. Left, inductively coupled transponder with antenna coil; right, microwave transponder with dipolar antenna 5

Figure 2.4: A typical RFID tag-reader

Section 2.4: Classification of RFID Transponder Systems

Figure 2.5: The various features of RFID systems

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RFID systems exist in countless variants. RFID systems can be classified in many ways according to the operation type, data quality, frequency range of operation, programmability, data carrier's operating principle, nature of power supply, data transfer etc. (Figure 2.5).  In full and half duplex systems the transponder‘s response is broadcast when the reader‘s RF field is switched on. In contrast, sequential procedures employ a system whereby the field from the reader is switched off briefly at regular intervals. These gaps are recognised by the transponder and used for sending data from the transponder to the reader.  The data capacities of RFID transponders normally range from a few bytes to several kilobytes. So-called 1-bit transponders represent the exception to this rule. A data quantity of exactly 1-bit is just enough to signal two states to the reader: ‗transponder in the field‘ or ‗no transponder in the field‘. For this reason, vast numbers of 1-bit transponders are used in Electronic Article Surveillance (EAS) to protect goods in shops and businesses.  The possibility of writing data to the transponder provides us with another way of classifying RFID systems. In very simple systems the transponder‘s data record, usually a simple (serial) number, is incorporated when the chip is manufactured and cannot be altered thereafter. In writable transponders, on the other hand, the reader can write data to the transponder, the data are stored in EEPROMs, FRAMs or SRAMs. However, these have the disadvantages of high power consumption during the writing operation and a limited number of write cycles (typically of the order of 100,000 to 1,000,000).  RFID transponders can be programmable and non-programmable. In programmable systems, write and read access to the memory and any requests for write and read authorisation must be controlled by the data carrier‘s internal logic. In the simplest case these functions can be realised by a state machine. However, state machines have their inflexibility regarding changes to the programmed functions. The use of a microprocessor improves upon this situation considerably. Also there are transponders that can store data by utilising physical effects. This includes the read-only surface wave transponder (SAW).  Passive transponders do not have their own power supply, and therefore all power required for the operation of a passive transponder must be drawn from 7

the (electrical/ magnetic) field of the reader. Conversely, active transponders incorporate a battery, which supplies all or part of the power for the operation of a microchip.  RFID transponders can be with-chip or chipless. With-chip versions incorporate ASICs whereas chipless RFID tags do not require microchips in the transponder.  One of the most important characteristics of RFID systems is the operating frequency and the resulting range of the system. The operating frequency of an RFID system is the frequency at which the reader transmits. The transmission frequency of the transponder is disregarded.  The different procedures for sending data from the transponder back to the reader can be classified into three groups: (i) the use of reflection or backscatter (the frequency of the reflected wave corresponds with the transmission frequency of the reader → frequency ratio 1:1) or (ii) load modulation (the reader‘s field is influenced by the transponder → frequency ratio 1:1), and (iii) the use of sub-harmonics (1/n fold) and the generation of harmonic waves (nfold) in the transponder.  According to construction formats RFID tags can be of different types: Disk and coins, keys, plastic/glass housed, clocks, ID-1 contact-less smart card, coilon-chip, smart label and others (Figure 2.6).

Figure 2.6: Different transponder/tag construction formats

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 RFID tags are also commercially divided into 6 classes (Figure 2.7).

Figure 2.7: Classes of RFID tags

Section 2.5: Fundamental Operating Principles of RFID This section describes the basic interaction between transponder and reader, in particular the power supply to the transponder and the data transfer between transponder and reader (Figure 2.8). The fundamental interaction procedures are briefly discussed here.

2.5.1: Radio Frequency The radio frequency (RF) procedure is based upon LC resonant circuits adjusted to a defined resonant frequency fR. (Figure 2.9). If the LC resonant circuit is moved into the vicinity of the magnetic alternating field, energy from the alternating field can be induced in the resonant circuit via its coils (Faraday‘s law). If the frequency fG of the alternating field corresponds with the resonant frequency fR of the LC resonant circuit the resonant circuit produces a sympathetic oscillation.

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Figure 2.8: Different operating principles of RFID systems

2.5.2: Microwaves EAS systems in the microwave range exploit the generation of harmonics at components with nonlinear characteristic lines (e.g. diodes). The Nth multiple of the output frequency is termed the Nth harmonic (Nth harmonic wave), the output frequency itself is termed the carrier wave or first harmonic. Capacitance diodes 10

are particularly suitable nonlinear energy stores for frequency multiplication. Figure 2.10 shows a transponder being placed within the range of a microwave transmitter operating at 2.45 GHz. The second harmonic of 4.90 GHz generated in the diode characteristic of the transponder is re-transmitted and detected by a receiver, which can then trigger an alarm system.

Figure 2.9: Operating principle of the EAS radio frequency procedure

Figure 2.10: Basic circuit and typical construction format of a microwave tag

2.5.3: Frequency Divider This procedure operates in the long wave range at 100–135.5 kHz. The security tags contain a semiconductor circuit (microchip) and a resonant circuit coil made 11

of wound enamelled copper. The resonant circuit is made to resonate at the operating frequency of the EAS system using a soldered capacitor. These transponders can be obtained in the form of hard tags (plastic) and are removed when goods are purchased. The microchip in the transponder receives its power supply from the magnetic field of the security device (reader). The frequency at the self-inductive coil is divided by two by the microchip and sent back to the security device. The signal at half the original frequency is fed by a tap into the resonant circuit coil (Figure 2.11).

Figure 2.11: Basic circuit diagram of the EAS frequency division procedure: security tag (transponder) and detector (evaluation device)

2.5.4: Electromagnetic types Electromagnetic types operate using strong magnetic fields in the NF range from 10 Hz to around 20 kHz. The security elements contain a soft magnetic amorphous metal strip with a steep flanked hysteresis curve. The magnetisation of these strips is periodically reversed and the strips taken to magnetic saturation by a strong magnetic alternating field. The markedly nonlinear relationship between the applied field strength H and the magnetic flux density B near saturation (Figure 2.12), plus the sudden change of flux density B in the vicinity of the zero crossover of the applied field strength H, generates harmonics at the basic frequency of the security device, and these harmonics can be received and evaluated by the security device.

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Figure 2.12: Typical antenna and tag design: electromagnetic type, BH curve

2.5.5: Acoustomagnetic These systems contain two metal strips, a hard magnetic metal strip permanently connected to a plastic box, plus a strip made of amorphous metal, positioned such that it is free to vibrate mechanically. Ferromagnetic metals (nickel, iron etc.) change slightly in length in a magnetic field under the influence of the field strength H. This effect is called magnetostriction and results from a small change in the interatomic distance as a result of magnetisation. In a magnetic alternating field a magnetostrictive metal strip vibrates in the longitudinal direction at the frequency of the field. Acoustomagnetic security systems (Figure2.13) are designed such that the frequency of the magnetic alternating field generated precisely coincides with the resonant frequencies of the metal strips in the security element. If a security element is within the field of the generator coil this oscillates like a tuning fork in time with the pulses of the generator coil. The transient characteristics can be detected by an analysing unit.

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Figure 2.13: Acoustomagnetic system comprising transmitter and detection device

2.5.6: Inductive Coupling An inductively coupled transponder comprises an electronic data-carrying device, usually a single microchip, and a large area coil that functions as an antenna. They are almost always operated passively. All the energy needed for the operation of the microchip is provided by the reader (Figure 2.14). For this purpose, the reader‘s antenna coil generates a strong, high frequency electromagnetic field, A small part of the emitted field penetrates the antenna coil of the transponder, which is some distance away from the coil of the reader. A voltage is generated in the transponder‘s antenna coil by inductance. This voltage is rectified and serves as the power supply for the data-carrying device (microchip). The data transfer between transponder and reader is carried out using load modulation. Switching a load resistor on and off at the transponder‘s antenna performs amplitude 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, this data can be transferred from the transponder to the reader. Load modulation can also be achieved using subcarriers when operating distance is larger.

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Figure 2.14: Power supply to an inductively coupled transponder from the energy of the magnetic alternating field generated by the reader

2.5.7: Electromagnetic Backscatter Coupling Backscatter transponders often have a backup battery to supply power to the transponder chip. To prevent this battery consumption if the transponder moves out of range of a reader, then the chip automatically switches over to the power saving ‗power down‘ mode. In this state the power consumption is a few μA at most. However, the battery of an active transponder never provides power for the transmission of data between transponder and reader, but serves exclusively for the supply of the microchip. Data transmission between transponder and reader relies exclusively upon the power of the electromagnetic field emitted by the reader. The data transfer between transponder and reader (Figure 2.15) depends on modulated reflection cross-section (A proportion of the incoming power P1 is reflected by the antenna and returned as power P2. The reflection characteristics (reflection cross-section) of the antenna can be influenced by altering the load connected to the antenna. 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 amplitude of the power P2 reflected from the transponder can thus be modulated (modulated backscatter).

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Figure 2.15: Operating principle of a backscatter transponder. The impedance of the chip is ‘modulated’ by switching the chip’s FET

2.5.8: Close Coupling Close coupling systems are designed for ranges between 0.1 cm and a maximum of 1 cm. The transponder is therefore inserted into the reader or placed onto a marked surface (‗touch & go‘) for operation. The functional layout of the transponder coil and reader coil corresponds with that of a transformer (Figure 2.16). The reader represents the primary winding and the transponder coil represents the secondary winding of a transformer. A high frequency alternating current in the primary winding generates a high frequency magnetic field in the core and air gap of the arrangement, which also flows through the transponder coil. This power is rectified to provide a power supply to the chip.

Figure 2.16: Operating Close coupling transponder in an insertion reader with magnetic coupling coils

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For Magnetic coupled and Capacitive coupled close coupling systems load modulation with subcarrier is used for data transfer in close coupling systems.

2.5.9: Electrical Coupling An electrically coupled system uses electrical (electrostatic) fields for the transmission of energy and data (Figure 2.17).

Figure 2.17: Electrically coupled system

2.5.10: Other Types of Systems The types of RFID systems discussed so far, employ microchips in their transponder systems for their operation. But there are other variants of RFID transponders that do not require microchips to operate, that is why they are called "Chipless RFID" transponders. Among the chipless tags available so far, most popular is the SAW (Surface Acoustic Wave) transponder. The different types of Chipless RFIDs will be discussed in detail in the next chapter.

Section 2.6: RFID Reader Architecture Readers in all systems can be reduced to two fundamental functional blocks: the control system and the HF interface, consisting of a transmitter and receiver (Figure 2.18). The HF interface is shielded against undesired spurious emissions 17

by a tinplate housing. The control system comprises an ASIC module and a microcontroller. In order that it can be integrated into a software application, this reader has an RS232 interface to perform the data exchange between the reader (slave) and the external application software (master).

Figure 2.18: Block diagram of a reader consisting of control system and HF interface. The entire system is controlled by an external application via control commands

2.6.1: HF Interface The reader‘s HF interface (Figure 2.19) performs the following functions:  generation of high frequency transmission power to activate the transponder and supply it with power;  modulation of the transmission signal to send data to the transponder;  reception and demodulation of HF signals transmitted by a transponder.

Figure 2.19: Block diagram of an HF interface for an inductively coupled RFID system

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2.6.2: Control Unit The reader‘s control unit (Figure 2.20) performs the following functions:  communication with the application software and the execution of commands from the application software;  control of the communication with a transponder (master–slave principle);  signal coding and decoding. In more complex systems the following additional functions are available:  execution of an anti-collision algorithm;  encryption and decryption of the data to be transferred between transponder and reader;  performance of authentication between transponder and reader.

Figure 2.20: Block diagram of the control unit of a reader. There is a serial interface for communication with the higher application software

Section 2.7: RFID Middleware Middleware is the software component between the RFID reader hardware and RFID application software. RFID middleware filters, formats, and converts lowlevel RFID hardware communication with the tags into usable event information, so that the data can be processed by a software application. The function of the RFID middleware is similar to the compiler inside a computer system. In an RFID system, middleware translates machine information into tag event information. There are different types of tag event information. The most common one is ―reader reads a tag.‖ This information can include some other useful parts depending on the specific reader model used, such as tag ID, zone ID, and time stamp. The reader generates huge amounts of such event information. 19

Section 2.8: RFID Frequency range, Application, and Existing standards The Table 2.2 shows application of RFIDs operating in different frequency regions along with read range, percentage wise market shares, coupling mechanisms etc. of the same. Also the standards imposed on different sectors of RFID are listed down.

Table 2.2: Tag details

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Chapter 3

Introduction to Chipless RFID Section 3.1: Introduction Barcode labels have been used to track items and stocks for sometime after their inception in the early 1970s. Though barcodes are printed in marks and spaces and very cheap to implement, they impose undeniable obstacles in terms of their short range readability, line of sight limitation and un-automated tracking. These limitations are costing large corporations millions of dollars per annum. The growing tendency today is to replace the barcodes with RFID tags, which have unique ID codes for individual items that can be read at a longer distance. The only reason why RFID tags have not fully replaced the barcode is the price of the tag. The cost of an existing RFID tag is still much higher when compared to the price of the barcode. The main cost of an RFID tag comes from the chip embedded as the information carrying and processing device in the tag. Huge investments and investigations focus on lowering the price of the RFID chip. However, the price of the RFID tag is still not competitive when compared with the cost of the barcode. The recent development of chipless tags without silicon integrated circuits (ICs) has lowered the cost of the tags comparable to that of the barcode. However, the technology is still at conceptual level. The next ten years will see a rapid gain in market share of chipless tags. The numbers sold globally will rise from 5 million 0.4% in 2006 to 267 billion 45% in 2016. By value, chipless versions will rise from $1.2 million 0.1% in 2006 to $1.39 billion - a more modest 13% of all income from RFID tags in 2016 because most of the increase in penetration will be by price advantage. The following sections provide a brief outlook of reported chipless RFID systems that can be found in open literatures today.

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Section 3.2: Chipless RFID Transponders There have been a few reported chipless RFID tag developments in recent years. However, most of them are still reported as prototypes and only a handful is considered to be commercially viable or available. The challenge that researchers face when designing chipless RFID transponders is how to perform data encoding without the presence of a chip. In response to this problem, two general types of RFID transponders can be identified: time domain reflectometry (TDR)-based and spectral (frequency) signature-based chipless RFID transponders. Figure 3.1 shows the classification of reported chipless RFID transponders.

Figure 3.1: Classification of Chipless RFID Transponders

3.2.1: TDR-based chipless RFID transponders These are interrogated by sending a signal by the reader in the form of a pulse and listening to the echoes of the pulse sent by the tag. This way a train of pulses is created, which can be used to encode data. Various RFID transponders have been reported using TDR-based technology for data encoding. It can be further classified into non-printable and printable TDR-based transponders.

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3.2.1.1: Non-printable TDR- based chipless transponders The example of a non-printable TDR-based chipless RFID transponder is the surface acoustic wave (SAW) tag (Figure 3.2), which is also the commercially most successful type. SAW tags are excited by a chirped Gaussian pulse sent by the reader centred around 2.45 GHz. The interrogation pulse is converted to a SAW using an interdigital transducer (IDT). The SAW propagates across the piezoelectric crystal and is reflected by a number of reflectors, which creates a train of pulses with phase shifts . The train of pulses is converted back to an EM wave using the IDT and detected at the reader end, where the tag‘s ID is decoded.

Figure 3.2: SAW transponder operation

3.2.1.2: Printable TDR- based chipless transponders Can be of two types,

3.2.1.2.1: Thin-Film-Transistor Circuits (TFTC) based transponder TFTC transponders are printed at high speed and on low cost plastic film. TFTC tags offer advantages over active and passive chip-based transponders due to their small size and low power consumption. They require more power than other chipless tags, but offer more functionality. However, low cost manufacturing processes for TFTC tags have not been developed yet. Another issue is the low electron mobility, which limits the frequency of operation up to several MHz.

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3.2.1.2.2: Delay-line based transponder These chipless RFID tags operate by introducing a microstrip discontinuity after a section of delay line. The trans-ponder is excited by a short pulse (1 ns) EM signal. The interrogation pulse is received by the transponder and reflected at various points along the microstrip line creating multiple echoes of the interrogation pulse. The time delay between the echoes is determined by the length of the delay line between the discontinuities. This type of tag is a replica of the SAW tag using microstrip technology, which makes it printable. Although initial trials and experiments of this chipless technology have been reported, only 4 bits of data have been successfully en-coded, which shows limited potential of this technology. A variant of this type is shown in Figure 3.3.

Figure 3.3: Principle of utilization of group delay in chipless tags. a) structure of the proposed tag b) group delay curve in frequency domain c) corresponding time delay

3.2.2: Spectral signature-based chipless transponders Encodes data into the spectrum using resonant structures. Each data bit is usually associated with the presence or absence of a resonant peak at a predetermined frequency in the spectrum (Figure 3.4). So far, five types of spectral signaturebased tags have been reported and all five are considered to be fully printable.

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Figure 3.4: Spectral signature based chipless RFID system We can distinguish two types of spectral signature tags based on the nature of the tag: chemical tags and planar circuit tags.

3.2.2.1: Chemical transponders Designed from a deposition of resonating fibres or special electronic ink. Two classes of chemical transponders exit.

3.2.2.1.1: Nanometric material tag These tags (Figure 3.5) consist of tiny particles of chemicals, which exhibit varying degrees of magnetism and when electromagnetic waves impinge on them they resonate with distinct frequencies, which are picked up by the reader. They are very cheap and can easily be used inside banknotes and important documents for anti-counterfeiting and authentication. In addition, these tags can work on low grade paper and plastic package material. But unfortunately, they only operate at frequencies up to a few kHz, although this gives them very good tolerances to metal and water.

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Figure 3.5: Nanometric materials tag

3.2.2.1.2: Ink-tattoo chipless tags Use electronic ink patterns embedded into or printed onto the surface of the object being tagged (Figure 3.6). The system operates by interrogating the ink-tattoo tag by a high frequency microwave signal (>10 GHz) and is reflected by areas of the tattoo, which have ink creating a unique pattern which can be detected by the reader. The reading range is claimed to be up to 1.2 m (4 feet). In the case of animal ID, the ink is placed in a one-time-use disposable cartridge. For non-animal applications, the ink can be printed on plastic/paper or within the material.

Figure 3.6: Ink-tattoo RFID tag

3.2.2.2: Planar circuit chipless RFID transponders Designed using standard planar microstrip/co-planar waveguide/stripline resonant structure, such as antennas, filters and fractals. They are printed on thick, thin and flexible laminates and polymer substrates. Several configurations of this type are possible. 26

3.2.2.2.1: Capacitively tuned dipoles The chipless tag consists of a number of dipole antennas, which resonate at different frequencies. When the tag is interrogated by a frequency sweep signal, the reader looks for magnitude dips in the spectrum as a result of the dipoles. Each dipole has a 1:1 correspondence to a data bit. Issues regarding this technology would be: tag size (lower frequency longer dipole—half wavelength) and mutual coupling effects between dipole elements.

3.2.2.2.2: Space-filling curves The tags represent a frequency selective surface (FSS), which is manipulated with the use of space-filling curves (such as Peano and Hilbert curve, etc.). Only 3 bits of data are reported with this type.

Figure 3.7: (a) First three orders of Peano and Hilbert Space filling curves, (b) Frequency response of an array of 2nd order Peano curve elements

3.2.2.2.3: LC resonant chipless tags Comprise a simple coil, which is resonant at a particular frequency (Figure 3.8). These transponders are considered 1-bit RFID transponders. The operating principle is based on the magnetic coupling between the reader antenna and the LC resonant tag. The reader constantly performs a frequency sweep searching for transponders. Whenever the swept frequency corresponds to the transponder‘s resonant frequency, the transponder will start to oscillate producing a voltage dip across the reader‘s antenna ports. The advantage of these tags is their price and simple structure (single resonant coil), but they are very restricted in operating 27

range, information storage (1 bit), operating bandwidth and multiple-tag collision. These transponders are mainly used for electronic article surveillance (EAS) in many supermarkets and retail stores.

Figure 3.8: LC resonant chipless tag and frequency response

Section 3.3: Chipless RFID Readers RFID readers are devices that perform the interrogation of RFID transponders. In a chipless RFID system, the RFID reader detects the tag by using signal processing demodulation techniques to extract data from the transponder‘s signal. A chipless tag cannot generate a signal without the reader sending an interrogation signal to the transponder. Therefore, the reader and transponders are in a master–slave relationship, where the reader acts as a master and the transponders as slaves. Nevertheless, RFID readers themselves are in a slave position as well. A software application, also called middleware, processes data from the RFID reader, acts as the master unit and sends commands to the reader as shown in Figure 3.9.

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Figure 3.9: Master–slave principle between the application software and reader, and the reader and transponders

Figure 3.10: Block diagram of typical RFID reader

3.3.1: Reader Architecture An RFID reader consists of three main parts as shown in Figure 3.10. They are: 1. Digital/control section 2. RF section 3. Antenna

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3.3.1.1: Digital/Control Section The digital section of the RFID reader performs digital signal processing over the received data from the RFID transponder. This section usually consists of a microprocessor, a memory block, a few analogue-to-digital converters (ADCs) and a communication block for the software application (Figure 3.11).

Figure 3.11: Block diagram of a typical chipless RFID reader digital/control unit

3.3.1.2: RF Section The reader‘s RF section is used for RF signal transmission and reception and consists of two separate signal paths to correspond with the two directional data flows as shown in Figure 3.12. The local oscillator generates the RF carrier signal, a modulator modulates the signal, the modulated signal is amplified by the power amplifier, and the amplified signal is transmitted through the antenna. A directional coupler separates the system‘s transmitted signal and the received weak backscattered signal from the tag. The weak back-scattered signal is amplified using low noise amplifiers (LNA) before the signal is decoded in the demodulator. Different demodulation techniques are used when decoding the data received from the tag. Most RF sections are protected from EM interference by metal cages.

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Figure 3.12: Block diagram of the RF section of an RFID reader

3.3.1.3: Antenna A number of different reader antennas have been developed during the years based on microstrip patch antennas. The antennas may be mono-static or bi-static/ nearfield or far-field depending on the nature of application.

3.3.2: Classification of RFID Readers Figure 3.13 shows classification of RFID readers based on the power supply, communication interface, mobility, tag interrogation, frequency response and the supporting protocols of the reader.

Figure 3.13: Classification of RFID readers Table 3.1 illustrates the terminologies above. 31

Classification Criteria

Classes

Power supply

Powered from Network Battery Assisted

Communication Interface

Network Stationary Handheld

Mobility R F I D R E A D E R

Serial

Interrogation Protocol

Frequency Spectrum

Data Encoding Process

Nature Readers supplied power by a power cord connected to an appropriate external electrical outlet. The battery is mainly used to power up the motherboard of the reader. Use a serial communication link to communicate with their host computers or software applications. Connect to the host computer via a wired or wireless network. Fixed readers. Mobile readers.

Passive

Limited to “listening” and do not perform additional tag interrogations.

Active

True interrogators which interrogate and listen to tags.

Non-unique Frequency

Operate at a unique (or short bandwidth