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 data-

carrying 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

2

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.

3

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 Lon Theremin invented an espionage tool for the Soviet Union which

retransmitted incident radio waves with audio information. Similar technology,

4

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 semi-

passive, 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):

5

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

6

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

7

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 transponders response is broadcast when the readers 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 transponders 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 carriers 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

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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 readers 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 (n-

fold) 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, coil-

on-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 (Faradays 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.

10

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

11

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 100135.5 kHz. The security tags contain a semiconductor circuit (microchip) and a resonant circuit coil made

12

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.

13

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

readers 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

transponders 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 transponders antenna performs amplitude modulation of the voltage at the readers 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.

15

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 A 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).

16

Figure 2.15: Operating principle of a backscatter transponder. The impedance of

the chip is modulated by switching the chips 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

18

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 readers 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 readers 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 (masterslave 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 low-

level 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.

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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.

22

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 tags 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 signature-

based tags have been reported and all five are considered to be fully printable.

25

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.

26

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.

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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 dipolehalf 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 transponders resonant frequency, the transponder will start to oscillate producing a voltage dip

across the readers antenna ports. The advantage of these tags is their price and simple structure (single resonant coil), but they are very restricted in operating

28

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 transponders 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 masterslave 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: Masterslave 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 readers 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 systems transmitted signal and the received weak back-scattered 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.

31

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/ near-

field 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.

32

R F I D

R E A D E R

Classification Criteria

Classes Nature

Power supply Powered from

Network

Readers supplied power by a power cord connected to an appropriate external electrical outlet.

Battery Assisted The battery is mainly used to power up the motherboard of the reader.

Communication Interface

Serial

Use a serial communication link to communicate with their host computers or software applications.

Network Connect to the host computer via a wired or wireless network.

Mobility Stationary Fixed readers.

Handheld Mobile readers.

Interrogation Protocol

Passive Limited to listening and do not perform additional tag interrogations.

Active True interrogators which interrogate and listen to tags.

Frequency Spectrum

Non-unique Frequency

Operate at a unique (or short bandwidth

33

3.3.3: Universal Reader

Barcodes have been enjoying a robust global standard and have free movements

across boundaries. RFID, being an enabling and maturing technology, lacks this

flexibility. Manufacturers in different countries have been following their own

standards and procedures, but Gen 2 RFID tag systems have set standards so tags

can be read across boundaries. The universal reader is named RangeMaster. The RangeMasters system-level block diagram is shown in Figure 3.14. It comprises a Field Programmable Analog Array (FPAA) in conjunction with an RFID State

Machine, enabling RFID system engineers to develop a universal RFID reader

supporting multiple protocols and frequencies for future fixed, mobile and

handheld reader designs.

Figure 3.14: System-level overview of the RangeMaster-embedded RFID reader

Section 3.4: Chipless RFID Applications

Chipless RFIDs have various potential applications in the areas of,

1. Low-cost item tagging

2. Smart sensing techniques of temperature, pressure, humidity, light, pH etc.

3. Various industrial, bio-medical, supply-chain applications that are at present

driven by conventional RFIDs.

34

Chapter 4

A Chipless RFID Tag Design

Section 4.1: Proposed Chipless Tag

The review of available and reported chipless RFID transponders has shown the

lack of an operational fully printable multi-bit chipless RFID transponder. This

section presents a novel chipless RFID transponder based on multiresonators,

proposed by Karmakar et al. at Monash University, Australia. The main

components of the transponder are the transmitting (Tx) and receiving (Rx)

antennas and multiresonating circuit. Block diagram and signal flow diagram of

the integrated chipless RFID transponder with basic components are shown in

Figure 4.1.

The proposed chipless RFID transponder consists of a vertically polarized UWB

disc-loaded monopole receiving (Rx) tag antenna, a multiresonating circuit and a

horizontally polarized UWB disc-loaded monopole transmitting (Tx) tag antenna.

When the interrogation signal reaches the transponder, it is received using the

receiving monopole antenna and propagates further on towards the multiresonating

circuit. The chipless tag encodes data in the frequency spectrum, thus encoding the

spectrum with its unique spectral signature. The spectral signature is obtained by

the RFID reader by means of interrogating the tag by a multi-frequency signal. The

tag encodes its spectral signature into the interrogation signal spectrum using a

multiresonating circuit, which is a multi-stop band filter. The multiresonator is a

set of cascaded spiral resonators designed to resonate at particular frequencies and

create stop bands. The stop band resonances introduce magnitude attenuation and

phase jumps to the transmitted interrogation signal at their resonant frequencies,

which are detected as abrupt amplitude attenuations and phase jumps by the RFID

reader. In order to provide isolation between the transmitting and receiving signal,

the reader and tag antennas are cross-polarized. As a result, cross-talk between the

transmitting and receiving antennas is minimized at the cost of introducing

restrictions in tag positioning and orientation.

The main differences between this spectral signature-based transponder and the

ones reported in the previous chapter are that we encode data in both amplitude

35

and phase, the transponder operates in the UWB region and that the tag responses

are not based on radar cross-section (RCS) backscattering, but on retransmission of

the cross-polarized interrogation signal with the encoded unique spectral ID.

(a)

(b)

Figure 4.1: (a) Chipless RFID transponder circuit block diagram, (b) Chipless

RFID system signal flow diagram

36

Section 4.2: Spiral Resonators

In this section, a microwave resonant structure which can meet the requirements as

in the previous section is investigated. The most suitable candidate that can meet

all the desirable features is the spiral resonator. When compared to other planar

circuits, such as defected ground structures (DGS), spiral resonators have the

advantage of having 550 times narrower 3 dB bandwidth. It can also be modified easily for data encoding.

Figure 4.2 shows the layout of a conventional spiral resonator. The microstrip line

and the spiral resonator are on the same plane (top layer) and are separated from

the continuous metallic ground plane (bottom layer) by a dielectric layer. The

spiral resonator is gap coupled to the 50 microstrip line. At its resonant frequency, the spiral resonator creates a stop-band effect.

Figure 4.2: Layout of spiral resonator placed next to microstrip line

4.2.1: The Stop-band Effect

The surface current distribution simulation is used in order to understand how the

stop-band effect is created at the spirals resonant frequency. Figure 4.3 shows (a) the peak surface current distribution of a spiral resonator at its resonant frequency

(2 GHz) and (b) at a non-resonant frequency (2.1 GHz). The simulation was

performed using CST Microwave Studio 2008. From Figure 4.3(a), it is clear that

the surface current distribution is greater around the spiral at its resonant

frequency. The spiral resonator creates a low impedance path to ground at its

resonant frequency and absorbs the majority of the current propagating from Port 1

37

to Port 2 of the microstrip line resulting in a stop-band effect. At non-resonant

frequencies, the spiral resonator couples almost none of the surface current

propagating between Port 1 and Port 2 as seen in Figure 4.3(b). When the spiral

resonator is coupled to the microstrip line, the entire circuit (microstrip line + gap

coupling + spiral resonator) is modelled as a parallel RLC due to its stop-band

characteristic. The resonant frequency fr of the spiral resonator coupled to a

microstrip line is given by,

For increasing attenuation of the individual spiral resonator we can use corner

coupling. Also by using the same spiral resonator coupled to the microstrip line

more than once, the attenuation can be increased. The disadvantage of this solution

is the layout size and inefficiency (same spiral repetition).

c)

Figure 4.3: CST surface current distribution of spiral resonator at (a) resonant

frequency of 2 GHz and (b) non-resonant frequency of 2.1 GHz, (c) Equivalent

circuit model of spiral resonator coupled to microstrip line

38

4.2.2: Design Problem Using Flexible Substrates

It is necessary to investigate the properties of microstrip spiral resonators on thin

flexible laminates, due to the fact that the chipless tag will ultimately be printed on

thin and flexible laminates such as paper and/or polymer/plastic. Chapter 5 deals

with this type of design. The main problems of designing spiral resonators on

flexible substrates are:

- resonators exhibit low Q factor due to thin substrate; and

- microstrip lines and tracks become thinner on thinner substrates.

In order to avoid the increase in metallic loss in the microstrip line due to the

decrease in dielectric thickness, CPW technology was used for designing high Q

spiral resonators (Figure 4.4). CPW technology uses spiral shapes etched out in

the stripline to create stop bands. CPW technology overcomes the problem of low

Q factor as found in microstrip technology.

Figure 4.4: Layout of (a) microstrip spiral resonator with different resonant

parameters, (b) spiral resonator etched out in a CPW strip line

4.2.3: The Multiresonator Cascaded Spiral Resonators

The multiresonating circuit is designed by cascading spiral resonators next to the

microstrip line (microstrip technology for PCB) or etching them out in the CPW

strip line (CPW technology for thin laminates) with different lengths so that a

multiple resonances occur. Figure 4.4(a) shows the top view of a 6-bit

multiresonator layout generated in ADS Momentum 2008. Figure 4.4(b) shows

the photograph of the fabricated 6-bit multiresonator on Taconic TLX-0 substrate.

The 6-bit multiresonator consists of six spiral resonators cascaded next to a 50 microstrip line. The multiresonator provides six distinguishable resonances

between 2 and 2.5 GHz. Each resonance is separated by approximately 100 MHz

39

from each other. In order to design the spirals at different frequencies, the length of

each spiral is varied so that the spirals resonant frequency is fine-tuned. Resonant frequency decreases with increase in length/number of turns of the spiral.

Figure 4.4: (a) Layout of 6-bit multiresonator in ADS Momentum 2008, (b)

Photograph of 6-bit multiresonator on Taconic TLX-0

Figure 4.5 shows the measured frequency response in both magnitude and phase

of the 6-bit multiresonator. From Figure 4.5, it is clear that at the resonant

frequencies of individual spirals of the multiresonator there is a magnitude dip and phase jump in the magnitude and phase of the spectrum of the multiresonator. These properties are used to encode data into the spectrum using

the multiresonator. The presence of a magnitude null (dip) and phase jump

represents logic 0, while the absence of a magnitude null and phase jump at a particular frequency represents logic 1.

CPW technology on thin flexible laminates is superior to microstrip technology.

Figure 4.6(a) shows the layout of coplanar waveguide (CPW) 3-bit multiresonator

in ADS Momentum 2008. Figure 4.6(b) shows the simulated frequency response

in both magnitude and phase of the 3-bit multiresonator.

40

Figure 4.5: Measured insertion loss and transmission phase of 6-bit

multiresonator

Figure 4.6: (a) Layout of (CPW) 3-bit multiresonator, (b) Measured insertion loss

and transmission phase of coplanar waveguide (CPW) 3-bit multiresonator

41

4.2.4: Encoding Data Using Novel Spiral Shorting Technique It is necessary to encode data into the tag in order for the tag to have a unique ID.

This can be done by introducing or removing the resonances of the multiresonator.

Figure 4.5 shows the S-parameter measurements of a multiresonator, which gives

a tag ID of 000000. In order to create a different ID, for example 101010, the

resonances at 2.1, 2.3 and 2.5 GHz need to be removed (Figure 4.7).

Figure 4.7: Measured insertion losses of chipless tags with different spectral

signatures

By removing the spiral, the resonance is removed. The other option is to short the

turns of the spiral as shown in Figure 4.8(a), thus shifting the resonance frequency

of the spiral up where it will be of no significance. The shift of the resonant

frequency with the shorting of the turns is shown in Figure 4.8(b). The advantage

of shorting turns in regards to removing the entire spiral from the layout is that it

enables future printing techniques to preserve the layout with all of the spirals

shorted and when encoding data the shorting can be removed via a laser or other

etching technique.

42

Figure 4.8: (a) Spiral shorting for microstrip (left) and CPW (right)

multiresonator, (b) Frequency shift of resonant frequency with short-circuited

spiral

Section 4.3: Tag Antenna

The system operates from 2GHz to 2.5GHz and hence requires antennas operating

in this frequency range. The circular microstrip UWB monopole antenna can be

chosen owing to its large frequency bandwidth and donut shape radiation pattern

(omni-directional in one plane). Figure 4.9 shows the UWB monopole antenna

design, the return loss an radiation patterns. Although the gain might not be high,

the radiation pattern showed that the transponder can be interrogated from almost

any angle by the RFID reader.

A prototype of the fully fabricated chipless RFID tag is shown in Figure 4.10. The

antenna and multiresonator sections can be distinctly identified.

43

Figure 4.9: (a) UWB monopole antenna design layout, (b) simulated return loss,

(c) simulated radiation pattern

Figure 4.10: Photograph of chipless RFID tag on Taconic TLX-0 laminate

44

Section 4.4: Proof-of-concept Designs

Utilizing the theory and techniques discussed in the previous section, following

proof-of-concept multiresonator designs have been attempted using High

Frequency Structural Simulator (HFSS) version 15.

(a)

(b)

Figure 4.12: (a) Design layout of 6-bit multiresonator with parameters on Taconic

TLX-0 ( = 2.45, h = 0.787 mm, tan = 0.0019), (b) 21 magnitude and phase

plots versus frequency showing amplitude dip at six resonant frequencies (m1-m6)

(Encoded tag ID 000000)

45

(a)

(b)

Figure 4.13: (a) Design layout of 6-bit multiresonator with two spirals shorted, (b)

21 magnitude and phase plots versus frequency for shorted spiral multiresonator,

two resonances got shifted owing to the spiral shorting (Encoded tag ID 000110)

Figure 4.12 shows the design layout and spectral response of the 6-bit

multiresonator system.

The shorted spiral design and its insertion loss versus frequency plot are shown in

Figure 4.13. Spiral shorting is done to shift the resonant frequencies of 2nd and

3rd resonators, resulting in only four resonances (m1-m4) within the frequency

range of interest. Encoded tag ID in this case is 000110.

46

(a)

(b)

Figure 4.14: (a) Design layout of 3-bit CPW multiresonator on Taconic TF-290

( = 2.9, h = 90 m, tan = 0.0028), (b) 21 plot versus frequency showing

amplitude dip at three resonant frequencies (m1-m3) (Encoded tag ID 000)

Figre 4.14 shows a 3-bit CPW multiresonator design along with the S21 plot versus

frequency. From the figure it is clearly noted that attenuation has increased to a

great extent than that for the microstrip counterpart and resulting 3-dB bandwidth

is less, implying higher Q-values resulting with this technique particularly for

designs using thin substrates.

47

(a)

(b)

Figure 4.15: (a) Design layout of 3-bit CPW multiresonator with one spiral

shorted, (b) 21 magnitude and phase plots versus frequency for shorted spiral

multiresonator, two resonances got shifted owing to the spiral shorting (Encoded

tag ID 010)

The shorted spiral design for CPW multiresonator and its insertion loss versus

frequency plot are shown in Figure 4.15. Spiral shorting is done to shift the

resonant frequencies of 2nd resonator, resulting in only two resonances (m1-m2)

within the frequency band of interest. Encoded tag ID in this case is 010.

48

Chapter 5

Paper Based RF Design

Section 5.1: Introduction

Recent research in printed electronics has revealed the potential of System-on-

Package (SoP) solutions on organic substrates applied to wireless sensors networks

(WSNs), Ultra Wide Band (UWB) modules, wireless transmitters using embedded

passives on organic substrates, and even miniature biomedical sensors among

others.

Paper is one of the cheapest organic materials available. It is flexible, of low

profile and recyclable. In addition, paper can be made hydrophobic, and/or fire

retardant by adding certain textiles to it. This can easily resolve any moisture-

absorbing issues from which fibre-based materials, such as paper, suffer. Last but

not least, paper is one of the most environmentally friendly materials. Thus the

paper-based design approach could potentially set the foundation for the first

generation of truly "green" RF electronics and modules.

Remarkably, it allows inkjet printing, a fast prototyping and packaging technique

which is similar to the one found in residential printers, but that is being refined

and optimized with more capable printers in laboratories so that it can be applied to

circuits. In terms of low-cost packaging, and the need for low profile transceivers,

inkjet printing on paper could be particularly appealing if three dimensional

antennas could be replaced with planar, printed antennas with similar performance.

This also enables components such as ICs, memory, batteries, and/or sensors to be

easily embedded in or on paper modules.

The vastly growing interest for cheaper, flexible and high performance RFID and

sensing applications have primarily led to the use of paper substrate. However the

development is still there in the primitive stage. In this chapter, printable paper

based antenna/RF circuit design methodology will be outlined in brief.

49

Section 5.2: Dielectric Characterization of Paper

There is wide availability of different types of paper, which vary in density,

coating, thickness, texture, and, implicitly, dielectric properties, including

dielectric constant and dielectric loss tangent. Due to this, dielectric RF

characterization of paper substrates becomes an essential step before any RF "on

paper" designs.

The most precise methods for determining RF characteristics of the substrate are

the resonator-based methods, including parallel-plate resonators, microstrip ring

resonators, and cavity resonators. The parallel-plate resonator method is usually

applied at low frequencies beneath the UHF band. In the UHF band and higher

frequencies, the microstrip ring resonator method provides dielectric information

at periodic resonant peaks.

Among the critical needs for the selection of the right type of paper for electronics

applications are the surface planarity, water repelling, lamination capability for 3-

D module development, via-forming ability, adhesion, and co-processability with

low cost manufacturing. For the RF characterization, a commercially available

paper with hydrophobic coating was selected. The thickness of a single sheet of

paper is 260 3 m.

Layout of the microstrip ring resonator is shown in Figure 5.1. The ring resonator

produces 21 results with periodic frequency resonances (Figure 5.2). Relative permittivity can be extracted from the location of the resonances of a given radius

ring resonator, while loss tangent is extracted from the quality factor of the

resonance peaks along with the theoretical calculations of the conductor losses.

Figure 5.1: Microstrip ring resonator configuration diagram.

50

Figure 5.2: Measured and simulated 21 of the ring resonator configuration A. Peak positions and -3-dB bandwidth at the three resonant frequencies were used to

extract the relative permittivity and the loss tangent of the paper substrate.

5.2.1: Extraction of

The relative permittivity can be extracted from the effective relative permittivity and the dimensions of the microstrip by using (1) as follows:

where is a function of the ring radius , the nth resonant frequency 0 obtained from measurement of the insertion loss, and the speed of light c in vacuum, as defined in (2) as follows:

and M in (1) is a function of the thickness of the paper h and of the fringing effects on the microstrip edges, which can be calculated as a function of h and conductor

thickness t as shown in (3) as follows:

51

in (3) is the effective strip width accounting for the nonzero strip thickness and is given by (4) as follows:

5.2.2: Extraction of tan

The loss in the rings occurs mainly due to the conductors, lossy dielectrics, and

radiation. The loss tangent of the paper substrate is a function of only the

attenuation due to the dielectric at the resonant frequency and is computed using (5) as follows:

where is the 0 wavelength of the free-space radiation from the rings at the resonant frequencies. was extracted by subtracting the attenuation due to the conductor and radiation from the total attenuation that occurs in the structure at the resonant frequencies.

Section 5.3: Inkjet Printing Technology

Inkjet printing is a direct-write technology by which the design pattern is

transferred directly to the substrate, and there is no requirement of masks compared

with the traditional etching technique, which has been widely used in industry.

Besides that, unlike etching, which is a subtractive method by removing unwanted

metal from the substrate surface, inkjet-printing jets the single ink droplet from the

nozzle to the desired position, therefore, no waste is created, resulting in an

economical fabrication solution.

52

Silver nano-particle inks are usually selected in the inkjet-printing process to

ensure a good metal conductivity. The conductivity of the conductive ink varies

from 0.4 ~ 2.5 107 S/m depending on the curing temperature and duration time. At lower temperature, large gap exists between the particles, resulting in a poor

connection. When the temperature is increased, the particles begin to expand and

gaps start to diminish (Figure 5.3). That guarantees a virtually continuous metal

conductor, providing a good percolation channel for the conduction electrons to

flow. Typically curing temperature of 120 and duration time of 2 h is used in fabrication to sufficiently cure the nano-particle ink. Figure 5.4 shows an RFID

antenna printed on paper.

Figure 5.3: SEM images of a layer of printed silver nanoparticle ink, after a 15-

min curing at 100 and 150, respectively

Figure 5.4: Photograph of UHF RFID antenna inkjet printed on paper and DMC-

11610 print head

Section 5.4: On Paper RFID Tag Design Example

Figure 5.5 shows a T-match folded bow-tie half-wavelength dipole antenna design

which can be fabricated on a commercial photo paper by the inkjet-printing

53

technology mentioned above. IC is placed in the centre of the T-match arms. The

T-match arms are also responsible for the matching of the impedance of the

antenna terminals to that of the IC through the fine tuning of the length L3, height

h, and width W3. In order to verify the performance of the inkjet-printed RFID

antenna, measurements were performed on a copper-metalized antenna prototype

with the same dimensions fabricated on the same paper substrate. Overall a good

agreement between the copper etched and the inkjet-printed antennas was observed

despite the higher metal loss of the silver-based conductive ink (Figure 5.6).

Figure 5.5: T-match folded bow-tie RFID tag module configuration

Figure 5.6: Return loss of the RFID tag antenna that covers the universal UHF

RFID band. Measurement results from the inkjet-printed tag and the heat-bonded

copper tag demonstrate a good agreement for both paper metallization

approaches.

54

Section 5.5: 3-D Paper-on-paper Integration

Limitations in integrating RF passive components using standard CMOS-based

technology have driven the trend towards hybrid packaging techniques involving

the integration of system-on-chip (SOC) chipsets with passives and power

amplifier modules on microwave substrates to form system-on-package (SOP)-

based miniaturized modules in the wireless industry. This trend has led to an

increase in the research on the use of relatively expensive laminated substrates,

such as ceramics (LTCC) and organics (LCP), which has driven up costs of

wireless front-end modules. The extremely low cost of paper and its feasibility for

making multilayer inkjet-printed passive structures offer a unique opportunity to

offset higher packaging costs involved in current wireless front-end modules.

Figure 5.7 demonstrates the suggested fabrication steps for the development of

multilayer (3-D) coplanar multilayer RF circuits on paper substrate.

Figure 5.7: Conceptual passive microwave circuit embedded process in paper

substrate

55

Chapter 6

Conclusion

Radio Frequency Identification (RFID) is a technology that has risen to

prominence over the past decade. The clear advantages of this technology over

traditional identification methods, along with mandates from supply chain giants

like Wal-Mart and the US Department of Defence, led to a large number of

research and commercialization efforts in the early 2000s.

Although this technique was used as early as World War II, RFID transponders

were expensive, large devices that remained confined to military applications.

However, the tremendous progress in VLSI technology along with the

establishment of standards in the early 2000s, enabled RFID tags to be

manufactured in high volumes resulting in a price point that initiated numerous

commercial applications. The main goal of commercial RFID systems is to

automate and enhance asset management by providing global asset visibility. This

ability of RFID systems finds various applications in diverse fields such as supply

chain management, indoor asset and personnel tracking, animal tracking, access

control, robotics and many more.

The immense commercial potential of RFID is mainly due to the numerous

advantages that the technology possesses over traditional identification

mechanisms such as barcodes. Some of these advantages are: (i) passive RFID tags

can be read at much greater distances than barcodes; (ii) there is no need for a line

of sight between the reader and tag; (iii) multiple tags can be read at much higher

rates than barcodes; (iv) RFID tags have much larger memory than barcodes which

allows storage of a lot more information than just the ID; and (v) the information

contained in the RFID tag can be modified dynamically using the interrogator.

However, almost a decade on, the early promise of widespread, ubiquitous

adoption of RFID is yet to materialize. This is due to a combination of several

technical and commercial factors. The technical imperfections and shortcomings

existing in present day RFID systems pose a very significant obstacle to the

widespread adoption of RFID. Also the cost of RFID tags is still much higher than

barcodes, making them virtually unaffordable for low cost item tagging and other

low cost applications.

56

In recent years, chipless RFID has been proposed as a low cost and competitive

replacement for the barcode. The cost cutting in chipless tags is accomplished by

doing away with the ASICs as in conventional RFIDs. The recently reported

chipless RFID tags are printed resonators, chemical fibres and TFTC organic tags.

However, these reported chipless tags have been stagnating in the prototyping

stage and have limitations in terms of reading range, size, data capacity, data

encoding, frequency of operation and finally, fabrication challenges. As for an

example, printed resonators have size restrictions, chemical fibres have reading

ranges up to a couple of millimetres, while TFTC has very low electron mobility

and can only operate in the kHz range and at best MHz frequency range.

Another aspect of chipless RFID system is the design of RFID readers for chipless

tags. Since the chipless RFID tags use unconventional methods for data encoding,

such as spectral signatures, conventional off-the-shelf RFID readers are not suitable for the new development. Hence, the RFID readers are needed to be

developed from scratch.

Hence, at present, there is a need for fully operational chipless tags that can get the

best of both worlds i.e., they should be functionally as good as conventional RFID

tags, and as cheap as barcodes. A direct way of making the tags cheaper is to make

them suitable for mass scale production. Another way of reducing the price of the

tags is to reduce the material cost. The use of paper as the substrate and the use of

inkjet printing technology for making the layout can serve both the purposes.

Besides, paper is bio-degradable, which makes it ideal choice for truly cheap,

flexible and "green" RFIDs.

57

Chapter 7

Future Work

In this chapter, future work would be proposed, which would include detailed

studies on the design and applications of different RFID systems, both with chip

and chipless variants. The work would fundamentally concentrate on chipless tag-

reader systems. Hitherto, a very small number of chipless RFID systems have been

reported and most of them are not commercially ready as yet because of their

limitations in many aspects including short range, low data capacity, fabrication

difficulties and so on.

Hence, an attempt would be made to develop a novel chipless RFID tag that can

overcome the shortcomings. The main idea would be to develop a spectral

signature-based RFID tag based on narrowband band-stop filters or

multiresonators. Both of these techniques are not much explored till today.

There would also be challenges to miniaturize the tag. The concept that could be

applied is to use a single antenna instead of two unlike most of the designs. A

single dual polarized antenna, instead of two cross polarized antennas for

transmission and reception, could significantly reduce the size of the tag.

The paper based RF antenna/circuit design techniques would also be explored,

which could dramatically reduce the cost of RFID tags by ensuring mass

production by the aid of inkjet printing technology. The ease of fabrication could

be achieved using this very technique. In addition, further studies on novel ways

for RF characterization of paper would be carried on simultaneously.

The project task would additionally include the design of Ultra Wide Band (UWB)

and Circularly Polarized (CP) antennas for application to chipless tags. The use of

large bandwidth antennas increases the data capacity of tags, which can also make

them capable of universal application. And circular polarization is the ideal choice

for any transponder system because it reduces the effect of orientation of antennas

for transmission and reception.

58

Chapter 8

References

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Websites:

1. www.ieeeexplore.ieee.org 2. www.springerlink.com 3. www.idtechex.com 4. www.centrenational-rfid.com 5. www.wikipedia.org