# Towards an Efficient Chipless RFID System for Modern Applications in IoT Networks
## 4 RFID System Validation  
According to the information encoding mechanism, the chipless RFID tags are classified into two main categories: time-domain tags and frequency-domain tags. For ro  
Figure 9: Simulated results of far field radiation pattern when the USRP antenna is operating at (**A**) 2.5 GHz; (**B**) 5 GHz; (**C**) 7.5 GHz; (**D**) 10 GHz. bustness, frequency-domain tags are preferred over time-domain as the later suffers from propagation delays. A typical frequency-based chipless RFID tag is comprised of multiple resonators tuned to various frequencies distributed in a particular bandwidth. The antenna should be capable of generating an interrogating signal with a bandwidth that can cover all the resonant frequencies of the tag, thus, requiring a wideband antenna unit. Validation of the RFID system also depends on the interaction of the interrogating signal with the tag and on the information retrieval mechanism. In a typical chipless RFID system, the tags can be deployed with readers in two modes; the retransmission mode and the radar cross-section (RCS) or backscattered mode [28, 29]. In these modes, the reader interrogates the tag with electromagnetic (EM) waves to detect the encoded data of the tag. On the one hand, RCS tags are smaller in size, but the detection process is more complicated, requiring a number of measurements. Additionally, RCS RFID systems can sometimes suffer from crosstalk, and in some cases, they involve a single antenna for both transmission and reception. On the other hand, the retransmission tags are larger in size with a detection mechanism that is much simpler, which allows the tag to retransmit the electromagnetic (EM) signal to the reader that can detect the tag information according to variations in the frequency signature in the form magnitude or phase deviation. Additionally, the crosspolarization between the reader and tag antennas ensures minimum crosstalk while maintaining isolation between the transmitted and received signals [30]. The advantages associated with retransmitted chipless tags make them an ideal choice for a modern chipless RFID system. However, the efficiency of such RFID systems depends upon the design of high gain UWB antennas and multi-resonator chipless tags with high coding capacity [31]. In the presented research, a measurement test bench is implemented using a vector network analyzer (VNA-MS4612A) and two sets of USRP antennas. The first set of USRP antennas connects the input-output ports of the resonator tag to act as the tag's transmitting and receiving antennas, respectively. This is shown in Figure 10. This arrangement ensures polarization mismatch between the transmitting and receiving antennas of the tag. The other two antennas function as reader antennas that are orthogonally connected to the ports of the VNA at a distance of 40 cm from the tag. To ensure accuracy in the measurement, the transmitting antenna of the reader should be copolarized with the receiving antenna of the tag. Similarly, the receiver antenna of the reader and the transmitting  
Figure 10: The transponder: A pair of USRP antennas act as the tag’s transmitting and receiving antennas. antenna of the tag should be copolarized. A schematic of the main modules needed for the validation procedure is illustrated in Figure 11, while the actual measurement setup is shown in in Figure 12.  
Figure 11: The schematic of the main modules used for the RFID system validation. Figure 12: The actual measurement setup for the RFID system validation. The S-parameter response (\(S_{21}\)) is measured for the proposed RFID system. It is compared with the simulated response and shown in Figure 13. The results are both in good agreement, thus, validating the specificity of the RFID tag to be deployed in IoT networks. The slight variation occurs due to fabrication, feed line losses, and possible mutual coupling among the antenna elements. Estimation of the received signal strength (RSS) is an important parameter to be quantified while designing an RFID system.