Chipless RFID Bar Code of the Future

Digital Object Identifier 10.1109/MMM.2010.938571

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.

Chipless RFID:

Bar Code of the Future

R

data capturing technique that utilizes radio fre-quency (RF) waves for automatic identifi cation of objects. RFID relies on RF waves for data transmission between the data carrying de-vice, 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 interro-gation signals to an RFID tag that is to be identi-fied; 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 com-puter 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 process-ing and is powered by extractprocess-ing power from the inter-rogation 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 transpon-ders, which use on-board power supply (such as batteries) are called active RFID tags. Passive RFID tags offer lower prices at

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 transpon-ders are found on today’s market and are comprehen-sively 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 chi-pless 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

1970s. Though bar codes are printed in marks and spaces and are very cheap to implement, they pres-ent undeniable obstacles in terms of their short-range readability and nonautomated tracking. These limi-tations 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 dis-tance. The obstacles of reading range and automa-tion 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 develop-ments 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 pre-sented in [11]. Fletcher advocates that application spe-cific IC (ASIC) design and testing along with the tag antenna and ASIC assembly result in a costly man-ufacturing 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 standard-ized 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 transis-tors, 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) Global Network Host Computer RFID Reader RFID Tag Clock Data

Figure 1. Block diagram of a typical RFID system.

ASIC Design ASIC Manufacturing ASIC Testing Antenna Manufacture Tag Assembly Conversion to Label/Package

to design an RFID ASIC with less than 8,000 transis-tors. Although this will reduce the price of the sili-con 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 elec-tronics 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 tradi-tional silicon ASICs have emerged. These tags, and therefore systems, are known as chipless RFID sys-tems. The expected cost of chipless RFID tags is below US$0.01. Most chipless RFID systems use the electro-magnetic (EM) properties of materials and/or design various conductor layouts/shapes to achieve particu-lar 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 consid-ered to be commercially viable or available. The chal-lenge 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 catego-rize 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 Refl ectometry-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 applica-tions. The disadvantages of these tags are the num-ber 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 TDR-based 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

TDR based Spectral

Signature Based

Nonprintable Printable _{Chemical Planar Circuits}

TFTC Delay-Line-Based Tags Nanometric Materials Capacitively Tuned Dipoles Space Filling Curves LC Resonant Ink-Tattoo Chipless RFID Amplitude/Phase Backscatter Modulation Based Left-Hand (LH) Delay Lines Multiresonator Based Stub-Loaded Patch Antenna Remote Complex Impedance Carbon Nanotube Loading Multiresonant Dipoles TDR Based SAW Tags

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

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].

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 functional-ity. 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 delay-line, 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 inter-rogation 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 Antenna

Interdigital Transducer Reflectors

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

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 Delay Line

Figure 6. Delay-line-based chipless tag with patch antenna and delay line [43].

Amplitude Input Signal Reflected Signal “Generated ID: 011” Reflected Signal “Generated ID: 110” Pulse Position Modulation Code Representation 000 001 010 011 011 100 101 110 110 111Time

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

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 orienta-tion 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 distin-guish 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 par-ticles 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 iden-tification 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 cre-ated 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 lim-ited 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 flex-ible laminates and polymer substrates. Capacitively tuned dipoles were first reported by Jalaly [50]. The

chipless tag consists of a number of dipole antennas, which resonate at different frequencies. The capaci-tively 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 correspon-dence 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

…

First Bit 11th Bit

Laminate (Dielectric) Dipole (Conductor)

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

–15 –20 –25 –30 –35 –40 –45 –50 RCS (dB) 0.5 0.6 0.7 0.8 0.9 Frequency (GHz) 1 y x Ey

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

Piano curves, which create five peaks in the radar cross-section (RCS) of the tag. The chipless tag was suc-cessfully 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 prin-ciple 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, informa-tion storage (1 bit), operating bandwidth, and multiple-tag collision. These multiple-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 Uni-versity [53]. The chipless tag comprises three main components: the transmitting (Tx) and receiving (Rx) antennas and multiresonating circuit. A block dia-gram of the integrated chipless RFID tag with basic components is shown in Figure 10.

The multiresonator-based chipless RFID tag con-sists of a vertically polarized UWB disc-loaded mono-pole 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

cascaded spiral resonators, which introduce attenu-ations and phase jumps at particular frequencies of the spectrum. After passing through the multireso-nating 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 contain-ing 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 multiresonator-based 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 multiresonator-based 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 sec-ond tag antenna with a novel multiresonant dipole antenna [59]. The multiresonant dipole antenna com-prises 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 encod-ing 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 bot-tom (ground) layer are fed by a prolongation of the ground plane with the prolongation impedance being 50 V. The half wavelength dipole anten-nas produce peaks in the return loss at their resonant frequencies. By removing any of the half wavelength dipoles, the corresponding resonant peak disappears without influencing the reso-nances of the other dipoles. The main benefit of using the multiresonant dipole antenna First Resonator Second Resonator Third Resonator Nth Resonator UWB Monopole Rx Antenna UWB Monopole Tx Antenna Vertical Polarization Horizontal Polarization Multiresonator

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

Amplitude-Phase-Backscatter-Modulation-Based Chipless Tags

Amplitude/Phase backscatter modulation-based chi-pless RFID tags are the third type of chichi-pless 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 per-formed by varying the amplitude or phase of the back-scattered signal based on the loading of the chipless tag’s antenna. The variation of the loading is not con-trolled 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 microstrip-based 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 num-ber of bits that can be detected, and that data encod-ing 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 technol-ogy. 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

periodical discontinuities. The received interrogation pulse is reflected upon reaching each discontinu-ity 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

–16 –14 –12 –10 –8 –6 –4 –2 0 3 4 5 6 7 Frequency (GHz) Magnitude of Spectr al Signature (dB)

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

–60 –40 –20 0 20 40 60 80 3 4 5 6 7 Frequency (GHz) Phase of Spectr al Signature ( °)

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

Rx UWB Monopole Tx Multiresonant Dipole Antenna R Feed Extension Spacing

Figure 14. Multiresonant-dipole-based chipless RFID tag [59] (red—top layer, yellow—bottom layer) (© 2009 EuMA, reprinted with permission).

Tag Tx Antenna Tag Rx Antenna Multiresonator with 35 Spirals

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

a higher order modulation scheme, such as quadra-ture phase shift keying (QPSK), which enables greater throughput but requires a higher signal-to-noise ratio for successful tag detection [62]. The QPSK modula-tor used within the chipless tag is based on a vari-able 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-mum scattering antenna (such as a dipole) [63]. The difference between scattering and minimum scat-tering antennas is that, when terminated with an open or short, the scattering antenna should scatter back the same power, irrespective of the type of loss-less 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 chi-pless 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 chi-pless 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 induc-tive loadings of their antennas, it is possible to cre-ate 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 sig-natures of different chipless RFID tags with different inductive loadings.

Stub-loaded-patch-antenna (SLPA)-based chi-pless 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 incor-porate another degree of diversity, such as cross-polarization 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 prin-ciples 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 induc-tances. The chipless tag is interrogated by transmit-ting n different continuous wave (CW) signals from the reader at n frequencies corresponding to the oper-ating frequencies of each patch antenna. When the tag Reflection Section Γ1 Γ2 Γ3 ϕ1 ejϕ1 ϑ1 ϕ2ϑ2 ϕ3ϑ3 T ⋅ ejϕ0 T ⋅ ejϕ0 _{Delay Line} Section Carrier Phase Carrier Envelope t ϕ1 + 2ϕ0 ϕ2 + 2ϑ1 + 4ϕ0 ϕ3 + 2(ϑ1 +ϑ2)+ 6ϕ0

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

RFID Reader Free Space Loss Scattering Antenna Inductive/ Reactance Load Zfreespace Zfreespace S21 S12 S11 S 22 _Γ Z0

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

60 40 20 0 Phase Ripple ( °) –20 –40 –60 6.9 7.1 7.3 7.5 7.7 7.9 Frequency (GHz)

Figure 17. Variation of the chipless tag’s phase signature with inductance loading [67] (© 2007 EuMA, reprinted with permission).

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 dif-ferentiation. 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 place-ment of its antennas.

Carbon-nanotube-loaded (CNL) chipless tags are a novel and unique example of RFID technol-ogy and nanotechnoltechnol-ogy combining to create a novel RFID tag and sensor module. The CNL chi-pless 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 technol-ogy 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 characteris-tics when placed in air and NH3 are shown in Fig-ure 20. From FigFig-ure 20, it is clear that the impedance of the SWCNT varies depending on the presence or absence of NH3 in the environment. The CNL chi-pless RFID tag operates by varying the amplitude of the backscattered signal, depending on the con-centration of NH3, as shown in Figure 21. Ampli-tude variation of the backscattered signal is due to the RCS variation influenced by the change of the impedance of SWCNT. The amplitude varia-tion of the backscattered power from the tag can be detected at the reader end and decoded to estimate the level of NH3.

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

Meandering O/C Stubs

L1 _{L2 L3}

Inset Length Inset Width

Element 1Spacing Element 2 Element 3

E-Plane H-Plane

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

15 mm 27 mm 25 mm 36 mm SWCNT SWCNT (a) (b) 118 mm

Figure 19. Carbon-nanotube-loaded chipless RFID tag on flexible laminate with (a) dimensions and (b) actual photograph [69]. 0 –5 –10 –15 –20 0.6 0.7 0.8 0.9 1 Frequency (GHz) P o w er Reflection Coefficient (dB) Air NH3 Flow

Figure 21. Power reflection coefficient of the carbon-nanotube-loaded chipless RFID tag before and after gas flow [69]. 125 100 75 50 25 0 75 50 25 0 –25 –50 Resistance ( Ω ) Resistance ( Ω ) 0 0.2 0.4 0.6 Frequency (GHz) 0.8 1 Resistance in NH3 Resistance in NH3

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

applications grows, there are a greater number of dif-ferent 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 classi-fies 14 different chipless tags in three main categories. The main classification of chipless tags is based on modulation techniques, which are TDR-based, spec-tral 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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