# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 1 Introduction  
Radio frequency identification (RFID) technology has been widely used in various fields, such as supply chains, transportation, personal identification, security systems, parking management and Smart-City application [1; 2; 3; 4; 5; 6]. The RFID system is mainly composed of a reader, a tag and a data processing system [7], which can extract the stored information from the tag in a remote manner. The reader is mainly composed of a continuous wave generating circuit, a power amplifier circuit, a receiving circuit, a circulator, an antenna and a digital processing circuit. The tag consists of a tag chip and a tag antenna. The tag chip is composed of a voltage multiplier, a modulation/demodulator, a clock circuit, a register and a logic control circuit. The data processing system mainly refers to the background computer system. For the radio frequency tag, with much opportunity to take the place of current optical barcode, the most difficult issue on the replacement is how to cut down the tag's cost to compete with the optical barcode while keeping its advantage of automatic remote reading [8]. Currently, main cost of the conventional RFID tag is related to the silicon chip in the tag [9]. So, one of the promising cost-down solutions is so-called chipless tag. Until now, a lot of possible solutions on the chipless tag have been proposed in time-domain (TD) [10; 11; 12; 13; 14; 15], space-domain [16; 17; 18] and frequency-domain (FD) [19; 20; 21; 22; 23; 24]. The chipless tags operated in the time domain include SAW chipless tags and microstrip delay chipless tags [10; 11; 12; 13; 14; 15]. The SAW chipless tag adopts a pair of passive dispersive SAW filters for pulse expansion and compression to improve anti-interference performance [10; 11]. On this sense, its coding capacity is high. However, submicron etching technology is used on the high-cost piezoelectric substrate. So, its cost is high large area. The other chipless tag on time domain is the microstrip delay chipless tag, with the ID generated at higher frequency [12; 13; 14; 15]. Its structure is a simple planar ones, which include antenna and ID generation circuit. The chipless tag based on transmission lines mainly used for detection, theft prevention and low-end commercial applications. However, its encoding capacity is limited to be only a few bits. [PERSON] et al. proposed the space angle-based chipless tag [16; 17; 18], in which the resonator was designed in V-shape and Y-shape. Its encoding was conducted by adjustment of the space angle between two arms of the V-shape and the Y-shape. The coding capacity can be above 3-bit. However, the structure is over-sensitive to the external factors and it is identified by measuring the scattered field in two orthogonal polarization directions. The method has high bit error rate (BER). The chipless tag based on frequency-domain is encoded by fulfillment of a resonator with specific geometry on the substrate. The spectral feedback of the UWB signal is changed accordingly. Its coding capacity depends on the number of the resonators and the available spectral scope. In References [19; 20; 24], a re-transmittable chipless tag is proposed for a microstrip transmission line loaded resonator. It requires two orthogonal UWB transceiving antennas connected by transmission lines. The microstrip resonators are loaded on both sides of the transmission line. The coding capacity of the chipless tag mainly depends on the number of the resonators and the available bandwidth. The literatures [21; 22; 23] showed a self-resonant chipless tag with reduced area because the two transmitting and receiving orthogonal ultra-wide band (UWB) antennas were neglected. Multi-slot structures were used in the chipless tag in the frequency-domain [25; 26]. [PERSON] proposed the circular ring slot and U-shape slot nested chipless tags [25; 26]. Due to the symmetry of the slot ring structure, the tag can be rotated at any angle horizontally. So, the reader antenna can receive the coding information in a sensitive manner [25].