Near-field chipless-RFID tags with sequential bit reading implemented in plastic substrates

Chipless radiofrequency identification (chipless-RFID) systems based on near-field 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 cm2 (corresponding to an information density of 7.4 bit/cm²), is demonstrated