# High-Density 3D Printable Chipless RFID Tag with Structure of Passive Slot Rings
## 4 Discussion
The twelve resonant frequencies are distinguished from each other in a clear manner. As far as authors know, this is the first time to fulfill a 3D printed tag without any requirement on the substrate, showing a wide application potential for our proposed RFID tag. This is a practical RFID tag with the potential ability to take the place of the current optical barcode. [PERSON] et al. also proposed the C-shaped slot chipless tag [26], in which the direction of excitation electric field should be aligned with the opening of C-shape. So, the application of the tag is limited because of the requirement for the alignment. The two corners of the resonator should be short-connected with suitable conductor when the logic is changed from \"1\" to \"0.\" On the scenario that the C-shape slot is adopted, any short-connection of the corners will have obvious influence on the resonant frequencies of the three straight line slots. The coding will be complex and not suitable for application. The encoding capacity is increased by adding the concentric rectangular slot rings. Simultaneously, the width of the slot is reduced to shrink down the area of tag. [PERSON] et al. proposed the spiral resonator chipless tag [43] but the structure is complex. The tag needs two orthogonal Tx-Rx antennas and the corresponding Tx-Rx antennas must be aligned with each other. Otherwise, the reader will not receive the encoding signal. [PERSON] et al. proposed C-shaped microstrip resonator chipless tag [22]. There is a critical constraint that the electric field excitation must perpendicular to the opening direction. In our design, the 2 th, 3 th and 4 th harmonics don't influence the coding condition. The encode capacity is 1.9 bits/GHz. The excitation can be introduced from any orientation. The most interesting point of our design is that no ground plane (back metal) is required anymore. In this way, our design can be printed on any substrate and fulfill the same function as that of the barcode. The conventional RFID reader operates in narrowband. It uses ASK demodulation algorithm in time domain. So, the cost is lower. The chipless RFID reader works in the UWB. The encoded data is extracted from the spectra of the backscatter signal. On this sense, the reader part is more complex that of the conventional RFID. On this point, the cost of the chipless RFID is higher. However, the cost of the tag used in the chipless RFID is much lower. It is also much easier to be used. Figure 18 is the block diagram of the chipless RFID reader. The UWB interrogation is generated by the impulse generator, which is controlled by the digital circuit. D/A convertor is used to transfer signal into digital ones. The interrogation signal is transmitted to the chipless tag through the UWB transmitting antenna. The backscattered signal is received by the UWB receiving antenna of the reader. The RF-to baseband processing circuit demodulates the encoding UWB signal from the LNA. After ADC conversion, the information in the tag is obtained by the signal processing technique. Figure 17: The simulation spectra of the twelve Rectangular Slot Rings when the FR4 substrate has different relative dielectric constants (\(\varepsilon_{r}\)), respectively. (**a**) \(\varepsilon_{r}=3\), (**b**) \(\varepsilon_{r}=4.4\), (**c**) \(\varepsilon_{r}=5.5\), (**d**) \(\varepsilon_{r}=6\). Although 3D printer and chipless UWB reader are somehow expensive, the chipless tag can be used for large scale fabrication. The cost of a chipless tag for 3D printing can be reduced to less than 0.1 cents. For the comparison, the cost of a chipless RFID system depends primarily on the large number of tags. The cost of a barcode is about 0.05 cents, while the cost of a conventional RFID tag is 3 cents [44]. Although the 3D printed RFID is slightly more expensive than that of a barcode, the multifunctions of the 3D printed RFID, such as automatic recognition, contactless reading, automatic tracking and working in extreme environments and so forth, make it a promising candidate for application.