K-State Resonators for High-Coding-Capacity Chipless RFID Applications

This paper presents a new, compact, high-coding-capacity resonator suitable for applications in chipless RFID tags. These tags consist of multiple resonators and two cross-polarized ultra-wide band antennas. Each resonator contains a U-shaped coupled microstrip frame with isolated (K-1) legs inside. One of the legs is designed to be connected to the U-frame via a metallic strip in order to adjust its resonance frequency. Using this feature, the frequency of each resonator can be reconfigured to be equal to one of the K-resonance frequencies. Therefore, each resonance presents one of the K-states of the resonator. This allows each resonator to represent more than one bit of information; this arrangement also permits N resonators tags to provide K
^N
codes. The structure can store large volumes of data in a small area and can simplify the detection process by only reading N resonance frequencies for each code. When N resonator tags are used in the proposed structure, the amount of data that can be encoded increases to 2
^{(log
K
/
log
2)N}
compared to 2
^N
in the case of conventional one-bit resonator (2-states) tags. An additional arrangement is also proposed and investigated in order to improve the spectral efficiency by allowing the bandwidth of each single resonator to be shared between two resonators. Several tags for codes with K = 8 are designed and implemented on the RT Duroid 5880 substrate as a proof-of-concept. An 8-state of the proposed resonator can be implemented in an area that is almost the same size as the area of a conventional 2-state resonator operating at 5 GHz. A satisfactory agreement between the empirical and simulated results is then confirmed.