Chipless RFID tags fabricated by fully printing of metallic inks

Botao Shao & Qiang Chen & Yasar Amin & Ran Liu & Li-Rong Zheng

Received: 20 June 2012 /Accepted: 24 May 2013 /Published online: 22 June 2013# Institut Mines-Télécom and Springer-Verlag France 2013

Abstract This paper reviews recent advances in fullyprinted chipless radio frequency identification (RFID) tech-nology with special concern on the discussion of codingtheories, ID generating circuits, and tag antennas. Two typesof chipless tags, one based on time-domain reflections andthe other based on frequency domain signatures, are intro-duced. To enable a fully printed encoding circuit, linearlytapering technique is adopted in the first type of tags to copewith parasitic resistances of printed conductors. Both simu-lation and measurement efforts are made to verify the feasi-bility of the eight-bit fully printed paper-based tag. In thesecond type of tags, a group of LC tanks are exploited forencoding data in frequency domain with their resonances.The field measurements of the proof-of-concept of the tagproduced by toner-transferring process and flexible printedcircuit boards are provided to validate the practicability ofthe reconfigurable ten-bit chipless RFID tag. Furthermore, anovel RFID tag antenna design adopting linearly taperingtechnique is introduced. It shows 40 % save of conductiveink materials while keeping the same performance for con-ventional half-wave dipole antennas and meander line an-tennas. Finally, the paper discusses the future trends ofchipless RFID tags in terms of fabrication cost, codingcapacity, size, and reconfigurability. We see that, coupledwith revolutionary design of low-cost tag antennas,fabrication/reconfiguration by printing techniques, movingto higher frequencies to shrink tag sizes and reducemanufacturing cost, as well as innovation in ID generatingcircuits to increase coding capacities, will be important

research topics towards item-level tracking applications ofchipless RFID tags.

Keywords Chipless RFID . Fully printing . Frequencyresonance . LC tank . Linearly tapering . Paper substrate .

Time-domain reflections

1 Introduction

Radio frequency Identification (RFID) technique is the useof radio frequency signals for access of information on thetags attached on objects. RFID system consists of two mainhardware components: a transponder/tag containing a se-quence of electronic codes used for identification of anobject and a device called interrogator/reader for collectinginformation from a tag. Inside an RFID tag, typically, is anintegrated circuit (IC) chip used for reading/writing andstoring functionalities.

RFID tags allow for automated tracking, as well as longrange and non-sight-of-light reading, which make them-selves a competitive replacement to barcodes that have beenpervasively used in recognition applications, for example,supply chains. The prime reason why RFID tags have notreplaced barcodes is the high cost, which includes the ex-penditure of chips, batteries, antennas, and assembly pro-cesses [1, 2]. So far, the extreme majority of RFID tags arepassive ones, which eliminate the use of batteries forpowering up the operation of IC chips, leading to the rela-tively lower cost.

Even so, these passive tags scarcely expend the applica-tions in the tracking and identification of item-level consum-er goods, where the cost instead of advanced functions is theforemost concern. In fact, with the evolution of silicon pro-cess, the prices of silicon IC chips attached on the passivetags have been substantially reduced, relative to a few de-cades ago. Further reducing the tag price, unfortunately, is

B. Shao :Q. Chen :Y. Amin : L.<R. ZhengiPack Center & ES Unit, KTH (Royal Institute of Technology),16440 Stockholm, Sweden

B. Shao (*) : R. Liu : L.<R. ZhengSchool of Information Science and Engineering, Fudan University,200433 Shanghai, Chinae-mail: botao@kth.se

Ann. Telecommun. (2013) 68:401–413DOI 10.1007/s12243-013-0378-3

fairly challenging in the foreseeable future due to two majorlimitations. One is the astoundingly increased capital invest-ment (tens of billions of dollars) spent to build up an ad-vanced foundry for manufacture of smaller silicon chips. Theother is the assembly process used to integrated silicon chipswith tag antennas and substrates. The assembly procedure isnot standardized yet; thus, the production efficiency is lim-ited. In addition, the assembling is increasingly tough due toboth the continuously shrunk size of chips and the flexibilityof substrates that make them stretchable all the time. Thepossibly lowest cost per tag is approximately 5 cents with theemployment of modern, inexpensive chips for passive RFIDtags [1, 3]. As a result, it poises an insurmountable obstacletoward the realization of item-level tags that are expected tobe less than 2 cents or even at sub-cent level [1, 4, 5].

In recent years, chipless RFID technique has attractedsignificant attention in the pursuing of low-cost RFID tags.By eliminating the use of silicon IC chips, chipless RFIDtags can offer more competitive price than normal silicon-ICbased tags do. Besides, chipless tags possess longer commu-nication range since it is unnecessary to use transistors as dosilicon-based tags where a threshold voltage is a requisite topower up IC chips [6]. Currently, the most successful com-mercialized chipless product is surface acoustic wave (SAW)tags [7, 8]. The most attractive merits of these tags exist inthe compatibility with present RFID band regulations(2.45 GHz) and allowing for large coding capacity (96-bit).The cost of the SAW tags, however, is close to the IC-basedcounterparts, since they necessitate the use of sub-microlithographic process and expensive piezoelectric substrates[2, 9].

A more aggressive approach is to develop printablechipless RFID tags. The low cost expectation is mainlydriven by the fact that printing techniques are high-

throughput processes, which can produce functional patternsat an astounding rate (tens or even hundreds of meters persecond). Additionally, a large amount of expensive electron-ic materials is likely to be saved in inkjet or gravure printingtechniques, since they are fully additive processes—solelydepositing material on demanded positions. Also, printingtechniques are compatible with cheap flexible substratesincluding plastic and paper sheets, holding high potential tofurther bring tag costs down [10–12].

As a matter of fact, numerous investigations on printablechipless RFID tags have been suggested in recent years. Oneof the most promising families is based on electromagneticproperties of metallic structures. Table 1 shows a summary ofthe recently reported chipless RFID tags based on metallicstructures. In the table, different chipless RFID tag solutionsare compared in terms of coding principles, components,operating frequencies, scalability, sizes, and coding capacity.According to adopted encoding principles, these tags can beclassified into three categories:

& Time–domain-based RFID tags [9, 13, 26],& Frequency domain footprint-based RFID tags [6, 18, 27],

and& Symbol-based RFID tags [5].

Time–domain-based RFID tags usually exploit transmis-sion lines (TLs) to propagate RF waves and utilize eithermicrowave circulators [26, 28] to introduce propagation de-lays or capacitors [13] to introduce reflections, for the en-cryption of binary ID codes. Another interesting time–do-main-based chipless RFID tag is reported in Sanming et al.[29], where coplanar waveguide lines terminated with open,short, or matched loads are connected with monopole anten-nas to encode ID data. The chipless RFID tag is capable ofbeing recognized not only by measuring radar cross section

Table 1 Summary of recently reported printable chipless RFID tags

Classes Components Operating frequency Scalability Size Coding capacity

Time domain TL, capacitor [13, 14] Various RFID bands N ∼4×6 cm2 8

Frequency domain Dipole barcode [15] 5–6 GHz Y N/A 5

Multiband RF barcode [16] 2.4–2.4835 GHz 5.15–5.35GHz 5.47–5.875 GHz

Y N/A 5

SRR array [17] 8–12 GHz Y ∼8.6×5.4 cm2 4

Coplanar strips [18] 2.5–7.5 GHz Y 4×2 cm2 22.9

Space filling curve [19] 3–4.5 GHz Y N/A 3

Single LC [20] 8.2 MHz Y ∼4×4 cm2 1

Spiral resonator [21] 3–7 GHz N ∼8.8×6.5 cm2 35

Elliptical dipole [22] ∼0–15 GHz N N/A 3

Complex impedance [23] 6.9–7.8 GHz N ∼0.17×0.43 cm2 4

Patch antenna [24] 2.17, 2.33, 2.52 GHz N 12.5×5.5 cm2 3

Dual polarized I slot [25] 6–13 GHz N 1.65×1.65 cm2 16

Symbol SAR code [5] N/A N 1.08×0.15 cm2 96

402 Ann. Telecommun. (2013) 68:401–413

magnitude in frequency domain but also by calculating time-domain reflections. For analysis simplicity, the tag is countedas time-domain chipless RFID tag in the paper. The lengthsof TLs are irrelevant to the operating frequencies of the tags,which means that these tags can operate in unlicensed fre-quency bands, especially industrial, scientific, and medicalradio (ISM) band that conventional chip-based RFID tags areworking with [28]. The virtue is particularly significant asRFID tags operating on other licensed bands are not allowedand impossible to be populated at the global level. Thecomparatively large size and limited demonstrated codingcapacity seems as the disadvantages of these time–domain-based tags. They are still, however, a promising choice forlow-cost item-level RFID solutions, particularly with theopen possibility for the enhancement of coding capacityand the miniaturization of tag size.

In frequency domain, it is likely to encode ID codes intospectrum signatures by placing inductors nearby a microstripline [30] or by assembling an array of metallic microwaveresonators with different resonant frequencies such asmicrostrip dipoles [15], split ring/four-square resonators [17],and “C” like resonators [18]. The ID codes can be recognizedby observing the presence of either resonant frequencies [18]or phases [24, 31], or the shift of phases and frequencies [18]. Itis believed that frequency–domain-based chipless RFID tagscan offer larger coding capacity in comparison with the time–domain-based counterparts. The tags with over 20- or 30-bitcoding capacity have been developed upon frequency-signature theory [18, 25, 32]. Principally, countless bits canbe encrypted in a frequency–domain-based chipless tag withsufficient number of resonators adopted to store data. In prac-tice, however, it is a challenging execution to code too largenumber of bits in such tags, primarily owing to the largeoccupation of RF spectrum. For instance, the dual polarizedchipless RFID tag recently proposed in Islam and Karmakar[33] allows for up to 16-bit coding volume but at the cost of theexploitation of 7 GHz band width, ranging from 6 to 13 GHz.Owing to less efficient usage of spectrum resource, frequency–domain-based chipless RFID tags can scarcely be deployed inISM band. A possible solution is to place frequency–domain-based signatures in ultra-wide-band (UWB), typically referringto the spectrum from 3.1–10.6 GHz [34]. To use UWB band,the corresponding regulations, especially the emitting powerrestrictions formulated by Federal Communication Committee(FCC), are mandatorily obeyed. The FCC power spectrummask under UWB is around −41.6 dBm/MHz, which is ruledin so a small magnitude as to avoid interference with othercharted radio applications. Conventional UWB signals withcontinuous wave emitting format, however, are hardly to beused in these tags owing to weak magnitude of the transmittedsignals under such restriction. A potential method is alterna-tively to use impulse-radio UWBwave with low duty cycle, sothat the energy in a transmitting signal is still intense in a short

period (for instance, 100 ps) and tags can have an acceptablereading range and signal-to-noise ratio while it offers largecoding capacity advantage [35]. Besides operating frequency,another important parameter for frequency-based chiplessRFID tags is the size mainly subject to the increasing numbersof used resonators. It has also a close association with theoperating frequencies. For instance, moving the frequency toupper frequency bands may lead to the size reduction, which iscalled scalability. The tags with scalability in the Table 1 haspromise to further shrink the sizes and fit them into a compactshape.

Besides above-mentioned two types of printable chiplessRFID tags, a novel, symbol-based RFID tag has been pro-posed by Inksure Company recently [5], in which the diffrac-tion of metallic bars are exploited to encode data. The objectdepth and orientation in relation to a reader can be detected bymonitoring and post-processing the diffracted radio waveswith mathematic models. Although up to 96-bit coding vol-ume is promising, the technique is still in the infancy.

These printable chipless RFID tags have demonstrated thepotential cost advantage compared with the IC-based coun-terparts. Nonetheless, the printable chipless RFID tags real-ized by all printing process and inexpensive paper substrateshave not yet been systematically reported. The present paperis intended to overview the relevant pioneering efforts madeby our group and at the same time predict the developmenttrend of printable chipless RFID tags. Firstly, a time–do-main-based chipless RFID tags fabricated by all printingprocess will be introduced [36]. Linearly tapering techniqueis employed in the tag to overcome excessive parasitic re-sistances of printed conductors on paper substrates. Besides,a frequency–domain-based chipless RFID tag is unveiled[37]. The tag contains a group of inductor–capacitor (LC)tanks with distinct resonance frequencies for storage of up toten-bit data. The layout of each LC coding tank isreconfigurable by using punching and filling processes,which may offer extra freedom in application and recycla-bility. In addition, an innovative tag antenna design tech-nique, linearly tapered antennas (LTA), is exhibited [38]. Theantennas allow for 40 % save in material consumption whilemaintaining the same performance compared with conven-tional dipole antennas and meander-line antennas (MLAs).

The paper is organized as follows. In Section 2, the time-domain reflection-based chipless tag will be introduced. Thetechnical challenges caused by inkjet printing technique willbe addressed and solved by using linearly tapering tech-nique. In Section 3, we will introduce an LC–tank-basedchipless tag. The coding principle and the fabrication processwill be presented. Measurement will be conducted to vali-date the possibility of the proof-of-concept design. InSection 4, the use of linearly tapering technique will beextended onto the design of RFID tag antennas for thesignificant reduction of material consumption to enable the

Ann. Telecommun. (2013) 68:401–413 403

realization of low-cost tag antennas. Section 5 will discussthe future trend of the development of chipless RFID tags,and “Section VI” will conclude this paper.

2 All-printed paper-based chipless RFID tag

The operation principle of the time-domain chipless RFIDsystem is shown in Fig. 1. Like other passive RFID tags, thesystem consists of a reader, which transmits interrogationalsignals and collects back-scattered RF signals, as well as a tag,which contains unique identification information. Moreover, anetwork server or computer connected to Internet is requiredto share the collected data with globally distributed terminals.But the tags in the RFID system, different from IC-basedRFID tags, are fully printed from metallic ink. Together withinexpensive paper substrates, it is promising to producechipless tags at sub-cent cost. In conventional RF circuits,matching status is pursued, and impedance mismatches areabout to be avoided with greatest effort for the transmission ofenergy without losses. In this tag, however, mismatches inimpedance are used for coding of binary digits of “0” or “1.”

Figure 2 shows the coding principle of the chipless tag. Astep signal propagates forward until it encounters a parallel-connected capacitor, where part of the signal will be reflectedback in the format of a spike-like voltage drop, and the rest willcontinuously march. The voltage drop is detected in the readerpart, representing the code “1”; otherwise, it represents “0”.

Figure 3 exhibits the circuit diagram of an eight-bit tagusing this coding principle. The microstrip line (ML) is usedto transmit RF signals, and eight capacitors behaving asencoding elements are placed with equal distance to producereflections. The circuit is terminated with a 50 Ω resistor atthe end to absorb redundant power.

The coding sequence of the ID generating circuit is shownin Fig. 4. The identical propagation delay between twoadjacent points is set. With all capacitors connected, a se-quence containing eight digits of “1” will be encoded in thetag. Similarly, disconnecting a specific capacitor from theML will enable the encoding of a digit of “0”.

The proof-of-concept of the tag ID generating circuit wasoriginally realized on conventional printed circuit boards

(PCBs) using lithography-based etching process in previouswork [14]. The geometries of two components, ML andshunt capacitors, were evaluated according to time-domainreflecting theories. The length of the ML, l, was calculatedusing the following expressions:

V ¼ l

t¼ c

ffiffiffiffiεr

p ð1Þ

l ¼ cffiffiffiffiffiffiffiffiεreff

p t ð2Þ

where c=3×108 m/s is the velocity of light in vacuum, ɛreff=3.48 is the effective relative permittivity, and t is the flighttime. The width of the 50Ω ML was determined with ana-lytic formulas or LineCalc program embedded in the simu-lation tool of advanced design system (ADS).

The amplitude of the backward and forward waves, rep-resented by |Ab| and |Af| respectively, at a certain discontinu-ity point depends on the incoming signal amplitude, |Ai|, andthe reflection coefficient, τ, at that point:

Af

�� �� ¼ Aij j 1−τj j ð3Þ

Abj j ¼ Aij j τj j ð4ÞThe reflection wave at each encoding point holds the

same amplitude of |Ab|. τ is related to discontinuity imped-ance by

τj j ¼ Zl−Z0

Zl þ Z0

����

���� ð5Þ

Zl ¼ 1

jωCð6Þ

Fig. 1 Schematic block for thefully printed paper-basedchipless RFID tags

Fig. 2 ID data encoding based on capacitive impedance discontinuity

404 Ann. Telecommun. (2013) 68:401–413

where Zl is the capacitive impedance, Z0=50 Ω is the char-acteristic impedance of the ML. With the two equations, thevalue of each capacitor was determined.

The resultant layouts of Rogers 4350 PCB-based chiplesstag circuit are shown in Fig. 5. Figure 5a presents a four-bittag, where lumped capacitors were utilized, while Fig. 5bdisplays an eight-bit one with the use of planar capacitors.The simulation and measurement were conducted to verifythe technical feasibility of the tags. Figure 6a exhibits theeight-bit tag response to a sinusoidal interrogation signalpossessing 0.4 ns pulse width and 10 ns period in simulation.The corresponding time-domain reflectometry (TDR) mea-surement results are shown in Fig. 6b, which was obtainedwhen a 200 mV square signal with 25 % duty cycle and20 ms period were input. The good consistence between thesimulation and the measurement results confirmed the vali-dation of the time–domain-based tag. The next step istargeting to the realization of such a tag on paper substrateby using fully inkjet printing technique.

Nevertheless, it is impossible to plainly transfer the time-domain circuit onto paper substrates, due to the existence oftwo technical challenges associated with printed conductinglines. One is the thin thickness of the printed metallic tracks(∼0.5–1 μm). It gives rise to significantly severer parasiticresistance compared with the 18- or 35-μm-thick copperlines on PCBs. The other challenge is the inferior electricalconductivity. It is well known that the conductor lines pro-duced by metallic inks hold worse conductivity than the bulkcounterparts. According to experimental measurements, theprinted silver lines on HP photopaper (in the thickness of280 μm and the relative permittivity of 2.8) with 100 °C and30 min sintering process only allows for a conductivity ofapproximate 3×106 S/m, which is less than one twentieth ofbulky silver conductor. It has been proven that prolongingthe sintering time is not an effective pathway to noticeablyimprove the conductivity. A higher sintering temperature

will lead to a better conductivity, but paper substrates, par-ticularly for coated papers, can barely survive at tempera-tures higher than 100 °C due to the thermal instability ofsurfaces, for instance, cocking or rupturing problems. Con-sequently, the paper-based printed lines contain enormousparasitic resistance under both DC and RF conditions. Toreveal how large such parasitic effect is, simulations andmeasurements were performed by using the simulation toolof ADS and instrument of TDR, respectively. Figure 7 showsthe corresponding results. Figure 7a displays the top view ofa conventional microstrip line in the length of 130 mm,which has a uniform line width of 719 μm, so-called uniformmicrostrip line (UML). When a 0.2-V incident voltage isapplied, it is seen from Fig. 7b that the reflected voltagerapidly increases as the incident signal travels forward. Atthe end of the UML, a 0.25-V around voltage is observed,indicating that the total impedance has increased to 84 Ω.The measurement results which are in good consistence withthe simulation ones confirm the severe effect of parasiticresistance.

Overprinting process seems like a promising method toalleviate the effect at the first sight. Overprinting processemploys multilayer printing technique to increase the thick-ness of printed lines and thus improve the conductance.Nevertheless, it is an unfeasible way to adopt the techniquein the situation, since as many as 36 layers are demanded tomeet a comparable thickness (18 μm) to PCB lines, not tomention that a large thickness may substantially degrade theflexibility of printed tags.

Instead, linearly tapering technique is applied ontomicrostrip lines, so-called tapered microstrip line (TML), tosolve the problem. The structure of a TML is as shown atFig. 7c. The TML line width increases linearly as the lengthextends, intending to make the parasitic impedance increaseat a slower rate. Meanwhile, the characteristic impedancewill decrease with the increasing line width. With the taper-ing method, it appears possible to keep the total impedance at50 Ω for the printed MLs.

To verify the feasibility experimentally, an inkjet-printedpaper-based TML is designed, manufactured, and character-ized. The total length of the TML is 130 mm, and the initialline width is 719 μm while the ended width is 1678 μm. It isnoticed that the reflected voltage for TML is kept at 0.2 V forthe all time, as shown in Fig. 7d. The result proves that thetapering technique is an effective approach to produce aprinted 50-Ω ML. The realization of the 50-Ω ML is criticalnot only for implementation of the all-printed RFID tags inthis paper, but also for the realization of other RF circuits and

Fig. 3 Circuit schematic of theeight-bit chipless RFID tag

Fig. 4 Reading sequence of eight-bit ID codes based on flight delay

Ann. Telecommun. (2013) 68:401–413 405

systems to be manufactured on low-cost flexible sheets byprinting techniques.

Based on the 50-Ω TML, the tag prototype (excluding theantenna part) was realized on HP photopaper with Cabotnano-silver ink, as shown in Fig. 8. In the tag, the TMLlength was calculated as 260 mm, and the capacitors were0.48, 0.94, 1.84, 2.97, 6.06, 8.83, 11.6, and 18.5 pF, respec-tively. Furthermore, a reference tag adopting conventionalUML technique was simultaneously fabricated. It is worthnoting that Fig. 8 shows merely the tag prototype without a50 Ω resistor at the end of the transmission lines. Althoughadding a printed resistance is not difficult, we still usedlumped resistor for simplicity of experiment. The readabilityof the two tags was examined, and the measurement resultsare displayed in Fig. 9. Figure 9a shows the result of theUML tag configured with eight-bit codes, “01011000.” Dueto the limited thickness and electrical conductivity, theprinted ML suffered from the severe effect of parasitic resis-tance, which in turn affected the propagation of a time-domain signal. The magnitude will be attenuated on a con-ventional ML produced by inkjet printing technology, givingrise to the failure to recognize the signal based on a givenvoltage level. This is particularly true when the signal isreflected back since it undergoes attenuation twice. It isobvious that the codes can hardly be recognized due to thepresence of parasitic resistance. On the contrary, the TML tagoffers high-quality signals, as shown in Fig. 9b. Moreover, thetag simulations were also conducted for cross-validation. Theresults were in good consistence with the measurement ones,

thus confirming the feasibility of the all printed paper-basedchipless RFID tag.

The ID generating circuit demonstrated here is the firstone realized by using all printing process and paper sub-strate. Currently, the coding capacity is eight-bit. Even so,the volume is larger than other published time–domain-based chipless RFID tags [9, 26, 28]. Although it is stillinadequate for the item-level tracking of massive productslike supermarket consumer goods, the chipless tag may findapplications in the tracking and identification of indoorgoods, since it can offer 196 sequences of codes. Moreover,the tag ID coding circuit is 4.9×9.5 cm2, and the size will beslightly larger if integrated with a printed antenna. The sizecan be reduced by two means. One is to choose paper sub-strates with higher dielectric constant and thinner thicknessto decrease both the length of the delay line and the sizes ofcapacitors. The other is to increase the operating frequencyto reduce the size of capacitors. By combining these twomethods, it is possible to shrink the tag size below 3×6 cm2

(smaller than a credit card) for ease of tracking items.

3 LC–tank-based chipless RFID tag

Apart from the time–domain-based RFID tag, a ten-bit fre-quency–domain-based RFID tag containing LC resonatingtanks is also suggested. The tag system, as shown in Fig. 10,consists of two loop antennas—one is transmitting antenna(TA), and the other is receiving antenna (RA). In operation,

Fig. 5 Layout photograph ofthe rigid Rogers-based chiplessRFID tag. a Four-bit circuit, beight-bit coding circuit.

Fig. 6 a Simulation and b measurement results for the eight-bit tag based on Rogers substrate with planar capacitors

406 Ann. Telecommun. (2013) 68:401–413

the TA transmits a sweeping frequency wave as an interroga-tional signal. When an LC–tank-based chipless tag comes intothe detection zone, the interrogational signal is modulatedaccording to the ID data encoded in the tag. After RA capturesthe modulated signal and transports it to spectrum analyzercircuits in the reader, the ID data will be recognized in the lightof LC resonances in the received signal.

The tag is based on sympathetic oscillation principle. Onewell-known application of the principle is electronic surveil-lance article labels. The simplest format of such a label onlycontains one LC tank circuit resonating at a specific frequen-cy point called intrinsic resonance frequency. When an al-ternating magnetic wave penetrates such a label, part of theenergy will be absorbed and consumed by the LC tank. Bydetecting the received wave, it is possible to distinguishwhether the label is present in the detection zone. Such a

tag, however, suffers from the limited capability, merely one-bit, limiting the applications in item-level tracking.

In the present tag, an array of ten LC tanks is employed toencode ten-bit data, as shown in Fig. 11a. Different resonance

b

a

dc

Fig. 7 aUML structure, b TDRmeasurement and simulationresults for the paper-based UMLwith 130 mm, c TML structure,d TML results

Fig. 8 Photograph of all printed paper based eight-bit ID generatingcircuit

a

b

Fig. 9 Measurement results for eight-bit tag with a sequence of“01011000” codes. a UML tag and b TML tag

Ann. Telecommun. (2013) 68:401–413 407

frequencies, ranging from 10 to 100 MHz, are assigned todifferent LC tanks. Binary ID data can be encoded in the tagby controlling the working conditions of these tanks. Forexample, an LC tank with 50 MHz resonance frequency canrepresent the coding of the digit of “1,” if it resonates at thepoint; otherwise, it means that the digit of “0” is encoded.

Practically, the LC tank circuit can be designed withreconfigurability, as shown in Fig. 11b. The encoding pointin the layout is left unfilled during manufacturing to deacti-vate the LC tank, but they can be filled by inkjet printingprocess in the use to activate the coding circuit. Thereconfigurable design is expected to significantly increasethe use flexibility, particularly when the ID codes have to bedetermined on situations. Moreover, a reconfigurable tagmay be repetitively used by using mechanical punchingand filling processes.

Currently, the chipless tag was implemented on a flexiblePCB by using toner-transferring process. First, the layout of

the tag was printed onto photo-papers with a laser printer.Then the printed layout pattern was attached onto a coppersurface, and a heat iron was utilized to heat the backside ofthe pattern to transfer the pattern onto copper surface. Thecopper under the protection of the pattern was preservedwhile the copper uncovered by the pattern was etched inthe following etching process. Finally, drilling a via andfilling it with silver inks was conducted to form the resultantsample of a LC tank coding unit, as shown in Fig. 12a.

The measurement setup for the chipless RFID tag isshown in Fig. 12b. The TA and the RA were placed apartwith 21 cm distance, and both were connected with a vectornetwork analyzer (VNA) as parts of a reader. Between theTA and one port of the VNAwas employed a power ampli-fier. The resonances of LC tanks were monitored by observ-ing the variation of forward voltage gains, |ΔS21|. The mea-surement results are shown in Fig. 13a and b. It is observedthat all the codes are located at multiple frequencies of 10MHz.Two groups of codes, “0010110111” and “1111111111”, areclearly identified. Among all the codes, the minimum signal-to-noise ratio (SNR) is 10.24 dB, indicating the strong readabilityof the tag. All these results prove the feasibility of the LC–tank-based chipless RFID tag.

The tag operating frequency ranges from 10 to 100 MHz,which means that it can be used in metal/water containedenvironment without significant performance degradation.The tag size is approximate 10×20 cm2, which might beinappropriate for tagging small items like banknotes andpassports. Fortunately, the size is allowed to be considerablyshrunken by using two techniques. One is to use a sharedinductor and a group of different capacitors. The inductorcan connect to a different capacitor for encoding of a distinctdigit each time. The other technique is moving the tagresonances to higher frequencies. With increase of frequen-cies, the sizes of inductors and capacitors will considerablyreduce and at the same time leading to the tag cost reduction.

In the tag, inkjet printing technique is simply used forcoding, not to manufacture the whole layout. This is becausein the current low frequency band, the skin depth is muchlarger than the thickness of conventional printed tracks. Thethin thickness of inductors and capacitors produced by additiveprinting process will finally cause the poor quality factor andSNR of the tag. However, while shifting to higher frequencies,especially when the skin depth can compete with the thicknessof printed metallic lines, the tag with acceptable performance islikely to bemanufactured by high-throughput printing process-es. Benefiting from a higher operating frequency, it is possibleto further shrink the size of each coding tank and arrange moreunits within a specific area. Another conceivable optimizationmeasure is to replace the parallel-plate capacitor with a copla-nar interdigital one, thus realizing a single-plane chipless RFIDstructure that can be directly printed on consumer goods. Allthe schemes are promising to making the LC–array-based

Fig. 10 RFID system block diagram for LC resonance-based RFID tag

Fig. 11 a Chipless tag containing ten LC resonating circuits, breconfigurable LC layout design

408 Ann. Telecommun. (2013) 68:401–413

chipless RFID tag more compact in size, stronger in codingcapacity, and more inexpensive in cost.

4 Linearly tapered tag antennas

Apart from ID generating circuits, printed antennas will alsoplay an important role toward the realization of fully printedchipless RFID tags. A simple and cost-effective antennadesign method, LTA, is exhibited for significantly drivingdown the fabrication cost of tag antennas.

It is well known that a conventional dipole antenna used inRFID tags is with uniform line width. Yet, few considered thatwhy the uniform line are necessarily used. In fact, uniformantennas may not be the most cost-efficient way for a tag. Fora half-wave dipole antenna, the current distribution along thelength direction can be approximated by a cosine plot [39],

I zð Þ ¼ I0cosπz=l� � ð7Þ

where I0 is the current at a feed point, z the distance from thefeed point, and l the length of the antenna. The current distri-bution pattern is plotted in Fig. 14 with black circles, while thevariation of line width is denoted in the blue dashed lines. Inthis figure, the magnitudes of currents and line widths havebeen normalized to the ones at feeding point, i.e., the center ofthe antenna. It is evident that the same amount of metal hasbeen assigned at distinct positions, wherever it is at the feeding

Fig. 12 a Demo-of-concept forLC–tank-based chipless RFIDtag, b measurement setups. Thetag was inserted between twoantennas in vertical way duringmeasurement

a

b

Fig. 13 Measurement results for the tag with a “0011011110” and b“1111111111” codes

Ann. Telecommun. (2013) 68:401–413 409

point, where the maximum current flows, or at the two ends,where minor current runs. This indicates that plenty of expen-sive metallic materials may have been wasted on where minorcurrent flows. The situation is particularly challenging foritem-level tracking applications, where low-cost RFID tagsrequire a cost-effective antenna design.

To save materials, linear tapering technique is extendedonto the design of dipole antennas. The LTA is referred to asan antenna with linearly varied line width. As current densitydecreases, the antenna line width reduces, by which mate-rials are expected to be saved. The principle of the LTAs hasbeen shown in Fig. 14 by red solid lines. In essence, thetapering method is using linearly fitting technique to emulatethe tendency of current distribution. Compared with com-mon dipole antennas, the LTA allows for almost 50 % re-duction in antenna areas and as a consequence on cost.

To verify feasibility of the tapering technique, two 869-MHz planar dipole antennas, one in uniform line width andthe other in tapering width, were designed with the layouts asshown in Fig. 15a and b and simulated by using the electro-magnetic simulation tool—Ansoft Designer™. These anten-nas were assumed to be manufactured by using all ink-jetprinting process on polyimide substrate. In the simulation,the polyimide was utilized with a thickness of 51 μm and adielectric constant of 3.6. The line width of the uniformantenna was fixed at 5 mm, while for the tapered one theline width at the center point was 5 mm, and then the linetapered down until zero at the two ends. The antenna thick-ness was fixed at 0.5 μm, and the conductivity was assumed

as one third of bulk silver material, the best conductivityachieved with printing method [40]. The length of the ta-pered antenna was set longer than the uniform one to pull theresonant frequency of the antenna back to 869 MHz.

It was found that the current density of the tapered anten-na was more uniform than that of the conventional one. Thecurrent density in the conventional dipole antenna was from0.02–1.63 A/m, while the tapered antenna possessed thecurrent density from 1.55–2.14 A/m. The latter shows anevidently higher and more uniform current density. Addi-tionally, the two types of antennas were compared in theterms of radiation efficiency, antenna gain (dBi) and area.The results shown in Table 2 indicate that the tapered anten-na is able to reduce material consumption by 41 %, includingpolyimide and nano-silver ink, with negligible degradationin radiation efficiency and antenna gain.

A MLA was taken as another example to examine thetapering technique. Thanks to the electrically small size andlow profile, MLAs have been widely used in RFID tags. TwoMLAs, one with uniform width and the other with taperedline width, were designed and compared in simulation. Thelayouts are as shown in Fig. 16a and b, respectively. Theelectromagnetic simulations were conducted to the two an-tennas, and the results were summarized in Table 3. From theresults, it is found that as high as 47.5 % material consump-tion is saved at the cost of slight degradation in antennaperformance. These results further confirm the technicalfeasibility of the linearly tapering technique.

The drawback of the tapering technique exists in theantenna bandwidth. Under European RFID regulations,UHF antennas have a relatively loose requirement on thebandwidth (∼867.5–869 MHz), while in North AmericaContinent the regulatory band width is relatively wider(902–928 MHz). Two inkjet printed dipole antennas, oper-ating at 869 and 915 MHz, respectively, were evaluated. Thesimulation results showed that the linearly tapered 869-MHzantenna caused a decrease in bandwidth (−10 dB BW) from82 to 64 MHz, while the tapered 915-MHz one drops from90 to 71 MHz. Although the tapering technique negativelyinfluenced the antenna bandwidth, however, the reducedbandwidth is still much higher than the regulated values.Therefore, we can conclude that the application of linearlytapering technique on tag antennas is validate.

The tapering method, based on simple linear fitting prin-ciple, is pretty easy to be implemented in practice. The effect

Fig. 14 Working principle of a linearly tapered antenna

Fig. 15 a Conventional dipoleantenna and the surface currentdistribution, b linearly tapereddipole antenna and the currentdistribution

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in cost reduction, additionally, is significant. The taperedantennas will offer over 40 % reduction in material consump-tion while maintaining the similar performance with conven-tional counterparts, which will significantly drive down thecost of tag antennas and contribute to the realization of ultra-low-cost RFID tags.

5 Future trends

Presently, chipless RFID tags have become an importantresearch topic due to the immense business potential initem-level tracking applications. To realize such item-leveltags, however, a few technical challenges have to be solved,such as reducing fabrication cost, improving coding capaci-ty, shrinking geometries, enabling reconfigurability of IDgenerating circuits, as so on.

5.1 Low cost

The final cost for item-level chipless tags is most likelyless than one cent for the large volume deployments. Torealize the goal, on one hand, printable chipless RFIDtags have been regarded as one of the most promisingcandidates, particularly considering the potential to be

directly produced onto inexpensive paper or plastic pack-aging by using high-throughput printing techniques. Thetime–domain-based chipless RFID tag shown in the pa-per has demonstrated the feasibility of the solution. Toreduce cost, the sizes of the chipless RFID tags areexpected to be as small as possible for the lowering ofmaterial consumption and resultantly manufacture cost.On the other hand, the evolution of tag antenna designs,for instance, the suggested LTAs, is also expected tocontribute significantly in the driving down of the fabri-cation cost of printable chipless RFID tags.

5.2 Large coding capacity

One fact cannot be denied is that majority of chipless RFIDtags are suffering from the limited coding capacity, whichposes significant limitations for the applications. For item-level tracking, large coding capacity will be an indispensablefeature for identification of billions of consumer goods.Recent investigations [25, 32] have made some progress toimprove the coding capacity of chipless tags. Preradovicet al. presented a 35-bit multi-resonators based chiplessRFID tag using On–Off Keying modulation [41]. Morerecently, Vena et al. proposed the use of “C” resonators ascoding units to enable a 22.9-bit chipless RFID tag in acompact size by pulse position modulation coding method[18]. Hybrid coding method of frequency and phase varia-tions was used and demonstrated the great potential inachievement of large coding capacity given limited RF spec-trum. Exploiting dual-polarization theory seems also a prom-ising pathway to double the coding capacity of a chiplessRFID tag [25]. Even so, the chipless RFID tags with a largercoding capacity (probably 64 bits or more [2]) are stillanticipated for the tracking of trillions of consumer items tobe sold annually. The richest resource to enhance codingcapacity for a chipless RFID tag still exists in the proposalof innovative ID code generating circuits [18, 25, 42].

5.3 Compact size

The compact size of a chipless tag is not only preferable forattaching a tag onto small consumer products, such as booksand postal packages, but also desired for the driving down ofthe manufacture cost. Printable chipless RFID tags in thefuture are expected to be smaller than a credit card (5×8 cm2)or even a 2D barcode. One effective pathway is to move the

Table 2 Comparison of conventional and linearly tapered dipoleantennas

Js (Amps/m) η G (dB) Area (mm2)

Uniform 0.20–1.62 74.34 % 2.07 805.40

Tapered 1.52–2.10 73.87 % 2.06 474.45

Fig. 16 a Structure and geometries of conventional MLA, b structureof linearly tapered MLA

Table 3 Comparison of conventional and linearly tapered MLAs

Js (Amps/m) η G (dB) Area (mm2)

Uniform 0.19–2.51 69.67 % 1.64 1,175.00

Tapered 0.22–4.20 67.07 % 1.52 616.93

Ann. Telecommun. (2013) 68:401–413 411

operate frequency of a chipless tag to a higher frequencyband. This is particularly true for the reported chipless tagswith scalability summarized in Table 1. In fact, some com-panies has been trying to develop the chipless RFID tagsbased on tens of GHz or THz, such as InkSure and Vubiq [5],and some research institutes like Grenoble Institute of Tech-nology (Grenoble INP) [2]. With the enhancement of fre-quencies, it is likely to code larger amount of data within alimited size.

5.4 Reconfigurability

It is recognized that the chipless tags, also known as “RFbarcodes,” can rarely modify the encoded data oncemanufactured. Yet, it is still possible to reconfigure the IDdata with minor modification of layouts for their flexibleusage, like the LC–tank-based tag advised in the paper.Reconfigurability will enable customers to encode data ondemand of practical conditions. Reconfigurable tags assistedby mechanical punching and filling processes, furthermore,have potential to be recycled for repetitive use purpose [14,21], which is also helpful to significantly drive down tag costindirectly.

6 Conclusions

The recent advances of the state-of-the-art chipless RFIDtags fabricated by fully inkjet printing of metallic inks havenbeen reviewed. Two novel types of chipless RFID tags aredemonstrated. One is an eight-bit fully printed paper-basedRFID tag. The ID data are encoded in the tag by placing agroup of capacitors besides a ML as coding elements forintroducing reflections. Another is a ten-bit frequency–do-main-based chipless tag. The ID data were encoded into thetag by using a group of LC tanks with distinct resonancefrequencies. The feasibility of the two types of chiplessRFID tags was successfully validated by measurements andsimulations, respectively. Moreover, a LTA design techniquewas introduced. The simulation results show that over 40 %material consumption can be saved for the LTAs comparedwith conventional uniform dipole antennas and MLAs whilewithout significant degradation in the performance. Thesolution holds prodigious potential to be used for bringingdown the cost of printable chipless RFID tags that require afar-field antenna to communicate with readers. Finally, weaddressed the future trend of printable chipless RFID tags interms of fabrication cost, coding capacity, size, andreconfigurability. We believe that reconfigurability, scalabil-ity (moving to higher frequencies and shrinking tag sizes), aswell as large coding capacity enabled by innovation of IDgenerating circuits, will be important research topics towarditem-level tracking applications.

Acknowledgments This work was financially supported by Vinnova(The Swedish Governmental Agency for Innovation Systems) throughthe Vinn Excellence centers program iPack Center and by the EUproject CLIP of grant number 243557 through EU FP7-SME-2008-2.

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