Near-field chipless RFID encoders with sequential bit reading and high data capacity

This paper presents a novel and unconventional approach for the implementation of chipless RFID systems with high data capacity, suitable for authentication and security applications. Contrarily to previous time-domain or frequency-domain chipless RFID tags, where encoding is achieved either by generating defects (reflectors) in a transmission line (producing echoes in an input pulsed signal), or by etching multiple resonators (each tuned to a different frequency) in a dielectric substrate (providing a unique spectral signature), respectively, the chipless tags proposed in this paper consist of a set of identical resonators conveniently aligned and etched (or printed) on a dielectric layer (e.g., liquid crystal polymer, paper, etc). The resonators are located at predefined equidistant positions in such a way that the presence or absence of resonators in such positions corresponds to the `1' or `0' logic states, respectively. The reader is simply a coplanar waveguide (CPW) transmission line fed by a harmonic signal tuned to the frequency of the resonant elements. In a reading operation, the tag must be mechanically guided and transversally displaced over the CPW, so that the resonant elements modulate the amplitude of the feeding harmonic signal (through electromagnetic coupling) as they cross the axis of the CPW transmission line. This sequential bit reading alleviates the spectral bandwidth limitations of previous multi-resonator chipless RFID tags since the resonators are all identical in the proposed encoders. Therefore, the data capacity (number of bits) can be substantially enhanced since it is only limited by the area occupied by the resonant elements. The necessary close proximity between the tag and the reader is not an issue in certain applications such as authentication and security (e.g., secure paper), where the reading distances can be sacrificed in favor of a high number of bits. The design of 10-bit encoders based on this approach, and implemented by means of S-shaped split ring resonators (S-SRRs) etched on a flexible microwave substrate, is reported. The area of the encoders is as small as 1.35 cm
²
. The number of bits can be significantly increased by simply adding further S-SRRs to the codes. Thus, high data capacity can be achieved without penalizing the complexity of the reader.