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Chipless Tags, the Next RFID FrontierS. Tedjini, E. Perret, V. Deepu, and M. Bernier1IntroductionEven if the concept of RadioFrequencyIDentification (RFID) was introduced manydecades ago 1, it is still very attractive and fertile in term of R&D and new ap-plications 2. Today, RFID is seen as a very enabling technology and it is underconsideration for thousands of applications covering a large variety of domainsamong them: ID papers, security, access control, road toll, ticketing, pharmacy,logistic, manufacturing, gambling, etc.3 Due to their relative low cost and largedistance of communication, passive (i.e., batteryless) UHF tags are very promising.The crucial issues of cost, efficiency, reliability, security, and standards are underconsideration by several groups worldwide 3.Research onpassive tags, particularlyUHF tags, is still veryactive in orderto en-sure interoperability, low-cost requirement, and data security. The interoperabilityis needed since there are three frequency bands worldwide. Roughly, the operatingfrequency bands 865869MHz for Europe, 902928MHz, for Americas and USA,952954MHz in Japan, China, and most of Asia 4. The interoperability requiresthe developmentof efficient miniaturizedantennaable to coverthe three RFID UHFBands. Privacyis highlydependantonthe securityof the data containedin the RFIDChip. Today, different microelectronic technologies are available for manufacturingRFID chips: CMOS, ASIC, and EEPROM are the best known examples 4,5. Asthe IFF 6 (Identification of Friend or Foe) was the first application of RFID, dur-ing World War II, the Allies used the cryptography on IFF transponders. In todaysRFID, chip on-tag cryptography is generally desirable and can be implemented inmany applications. However, on-tag cryptography is still prohibitive when it vio-lates application requirements, such as power or cost constraints 7. The third issueconcerns the cost of the tag. In addition to the cost of the RFID IC and the antenna,the cost of the tag will dependon the connectionmanufacturingprocessbetweentheantenna and the IC RFID Chip. Even if much progress has been made in antennaS. Tedjini (?), E. Perret, V. Deepu, and M. BernierGrenoble-inp/LCIS, ESISAR, F 26902 Valence, Francee-mail: smail.tedjinilcis.grenoble-inp.frD. Giusto et al. (eds.), The Internet of Things: 20thTyrrhenian Workshop on DigitalCommunications, DOI 10.1007/978-1-4419-1674-7 23,c ? Springer Science+Business Media, LLC 2010239240S. Tedjini et al.cost reduction using low-cost substrate and ink printing 8 of at least a part of theantenna, the interconnection between antenna and RFID IC ports still demands bet-ter reliability 9,10.One alternative for better data security and low-cost objective is to considerChipless solutions. Recently many designs of chipless RFIDs in the microwaveregion have been reported in literature. Microwave tags or RF tags have obviousadvantages of longer range compared to optical barcodes and are easy to be fabri-cated by conventional lithographic techniques. This is a very interesting approachunder development in many labs.In this communication, we discuss different approaches and advances for UHFtag design. Section2 will discuss the traditional tags based on RFID chip. Section3is dedicated to chipless. Both RF and THz chipless possibilities are presented anddiscussed. Some simulation results of newly proposed structures are reported.2Traditional RFID TagNowadays, UHF Radio Frequency Identification (RFID) technology is generatingmuch interest in industrial and academic institutions owing to its immense potentialin tracking. This is why the design of UHF RFID tags with high performances andlow cost is a very active field of research.UHF tags with relatively low cost have been in place for the last 10 years. How-ever, the deployment in high-volume of this technology is still held back by therelatively high cost of these tags. This is a part of the situation, but it is not the onlyreason. Indeed, if we consider the case of HF RFID, one can notice that such tagsare relatively close to the UHF tags regarding manufacturing process, technologyand in situ materials. These tags are also composedof a silicon chip and metal stripsfor communicatingwith the reader, and thus the costs of both of these tags (UH andUHF) are comparable.But the field of application is quite different. The HF tags are used for short-range applications. However, if we consider the mass market, in spite of this realhandicap (for maintenance or handling reason, it is generally better to be able toread all tags involvedin a specific area remotely),HF tags are still the most involvedin this sector. The reason for this is not economical (tags prices are comparable)buttechnical. Indeed, in real situation, the HF tags are more robust than the UHF tags.This has an extremely important impact for the final user: without preliminary test,a unique HF tag can be used with a very great number of objects. For example, inpractice, the RFID system operationwill not be disturbed if the object, on which thetag is placed, contains metals or not.In the case of UHF, the environment (the object on which the tag is placed, aswell as the close environment) in which the tags are used affects their characteris-tics considerably. In particular, when the tag is placed in an environment differentfrom which it was specifically designed for, the performances of the system canbe rapidly damaged, and thus the potential for this technology is limited. However,Chipless Tags, the Next RFID Frontier241we remind that this application would be viable for detection in about 99% cases.General observations could lead one to notice that tracking objects have neither thesame electromagnetic properties, nor the same geometrical size. Moreover, consid-ering the trend of new applications, it is expected that the tags will be integrateddirectly in the object itself. Indeed, it is interesting to follow the object throughoutits different manufacture steps, but also throughout the supply chain.Therefore, it is important to develop new RFID tag design tools to promotethe deployment in mass-volume of the UHF RFID technology. The tool, on whichwe are working 11, is able to automatically design the tag according to user spec-ifications. The inputs by the user are the geometrical size of the final tag and theproperties of the UHF RFID chip frontend. Depending on the user requirements,on where the tag will be used (permittivity, metal, etc.), the application returns thetopology of the tag, which can be used directly for the manufacturing.Antennas design solutions rest on purely empirical design approaches, based onfoldeddipoleantennasandcurrentloop(formatchingandnearfieldconsiderations).We develop a new design approach where the user requirements take part in the de-sign of the antenna. The antenna topology is not defined a priori as in the classicalmethods. Original topologies of antennas are generated automatically and selectedaccording to the constraints. Some examples are given in Fig.1. This approach doesnot simply consist in optimizing a topology already selected, but in designing theantenna from beginning to end. Our approach combines general purpose software(for example, MATLABR?) with software dedicated to EM simulations (for exam-ple, ANSOFT DESIGNERR?). The main motivation for the integration of thesetwo types of software is to benefit from data processing, the very broad panel offunctions (for example functions of optimization), along with powerful and verygeneral-purpose electromagnetic simulators. All the commands are controlled fromMATLABR?what makes this approach very flexible. So, the designer can avoidmanual repetitive tasks as well as the tedious parameterizations.We use an optimization process based on the concept of Genetic Algorithms(GA) to satisfy the constraints set during the design process. The optimization con-sists of an iterative process, which first generates the antenna shape, then simulatesit and finely evaluates its performance according to the imposed constraints. Thus,the antenna shape changes during iteration on an evolutionary principle. This is re-peated until an antenna design, which satisfies the project specification (as goodFig. 1 Examples of automatic antenna designs: (1) symmetric (2) asymmetric (3) sectorizationmethod (meandered zone in dash line)242S. Tedjini et al.as possible) is obtained. This very flexible approach makes it possible to take intoaccount the different issues throughout the design process, and specially, the physi-cal environment of the tag 11.3Chipless SolutionsChipless RFID is an emerging area of RFID technology for ultra-low-cost RFIDapplications. However, it is currently confined to the unlicensed radio frequencybands. In this section, we present different approaches for chipless solutions, in-cluding some alternative to consider higher frequencies, namely the THz domain.3.1Chipless MethodsThe most popular tags on the market (the most sold too) are the passive tags. Thisfamily of tags has achieved tremendous growth, although unit costs still remainhigh, hinderingtheir development1. This is why tags without chip (chipless) haveappeared and making it possible to reduce the cost in a large way to make themcompatible with the Auto ID Center recommendations. In addition to the price ofthe chip, this approach allows to drive down chip-assembly cost. These elementsmake up to more than half of the price of the traditional tag 12.Chipless tags, also named “RF barcode”, are usually manufactured with lowcost materials, generally electromagnetic reflective or absorptive materials. Chip-less tags, compared to passive tags, generally have the following characteristics:?low cost, less than 5 cents in volume,?contactless, short ranges less than 1m,?better reliability: thermal and mechanical behavioursHowever, these advantages should be balanced with the limited storage capacity(a few tens of bits) and the non-rewriteablecharacteristic (Read-OnlyTags) of thesedevices.Anotherdrawbackis thecostofthereader,whichcouldbehighercomparedto chip-based readers.Chipless tags are composed of different families, based on various approaches.The most promising are based on:1. The acousto-optics properties of materials, more precisely on surface acousticwave (SAW) 13. This approach already commercialized is by far the most ma-ture chipless RFID technology.2. Printed organic transistors. This prospective approach is mainly based on thesame principle of passive RFID 14,3. The electromagnetic properties of RF in passive microwave integrated circuits1521. This approach is very promising but still in the developing stage.Thequestionis: howis itpossibletoencodeinformationusingultralow-costpassiveRF devices? The principle of the information encoding, which consists in encodingChipless Tags, the Next RFID Frontier243the identificationnumberofthe tag, is based onthe generationofa specific temporalor frequencyfootprint. This temporal footprint can be obtained by the generation ofechoes due to the reflection of the incidental impulse. In the frequency domain, onecan characterize the spectrum of the tag backscatter signal. There are several waysto encodebinarydata. Two easy-to-implementapproachesfor informationencodingconsist of:?locating the presence or absence of a specific signal which is known to occur ata given time or frequency (such as On-Off Keying modulation (OOK).?measuring the gap (in time or in frequency) between two characteristic signals(such as pulse position modulation (PPM).The signals are generally electromagnetic waves, one can use the amplitude or thephase to encode the information. In the temporal domain, the design of devicesrests ontheconceptofreflectingsignalsduetodiscontinuities.Thesediscontinuitiescan be typically due to a rough variation of the geometries of the transition line(microwave approach) or of the medium (optic approach). A simple technique is toplace a number of discontinuities at different distances in order to obtain a specificsignal where the information is encoded by the temporal gap between the impulses.These discontinuities can be easily realized with localized 15 or distributed 16capacitances placed on a transmission line.In the frequency domain, it is possible to encode the information by taking intoaccount the amplitude variations in the frequency of the backscattering wave. Suchwork has been done by placing resonatingelements near a transmission line 17,18or by exploiting the resonance frequency of a network of dipoles 19, 20. Somestudies have shown that it is particularly interesting to encode information by thephase wave variations 21,22.The great advantage of these devices is that they can be manufactured on the topof low-cost dielectric substrates. However, it has low data capacity and dimensionsare usually quite larger (about tenth of cm2/.The introduction of 2D structures could tackle these limitations. We think alsothat these different principles presented earlier can be transferred to higher frequen-cies in order to offer miniaturized tag solutions with higher capacities. Some yearsago, devices based on holographic principles 23 have been investigated. Such asolution requires imaging technique in order to read the information.3.2RF ChiplessIn the amplitude approach, an array of microstrip dipoles behaving as band passor band stop filters tuned to certain predetermined frequencies is used to representdata as given in 19. Another method is to use capacitive tuned split microstripresonators. Here, the capacitance of each element of the dipole array is variedby changing the dimension of the split at the center, thereby obtaining the de-sired tuning 20. The aforementioned two techniques are based on the bistatic S21244S. Tedjini et al.measurements. These methods posse certain difficulties owing to multipath effects,mutual coupling, requirement of large bandwidth and few number of data bits.Preradovic et al. has presented a fully passive chipless RFID system using boththe amplitude and phase of the spectral signature 17. This system uses a pair of or-thogonally polarized dual band antennas with wide bandwidth for the transmissionand reception of signals. A multi resonator circuit is used to encode the multi fre-quency encoder signal from the antenna. By varying the dimensions of each of thespiral resonator,the correspondingfrequencycan be varied.But in practical cases, ithas to be seen whether the signal to noise ratio is well maintained. This method re-quires a reference for performing the amplitude and phase of the signal. Mukherjeeet al. 22 has proposed a method based on the phase frequency signature by thereactive termination of the tag antenna.A microstrip based L-C ladder is used to en-codethe bits in phase frequencyprofile.Variousfullyprintablechipless RFID tagswith reduced cost are also available in literature. Inkjet printable eight bit tags havebeen realized in 16. This method uses a transmission line with capacitive discon-tinuities using SMT (Surface Mountain Technology) technology. Based on whetherthe capacitors are connected or not connected to the transmission lines, there willbe reflections or no reflections. This idea is used to code data.Inksure technologies has proposed an easily printable SAR based RFID tagwhich has a read range of one foot 24. Post processing is done in the receiveddata to account for the diffraction effects between elements in the tag in order toincrease the data accuracy.But evenwith all the aforementionedtechniques,the only commercialsuccessfulchipless RFID system is the one developed by RFSAW based on surface acousticwaves (SAWs) 13. An acoustic wave device uses a piezoelectric material to gener-ate the acoustic wave. When an oscillating electric field is applied to a piezoelectricacoustic wave sensor, an acoustic wave is created and propagates through the sub-strate. This is then converted back to an electric field. Based on the discontinuities,the reflected wave is modified, which can be analyzed to obtain the stored informa-tion. SAW tags are cost effective with large capacity of data storage.Although, SAW tags are fully functional and can replace the chipped tags, theydo not provide a fully printable solution due to their piezoelectric nature. Thus, theycannot be applied for low-cost substrates such as paper. This has further intensifiedthe need of an all printable and compact tag with large data storage capability andsmall bandwidth so as to work within the allocated RFID bands.We are working toward the design of an all printable and compact RF-RFID tag.As anexample,a twobitchipless RFIDtagbasedonthevariationsinphasehasbeenstudied. The phase variations are obtained by varying the topology of the reflectingelement. Dependingon the presence or absence of the tag, the phase of the reflectedwave is altered. Figure2 shows the phase of the reflected signal. It can be seen thatlarge noticeable variations in phase is produced for various configurations of thereflecting element, due to changes in the current distribution.It is also interesting to note that there are variations in the resonance (Fig.3),which is obvious owing to changes in current distribution. This data can also beused to double check the received information.Chipless Tags, the Next RFID Frontier245Fig. 2 Phase variations with different configurations corresponding to different codesFig. 3 Variation in resonance for different configurations246S. Tedjini et al.3.3Towards THz ChiplessAs shown previously, surface information would be carried out by RF structure thatwould be read by RF signals. We discuss in this part a different approach of theRFID where the information is carried out by a multi-layer structure whose dimen-sions would be compatible with the terahertz (THz) frequencies. In this case, theinformationcodedinvolumewouldbereadbyTHzsignals.Theuserwill haveathisdisposal three possibilities of memorizing his information: either on the surface ofthe tag using RF signal (RFID), or in the volume or both. This latter solution bringsflexibility and permits to deal with much more protected data. This new family oftags constitutes powerful communicating objects and ensures an optimal manage-ment of energy, since they are passive devices based on reflections. Moreover, theirpassive character makes them no forgeable, and the THz information coded in theirvolume makes them unreadable unless you are authorized.It exists different approaches to code the THz information. This can be achievedeitherusingthetemporalresponseofthemultilayerstructure,orits frequencysigna-ture. Indeed, a reflection coefficient appears at each interface of the structure sincethecharacteristicimpedanceofadielectricmaterialdepends(intheTHzdomain)onits optical properties (permittivity and magnetic permeability), then an electromag-netic (EM) signal, emitted by a pulse source, is partially reflected at each interfaceof the multilayer structure generating echoes whose time delay and magnitudes de-pendonthe geometricalandopticalpropertiesofeachlayer. Inturn,a detectorreadsa temporal signature: the tag information is coded in the time domain. The reader isthen able to detect two consecutive echoes as long as the pulse duration of the in-coming wave is smaller than its back and fourth travel time within the thinner layer.Taking this into account, the source used must generate a pulsed signal with pulsedurations not greater than few tens of femtoseconds for a 1mm-thick tag. For suchtag, the system requires a THz source.Nevertheless, it is still possible to get specific information about the tag even ifthe THz source provides a continuous signal (CW source). Indeed the total EMwave reflected by the whole multilayer structure is the sum of all the reflectedwaves having been reflected at each interface. Since these reflected waves have thesame frequencies but different magnitudes, the intensity of the total reflected (ortransmitted) wave is due to the interferences between these phase-shifted reflectedwaves. As longas the relativephaseshifts betweenthemultiplereflectiondependonthe frequency of the CW incoming wave, on the one hand, and on the geometricaland optical properties of the tag, on the other hand, a frequency sweep of the CWsource involves specific intensity modulation of the total reflected (or transmitted)EM wave. In turn, a detector reads a spectral signature: the tag information is codedin the frequency domain. Below we present some results obtained with this latterapproach, and more specifically how it is possible to code several bits with a simplemultilayer structure as a RFID chipless tag in the THz domain.The studied THz tag on Fig.4 consists of non-magnetic dielectric media ar-rangedinawell-definedorder,ensuringthreedifferentfunctions,whicharerequiredto identify precisely the tag and its information. The periodical stack of layersChipless Tags, the Next RFID Frontier247Fig. 4 Schematic of the THz chipless tag structureFig. 5 Transmission coefficient and phase (dashed line) of a TEM ( D 0) plan wave out comingfrom the THz tag, for different defect layer thicknessesA and B is well-known as Bragg mirror. This periodic structure has a transmissioncoefficient, which depends on the frequency of the incident signal.As depicted in Fig.5b, this Bragg mirror prevents an incoming wave from beingtransmitted throughthe structure if its wavelength is included within a certain band-width whose spectral characteristics (rejection level, position and width) depend onthedielectricandgeometricalpropertiesofeachperiodicallystackedlayersAandB.The Bragg mirror is a 1-D photonic crystal, presenting a Photonic Band Gap(PBG) that allows coding spectral information. Indeed, introducing a layer C (de-fect layer Fig.4) embeddedby two 1-D photonic crystals, one creates a Fabry-P erot248S. Tedjini et al.cavity 25 having frequency-dependent reflectivity since the bandwidth separatingtwo consecutivetransmissionpeaksdependontheopticallengthofthedefect.Thus,either none or several peaks occur within the PBG. Then, presence or absence ofthosetransmissionpeaks,calleddefectmode,codetheusefulinformation.As exam-ple of results, we consider the multi-layer structure on Fig.4, to develop a THz tagwhose spectral signature presents an orientation-free dependence: the reader mustidentify the tag regardless its relative orientation. The spectral response of the de-veloped structure must be also independentof the polarizationstate of the incomingTHz EM wave in order to fit the RFID applications requirements. The transmittedand reflected EM waves (Et and Er, respectively) are numerically calculated withthe transfer matrix method 26, considering TE or TM planar incoming wave Ei,with any orientation about the tag surface. The simplest way to code the infor-mation is to read the presence/absence of a defect mode within the PBG. Figure5shows the spectral signatures of 4 tags having the same layers dimensions exceptfor defect thickness.TheabsenceofdefectmodeinthebothhalfbandwidthofthePBG (Fig.5a)couldbe interpreted as “00”. Therefore Fig.5b sets for “10”, Fig.5c for “01” and Fig.5dfor “11”. It means that, the number of states one can identify is given by the numberof defect peaks measurable by a reader having a certain spectral resolution. In turn,to improve the number of states, one should first improve the spectral resolution ofthe reader and/or broaden the PBG bandwidth. This latter solution can be achieveddeveloping, for example, Bragg mirrors with metamaterials.4ConclusionThe RFID technology is expanding rapidly and applied in many domains. It is be-coming a part of our everyday life. The variety of applications and environmentsrequires the development of quite different tags in order to meet the needs and theconstraints of each situation. The tags can be grouped in two families regardingtheir composition. The first family is the chipped tag, based on the use of an ICchip, which contains the information. These tags are quite developed and are avail-able in many different formats. Chipped tags continue to be investigated in order toovercome some limits such as data security, reliability and low-cost needs. The sec-ond family is the chipless form. These tags do not use IC chip and the informationis directly coded on the surface and/or in the volume of the structure. These tags,also known as RF barcodes, are very attractive in term of cost and data security.Many research projects worldwide are dedicated to the developmentof efficient andversatile chipless tags.Chipless tags are less than 1% of the RFID market today. Due to their low costandgreatflexibility,somemarketprojectionsshow that chiplesstags will reach60%of the RFID market before the end of the next decade. This is the reason why wecan consider chipless as the next RFID frontier.Chipless Tags, the Next RFID Frontier249Acknowledgments The authors would like to thank Prof. L. Duvillaret and Dr. F. Garet, forguidance and fruitful discussions on part of this work and H. Chaabane for his help. The authorsare grateful to Grenoble-inp for supporting this project via the BQR.References1. Stockman H (1948) Communication by means of reflected power. Proceeding of the IRE, pp119612042. Landt J (2001) Shrouds of time: the history of RFID. /technologies/rfid/resources/shrouds of time.pdf3. IDTechEx Knowledgebase. www.IdtechE4. Finkenzeller K (2004) RFID handbook: fundamentals and applications. Wiley5. Preradovic S, Karmakar NC, BalbinI (2008) RFID transponders. IEEE Microw Mag 2: 901036. (2005) Identification friend or f
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