Abstract-A high data capacity chipless radiofrequency identification (chipless-RFID) system, useful for security and authentication applications, is presented in this paper. Reading is based on near-field coupling between the tag, a chain of identical split ring resonators (SRRs) printed on a (typically flexible) dielectric substrate (e.g., liquid crystal polymer, plastic, paper, etc.), and the reader. Encoding is achieved by the presence or absence of SRRs at predefined (equidistant) positions in the chain, and tag identification is based on sequential bit reading. Namely, the tag must be longitudinally displaced, at short distance, over the reader, a microstrip line loaded with a SRR and fed by a harmonic signal. By this means, the harmonic signal is amplitude modulated, and the identification (ID) code is contained in the envelope function, which can be obtained by means of an envelope detector. With this system, tag reading requires proximity with the reader, but this is not an issue in many applications within the domain of security and authentication (e.g., secure paper for corporate documents, certificates, etc.). Several circularly-shaped 40-bit encoders (implemented in a commercial microwave substrate), and the corresponding reader, are designed and fabricated as proof-of-concept demonstrators. Strategies for programming the tags and a first proof-of-concept chipless-RFID tag fabricated on plastic substrate through inkjet printing are included in the paper.

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This paper presents a time-domain, chipless-RFID system with 80-bit tags inkjet-printed on ordinary DIN A4 paper. The tags, consisting of a linear chain of resonant elements (with as many resonators as the number of identification bits plus header bits), are read sequentially and by proximity (through near-field coupling). To this end, a transmission line, fed by a harmonic (interrogation) signal tuned to the resonance frequency of the tag resonators (or close to it), is used as a reader. Thus, during reader operation, the tag chain is mechanically shifted over the transmission line so that the coupling between the line and the functional resonant elements of the tag chain is favored. Logic states that '1' and '0' are determined by the functionality and non-functionality (resonator detuning), respectively, of the resonant elements of the chain. Through near-field coupling, the transmission coefficient of the line is modulated and, as a result, the output signal is modulated in amplitude (AM), which is the identification code contained in the envelope function. As long as the tags are inkjet-printed on ordinary DIN A4 paper, the cost is minimal. Moreover, such tags can be easily programmed and erased, so that identical tags can be fabricated on a large scale (and programmed at a later stage), further reducing the cost of manufacture. The reported prototype tags, with 80 bits of information plus four header bits, demonstrate the potential of this approach, which is of particular interest to secure paper applications.

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Chipless radiofrequency identification (chipless-RFID) systems based on nearfield coupling between the tag and the reader and sequential bit reading, with tags implemented on plastic substrates, are presented in this paper. In the proposed system, the tag is a set of identical resonant elements (S-shaped split ring resonators-S-SRRs), inkjet-printed on a plastic substrate (PEN), forming a resonator chain. The presence or absence of resonant elements at predefined and equidistant positions in the chain determines the logic state '1' and '0', respectively, associated with each resonant element. The reader is a coplanar waveguide (CPW) transmission line fed by a harmonic signal tuned to the resonance frequency of the resonant elements of the chain. Tag reading is achieved by displacing the chain of resonant elements above the CPW transmission line, in close proximity to it, so that near-field coupling between the CPW transmission line and the resonant elements of the tag results. By this means, the injected carrier signal is amplitude modulated, provided the transmission coefficient of the line varies with the presence or absence of resonant elements in the chain, and the identification (ID) code is contained in the envelope function. The functionality of the proposed system, with 10-bit tags occupying an area of 1.35 cm 2 (corresponding to an information density of 7.4 bit/cm 2), is demonstrated.