Chipless RFID Bar Code of the Future

R

adio-frequency identification (RFID) 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 [1], [2]. A typical RFID system is shown in Figure 1. An RFID system consists of three major components: a reader or interrogator, which sends the interrogation signals to an RFID tag that is to be identified; an RFID tag or transponder, which contains the identification code; and middleware software, which maintains the interface and the software protocol to encode and decode the identification data from the reader into a mainframe or personal computer. The RFID reader can read tags only within the reader’s interrogation zone. The reader is most commonly connected to a host computer, which performs additional signal processing and has a display of the tag’s identity [3]. The host computer can also be connected via the Internet for global connectivity/networking. The vast majority of RFID transponders (or tags) are usually comprised of an antenna and integrated circuit (IC) [4]. The IC performs all of the data processing and is powered by extracting power from the interrogation signal transmitted by the RFID reader. These transponders are called passive due to the fact that they do not have any on-board power supply. RFID transponders, which use on-board power supply (such as batteries) are called active RFID tags. Passive RFID tags offer lower prices at

Chipless RFID: Bar Code of the Future

© DIGITAL VISION

Stevan Preradovic and Nemai Chandra Karmakar Stevan Preradovic ([email protected]), and Nemai Chandra Karmakar ([email protected]) are with the Department of Electrical and Computer Systems Engineering, Bldg 72 Clayton Campus, Monash University, 3800 VIC Australia.

Digital Object Identifier 10.1109/MMM.2010.938571

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1527-3342/10/$26.00©2010 IEEE

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Clock RFID Reader Global Network

Data

RFID Tag

Host Computer

Figure 1. Block diagram of a typical RFID system. the cost of shorter reading ranges (up to 3 m) when compared to the more expensive long-range active RFID tags (read up to 100 m). Various other transponders are found on today’s market and are comprehensively presented in [5]. The cost of the entire RFID system is dependent on the cost of the tag, which is dependant on the cost of its IC [6]. Therefore, efforts have been put in developing chipless RFID tags with no ICs, which mean that the main cost of the tag is being removed. So far, the only commercially available chipless RFID tag is the surface acoustic wave (SAW) tag (developed by RF SAW) [7]. This article presents a comprehensive review of chipless RFID tags available on the market and reported in peer-reviewed journals and conferences. However, in the quest to be as comprehensive as possible the authors have also referenced online internet articles that report novel chipless RFID technologies.

Limitations of Bar Codes and Emergence of Chipless RFID Concepts Bar code labels have been used to track items and stocks for sometime after their inception in the early

ASIC Design

ASIC Manufacturing

ASIC Testing

Antenna Manufacture

Tag Assembly

Conversion to Label/Package

Figure 2. RFID label/tag manufacturing process.

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1970s. Though bar codes are printed in marks and spaces and are very cheap to implement, they present undeniable obstacles in terms of their short-range readability and nonautomated tracking. These limitations currently cost large corporations millions of dollars per annum [8]. The growing tendency today is to replace bar codes with RFID tags, which have unique ID codes for individual items that can be read at a longer distance. The obstacles of reading range and automation would be solved using RFID. The only reason why RFID tags have not replaced the bar code is the price of the tag. The cost of an existing RFID tag is still much higher when compared to the price of the bar code. The main cost of an RFID tag comes from the chip embedded as the information-carrying and processing device in the tag. Significant investments and research have been focused on lowering the price of the RFID chip. As a result, the price of the RFID tag has become lower [9]. However, the price of the RFID tag is still not competitive when compared to the cost of the bar code. The recent development of chipless tags without silicon ICs has lowered the cost of the tags to a level comparable to that of the bar code. Even though the technology is still in its infancy, a number of developments have already been made in the industry, which we overview here.

Difficulties of Achieving Low-Cost RFID The use of RFID instead of optical bar codes has not yet been achieved due to the greater price of the RFID tag (US$0.10) compared to the price of the optical bar code (US$0.5) [10]. The reasons why it is difficult to produce cheap RFID tags are comprehensively presented in [11]. Fletcher advocates that application specific IC (ASIC) design and testing along with the tag antenna and ASIC assembly result in a costly manufacturing process. This is why it is not possible to further lower the price of the chipped RFID tag. The basic steps for manufacturing a chipped RFID tag are shown in Figure 2. The design of silicon chips has been standardized for more than 30 years, and the cost of building a silicon fabrication plant is in the billions of U.S. dollars [12], [13]. Since silicon chips are fabricated on a wafer-by-wafer basis, there is a fixed cost per wafer (around US$1,000). As the cost of the wafer is independent of the IC design, the cost of the RFID chip can be estimated based on the required silicon area for the RFID chip. Significant achievements have been made in reducing the size of the transistors, allowing more transistors per wafer area [14]. Decreasing the amount of transistors needed results in an even smaller silicon area, hence a lower RFID chip price. As a result, great efforts have been made by the Massachusetts Institute of Technology (MIT)

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to design an RFID ASIC with less than 8,000 transistors. Although this will reduce the price of the silicon chip, its miniature size imposes limitations and further handling costs. The cost of dividing the wafer, handling the die, and placing them onto a label remains significant, even if the cost of the RFID chip were next-to-nothing. The cost of handling the die increases with the use of smaller-than-standard chips, simply because the electronics industry is not standardized for them. Hence, with highly optimized low transistor count ASICs, implemented assembly processes and extremely large quantities (over 1 billion) of RFID chips sold per annum, a minimum cost of US$0.05 is the reality for chipped RFID tags. Given the inevitable high cost of silicon chip RFID tags (when compared to optical bar codes), efforts to design low-cost RFID tags without the use of traditional silicon ASICs have emerged. These tags, and therefore systems, are known as chipless RFID systems. The expected cost of chipless RFID tags is below US$0.01. Most chipless RFID systems use the electromagnetic (EM) properties of materials and/or design various conductor layouts/shapes to achieve particular EM properties/behavior.

Review of Chipless RFID Tags There have been some reported chipless RFID tag developments in recent years. However, most are still reported as prototypes, and only a handful are considered to be commercially viable or available. The challenge for researchers when designing chipless RFID tags is how to perform data encoding without the presence of a chip. In response to this problem, three

general types of RFID tags can be identified as shown in Figure 3. Based on the open literature, it is possible to categorize chipless RFID tags in three main categories: • time domain reflectometry (TDR)-based chipless tags • spectral signature-based chipless tags • amplitude/Phase backscatter modulation-based chipless tags.

Time-Domain Reflectometry-Based Chipless Tags TDR-based chipless RFID tags are interrogated by sending a signal from the reader in the form of a pulse and listening to the echoes of the pulse sent by the tag. A train of pulses is thereby created, which can be used to encode data. The advantages of these tags when compared to chipped tags are low cost, greater reading ranges, and their applicability in localization/positioning applications. The disadvantages of these tags are the number of bits that can be encoded and high-speed RFID reader RF front-ends required for generating and detecting short ultrawideband (UWB) pulses. Various RFID tags have been reported using TDR-based technology for data encoding. We can distinguish between nonprintable and printable TDRbased tags. An example of a nonprintable TDR-based chipless RFID tag is the SAW tag, for example, developed by RFSAW Inc. [15]. SAW tags are excited by a chirped Gaussian pulse sent by the reader centered around 2.45 GHz [16]–[20]. A SAW tag is shown in Figure 4. The interrogation pulse is converted to a SAW using

Chipless RFID Tags

Spectral Signature Based

TDR TDR Based based

Nonprintable SAW Tags

Printable TFTC Delay-LineBased Tags

Chemical

Amplitude/Phase Backscatter Modulation Based

Planar Circuits

Left-Hand (LH) Delay Lines

Nanometric Materials

Capacitively Tuned Dipoles

Stub-Loaded Patch Antenna

Ink-Tattoo Chipless RFID

Space Filling Curves

Remote Complex Impedance

LC Resonant

Carbon Nanotube Loading

Multiresonator Based Multiresonant Dipoles

Figure 3. Classification of chipless RFID tags. TDR: Time-domain reflectometry; SAW: surface acoustic wave; TFTC: thin-film-transistor circuit. December 2010

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Reflectors

Antenna

Amplitude Input Signal 011

Reflected Signal “Generated ID: 011”

1

1

0

11

11

0

10

0

1

10

01

1

01

0

Pulse Position Modulation Code Representation

00

Figure 4. Circuit architecture of a surface acoustic wave tag [5].

110

Reflected Signal “Generated ID: 110”

00

Interdigital Transducer

Time

Figure 7. Interrogation and coding of delay-line-based chipless tag [43].

Figure 5. Organic-thin-film-transistor circuit printed on flexible plastic film. [Courtesy of National Institute of Advanced Industrial Science and Technology (www.aist.go.jp) (www.aist.go.jp/aist_e/latest_ research/2008/20080728/20080728.html), reprinted with permission.]

Antenna

Transmission Delay Line

Place for Sensor Integration

Figure 6. Delay-line-based chipless tag with patch antenna and delay line [43]. an interdigital transducer (IDT). The SAW propagates across the piezoelectric crystal and is reflected by a number of reflectors, which create a train of pulses with phase shifts [21]–[28]. 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 [29]–[38].

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Printable TDR-based chipless tags can be found either as thin-film-transistor circuit (TFTC) or microstrip-based tags with discontinuities. TFTC tags are printed at high speed on low-cost plastic film [39]. TFTC tags offer advantages over active and passive chip-based tags 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 yet been developed. Organic TFTC could provide a cost-effective solution [40]. One of the institutes working on organic TFTC development is the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. An organic TFTC printed on flexible plastic film is shown in Figure 5. Another issue is the low electron mobility, which limits the frequency of operation up to several megahertz. Delay-line-based chipless tags operate by using a microstrip discontinuity after a section of delayline, as reported in [41]–[43]. A delay-line-based chipless tag is shown in Figure 6. The tag is excited by a short pulse (usually 1 ns) EM signal. The interrogation pulse is received by the tag and reflected at various points along the microstrip line creating multiple echoes of the interrogation pulse, as shown in Figure 7. 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 on this chipless technology have been reported, only 4 bits of data have been successfully encoded, which shows the limited potential of this technology.

Spectral-Signature-Based Chipless Tags Spectral signature-based chipless tags encode 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

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Dipole (Conductor)

Laminate (Dielectric) …

First Bit

11th Bit

Figure 8. Capacitively tuned dipoles arranged as a 11-bit chipless RFID tag.

chipless tag consists of a number of dipole antennas, which resonate at different frequencies. The capacitively tuned dipole tag is shown in Figure 8. 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 include tag size (lower frequency longer dipole—half wavelength) and mutual coupling effects between dipole elements. Space-filling curves used as spectral signature encoding RFID tags were first reported by McVay [51]. The tags are designed as Piano and Hilbert curves with resonances centered around 900 MHz. The tags represent a frequency selective surface, which is manipulated with the use of space-filling curves (such as the Hilbert and Piano curves). The space-filling curve exhibits an interesting property of resonating at a frequency, which has a wavelength much greater than its footprint. This is an advantage since it allows the development of small footprint tags at UHF ranges. Figure 9 shows the 5-bit space-filling curve chipless tag, which comprises an array of five second-order

y

–15

x Ey

–20 –25 RCS (dB)

spectrum. The advantages of these tags are that they are fully printable, robust, have greater data storage capabilities than other chipless tags, and are low cost. The disadvantages of these tags are large spectrum requirements for data encoding, chipless tag orientation requirements, size, and wideband dedicated RFID reader RF components. So far, seven types of spectral signature-based tags have been reported, and all seven are considered to be fully printable. We can distinguish two types of spectral signature tags based on the nature of the tag: chemical and planar circuit. Chemical tags are designed from a deposition of resonating fibers or special electronic ink. Two companies from Israel use nanometric materials to design chipless tags. These tags consist of tiny particles of chemicals, which exhibit varying degrees of magnetism, and, when EM waves impinge on them, they resonate with distinct frequencies, which are picked up by the reader [44]. They are very cheap and can easily be used inside banknotes and important documents for anticounterfeiting and authentication. CrossID, an Israeli paper company, claims to have 70 distinct chemicals, which would provide unique identification in the order of 270 (more than 1,021) when resonated and detected suitably [45]. Tapemark also claims to have nanometric resonant fibers, which are 5 µm in diameter and 1 mm in length [46]. These tags are potentially low cost and can work on low-grade paper and plastic packaging material. Unfortunately, they only operate at frequencies up to a few kilohertz, although this gives them very good tolerances to metal and water. Ink-tattoo chipless tags use electronic ink patterns embedded into or printed onto the surface of the object being tagged. Developed by Somark Innovations [47], the electronic ink is deposited in a unique bar code pattern, which is different for every item. 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 reader detection is based on spatial diversity created by the presence or absence of ink particles on the tagged surface. The reading range is claimed to be up to 1.2 m (4 ft) [48], [49]. In the case of animal ID, the ink is placed in a one-time-use disposable cartridge. For nonanimal applications, the ink can be printed on plastic/paper or within the material. Based on the limited information available for this technology (which is still in the experimental phase) we assume that it is spectral signature based. Planar circuit chipless RFID tags are designed using standard planar microstrip/coplanar waveguide/ stripline resonant structures, such as antennas, filters, and fractals. They are printed on thick, thin, and flexible laminates and polymer substrates. Capacitively tuned dipoles were first reported by Jalaly [50]. The

–30 –35 –40 –45 –50 0.5

0.6

0.7 0.8 Frequency (GHz)

0.9

1

Figure 9. Five-bit piano-curve-based tag and tag radarcross-section spectral signature [51].

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cascaded spiral resonators, which introduce attenuations and phase jumps at particular frequencies of the spectrum. After passing through the multiresonating circuit, the signal contains the unique spectral signature of the tag and is transmitted back to the transmitter using the Tx monopole tag antenna. The Rx and Tx tag antennas are cross-polarized in order to minimize interference between the interrogation signal and the retransmitted encoded signal containing the spectral signature. Figure 11 shows a 35-bit tag designed on Taconic TLX-0 (er 5 2.45, h 5 0.787 mm, tan d 5 0.0019). The main differences between the multiresonatorbased tag and those reported in the previous sections are that the tag encodes data in both amplitude and phase (Figures 12 and 13), the tag operates in the UWB region, the tag supports simple spiral shorting data encoding [58] and the tag responses are not based on RCS backscattering but on retransmission of the cross-polarized interrogation signal with the encoded unique spectral ID. The chipless tag is designed for printing on the Australian polymer banknote as an anticounterfeiting security feature. The Multiresonant dipole-based chipless RFID tag is based on a similar concept as the multiresonatorbased chipless tag. However, the tag’s designers seek to build on the concept of the multiresonator tag by replacing the stop-band spiral resonators and the second tag antenna with a novel multiresonant dipole antenna [59]. The multiresonant dipole antenna comprises a set of parallel loop antennas, which resonate at different frequencies. Each loop antenna corresponds to a single bit of data. The multiresonant dipole-based chipless RFID tag is shown in Figure 14. From Figure 14, it is clear that the tag receives the reader’s wideband interrogation signal by the Rx UWB monopole antenna and retransmits only certain frequencies, hence encoding a unique spectral signature in the response signal sent by the Tx multiresonant dipole antenna. The multiresonant dipole antenna comprises a series of folded half-wave dipole antennas. The dipole arms etched out in the bottom (ground) layer are fed by a prolongation of the ground plane with the prolongation impedance being 50 V. The half wavelength dipole antennas produce peaks in the return loss at their resonant UWB frequencies. By removing N th Monopole any of the half wavelength Tx Resonator Antenna dipoles, the corresponding resonant peak disappears without influencing the resonances of the other dipoles. Horizontal Polarization The main benefit of using the multiresonant dipole antenna

Piano curves, which create five peaks in the radar cross-section (RCS) of the tag. The chipless tag was successfully interrogated in an anechoic chamber. Only 5 bits of data have been reported to date. The advantage of the tag is its compact size due to the properties of the space-filling curves. However the disadvantage of the tag is that it requires significant layout modifications in order to encode data. LC Resonant chipless tags comprise a simple coil, which is resonant at a particular frequency. These tags are considered 1-bit RFID tags. 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 tags. Whenever the swept frequency corresponds to the tag’s resonant frequency, the tag 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 range, information storage (1 bit), operating bandwidth, and multipletag collision. These tags are mainly used for electronic article surveillance (EAS) in many supermarkets and retail stores [52]. The Multiresonator-based chipless RFID tag was designed and patented by the authors at Monash University [53]. The chipless tag comprises three main components: the transmitting (Tx) and receiving (Rx) antennas and multiresonating circuit. A block diagram of the integrated chipless RFID tag with basic components is shown in Figure 10. The multiresonator-based chipless RFID tag consists of a vertically polarized UWB disc-loaded monopole Rx tag antenna, a multiresonating circuit, and a horizontally polarized UWB disc-loaded monopole Tx tag antenna [54]–[57]. The tag is interrogated by the reader by sending a frequency swept continuous wave signal. When the interrogation signal reaches the tag, it is received using the Rx monopole antenna and propagates towards the multiresonating circuit. The multiresonating circuit encodes data bits using

UWB Monopole Rx Antenna

Vertical Polarization

Multiresonator First Resonator

Second Resonator

Third Resonator

Figure 10. Chipless RFID tag circuit block diagram.

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Multiresonator with 35 Spirals Tag Rx Antenna

Magnitude of Spectral Signature (dB)

Tag Tx Antenna

0 –2 –4 –6 –8 –10 –12 –14 –16 3

Figure 11. Photograph of 35-bit chipless RFID tag (length 5 88 mm, width 5 65 mm).

4

5 6 Frequency (GHz)

7

Figure 12. 35-bit magnitude response of the multiresonator-based chipless RFID tag.

is that the size of the entire tag can be reduced and spatial efficiency is enhanced.

Amplitude-Phase-BackscatterModulation-Based Chipless Tags Amplitude/Phase backscatter modulation-based chipless RFID tags are the third type of chipless RFID tags presented in this article. These tags require less bandwidth for operation than TDR-based and spectral signature-based chipless tags. Data encoding is performed by varying the amplitude or phase of the backscattered signal based on the loading of the chipless tag’s antenna. The variation of the loading is not controlled by an on/off switch between two impedances, but, instead, it is controlled by reactive loading of the tag’s antenna. The antenna loading influences the RCS of the antenna [60] in amplitude or phase, which can be detected by a dedicated RFID reader. The reactance of the load may vary due to the fact that the antenna load is an analog sensor or left-handed (LH) delay line, or that the antenna is terminated by a microstripbased stub reflector. The advantages of this type of chipless tag are that it operates over narrow bandwidths, and it has a simple architecture. The disadvantages are the number of bits that can be detected, and that data encoding is performed by a lumped/chipped component which increases its cost. Based on the data encoding antenna loading element we can distinguish between four types of different backscatter modulation-based chipless RFID tags. LH delay line loading of the chipless tags is one of the most recent developments of chipless tag technology. It utilizes analog circuits for phase modulation and increases the response time of the tag using the slow-wave effect of LH delay lines [61], which also minimizes the size of the tag. The operating principle of the chipless tag is presented in Figure 15. From Figure 15, it is clear that the chipless tag is interrogated by a band-limited pulse transmitted from the RFID reader. The interrogation pulse is received by the chipless tag antenna and propagates through a series of cascaded LH delay lines, which represent

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Phase of Spectral Signature (°)

80 60 40 20 0 –20 –40 –60 3

4

5 6 Frequency (GHz)

7

Figure 13. 35-bit phase response of the multiresonatorbased chipless RFID tag.

Rx UWB Monopole R

Feed Extension Tx Multiresonant Dipole Antenna Spacing

Figure 14. Multiresonant-dipole-based chipless RFID tag [59] (red—top layer, yellow—bottom layer) (© 2009 EuMA, reprinted with permission). periodical discontinuities. The received interrogation pulse is reflected upon reaching each discontinuity and the information is coded by the phase of the reflected signal with respect to a reference phase. The envelope of the reflected signals with encoded data maintain similar magnitudes (envelopes) while the phase variation differs due to different G1, G2, and G3 with phase values w0, w1 and w2, respectively. The LH delay line-based chipless tag encodes data using

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Γ1

Γ2

Γ3

ϕ1 ϑ1 e jϕ1

ϕ2 ϑ2

ϕ3 ϑ3

T ⋅ e jϕ0

T ⋅ e jϕ0

Delay Line Section

ϕ3 + 2(ϑ1 + ϑ2)+ 6ϕ0

ϕ1 + 2ϕ0 Carrier Phase

Reflection Section

ϕ2 + 2ϑ1 + 4ϕ0

t

Carrier Envelope

Figure 15. Operating principle of left-hand-delay-linebased chipless RFID tag [61].

Free Space Loss Zfreespace

S21

Zfreespace

S22

S11

Z0

S12

Γ

Inductive/ Reactance Load

Scattering Antenna

RFID Reader

Phase Ripple (°)

Figure 16. 2-port model of chipless RFID system based on remote measurement of complex impedance.

60 40 20 0 –20 –40 –60 6.9

7.1

7.3 7.5 7.7 Frequency (GHz)

7.9

Figure 17. Variation of the chipless tag’s phase signature with inductance loading [67] (© 2007 EuMA, reprinted with permission). a higher order modulation scheme, such as quadrature phase shift keying (QPSK), which enables greater throughput but requires a higher signal-to-noise ratio for successful tag detection [62]. The QPSK modulator used within the chipless tag is based on a variable reactive element, which minimizes the variation of the amplitude and maximizes the phase variation. Remote complex impedance-based chipless RFID tags comprise a printable antenna, which is loaded/ terminated with a lossless reactance. The tag antenna is chosen to be a scattering antenna (such as a patch antenna) instead of a typically used mini-

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mum scattering antenna (such as a dipole) [63]. The difference between scattering and minimum scattering antennas is that, when terminated with an open or short, the scattering antenna should scatter back the same power, irrespective of the type of lossless termination (including open and short), while the minimum scattering antenna will scatter almost no power back in open circuit conditions [64], [65]. This property of scattering antennas is reported by Mukherjee et al. in [66] to encode data by means of loading a scattering antenna with microstrip stubs, which represent different inductances, and therefore manipulating the phase component of the antennas RCS and backscattered signal. The chipless RFID system based on remote measurement of complex impedance can be modeled as a two-port network where the reader is considered to be the source while the reactive impedance is considered to be the load. Figure 16 shows the model of the chipless RFID system. The transmitted interrogation signal is defined by the S21 parameter while the S12 parameter is the backscattered chipless tag response signal with phase signature. By having chipless RFID tags with different inductive loadings of their antennas, it is possible to create different phase signatures in the backscattered signal, which can be used to identify each tag at the reader end [67]. The reactive loadings are designed to be microstrip stubs in order to make the tag fully printable and low-cost. Figure 17 shows the phase signatures of different chipless RFID tags with different inductive loadings. Stub-loaded-patch-antenna (SLPA)-based chipless RFID tags reported by Balbin et al. in [68] are a newer generation backscatter phase signature tags similar to the remote complex impedance based tag presented earlier. However, the SLPA-based tags are more robust and industry-suited since they incorporate another degree of diversity, such as crosspolarization diversity (besides the phase variation of the backscattered signal due to reactance loading) and multiple tag antennas. The operating principle of the SLPA chipless RFID tag is based on basic principles of vector backscattered signals from multiple planar reflectors. The SLPA-based tag is shown in Figure 18. The chipless tag antennas are multiple patch antennas, which are suited due to their scattering antenna properties as described earlier. The planar reflectors are in the form of meander stubs in order to minimize area and cost. The numbers of bits that can be encoded by the tag vary depending on the number of patches (n) and the available meander line inductances. The chipless tag is interrogated by transmitting n different continuous wave (CW) signals from the reader at n frequencies corresponding to the operating frequencies of each patch antenna. When the tag

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Meandering O/C Stubs

118 mm

L1 Inset Width Element 1

15 mm

SWCNT L2

Spacing

27 mm

Inset Length L3

36 mm

25 mm (a)

Element 2 Element 3 E-Plane H-Plane SWCNT

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125

75

100

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25

50

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25

–25

0

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0.2

0.4 0.6 0.8 Frequency (GHz)

Resistance in NH3 Resistance in Air

1

Resistance (Ω)

Figure 19. Carbon-nanotube-loaded chipless RFID tag on flexible laminate with (a) dimensions and (b) actual photograph [69].

Resistance (Ω)

is read by directive reader antennas, a bit sequence can be detected using the relative phase difference of the backscattered signals. The relative phase refers to the phase difference between the E-plane and H-plane signals at the reader, adding another degree of differentiation. It is important to notice that this type of chipless RFID tag requires interrogation and reading with a directional dual polarized reader antenna and not circularly polarized due to the tag’s operating principles. The SLPA-based chipless tag is suitable for conveyor belt applications due to the cascaded placement of its antennas. Carbon-nanotube-loaded (CNL) chipless tags are a novel and unique example of RFID technology and nanotechnology combining to create a novel RFID tag and sensor module. The CNL chipless RFID tag comprises a conformal UHF RFID antenna and a single-walled carbon nanotube (SWCNT) designed for toxic gas detection [69]. The CNL chipless RFID tag is shown in Figure 19. It is important to note that both the antenna and SWCNT were printed using inkjet printing technology for the first time. The chipless tag antenna is a bowtie meander-line dipole antenna. The SWCNT is placed between at the input port of the antenna in order to enable data encoding. The SWCNT is highly sensitive to the presence of ammonia (NH3), and its impedance characteristics when placed in air and NH3 are shown in Figure 20. From Figure 20, it is clear that the impedance of the SWCNT varies depending on the presence or absence of NH3 in the environment. The CNL chipless RFID tag operates by varying the amplitude of the backscattered signal, depending on the concentration of NH3, as shown in Figure 21. Amplitude variation of the backscattered signal is due to the RCS variation influenced by the change of the impedance of SWCNT. The amplitude variation of the backscattered power from the tag can be detected at the reader end and decoded to estimate the level of NH3.

(b)

–50

Resistance in NH3 Resistance in Air

Figure 20. Measured impedance characteristics of single-walled carbon nanotube in air and ammonia [69].

Power Reflection Coefficient (dB)

Figure 18. Stub-loaded-patch-antenna-based chipless RFID tag comprising three patch antennas loaded with meander line stubs [68].

0 Air NH3 Flow

–5 –10 –15 –20 0.6

0.7

0.8 0.9 Frequency (GHz)

1

Figure 21. Power reflection coefficient of the carbonnanotube-loaded chipless RFID tag before and after gas flow [69].

Conclusion An overview of reported chipless RFID tags in open literature and on the market has been presented. As the requirement for cheaper RFID tags for various

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applications grows, there are a greater number of different chipless RFID tags that can be classified in a wide range of different types. This article reports the first classification of chipless RFID tags, which classifies 14 different chipless tags in three main categories. The main classification of chipless tags is based on modulation techniques, which are TDR-based, spectral signature-based and amplitude/phase backscatter modulation-based chipless RFID tags. All three types of tags can be either printable or nonprintable, which determines their eligibility for certain applications, robustness and cost. Although the majority of chipless tags are still in prototyping stage it remains to be seen whether they will make it into the mainstream market. However, the progress of chipless RFID technology in recent years enthusiastically suggests that the best of chipless RFID is yet to come.

Acknowledgment This work was supported in part by the Australian Research Council under Discovery Grant DP0665523: Chipless RFID for Bar code Replacement.

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