# Passive RFID-Based Inventory of Traffic Signs on Roads and Urban Environments
## 4 System Validation
### Simulated In-Motion Results  
In this section, the feasibility of the proposed link is theoretically evaluated using the radio propagation model described in Section 3. The methodology is as follows: firstly, as a function of the location of the vehicle, we calculate the direct and reflected fields at the RFID tag using Equations (6)-(8). Secondly, using (14) we calculate the probability \(P_{downlink}\) that the power received at the tag is higher than the required threshold power. Thirdly, the calculations are repeated for the uplink (tag to reader) communication and the coverage probability \(P_{uplink}\) is obtained. Finally, assuming that both the downlink and the uplink are independent events, the probability that the overall communication will be correct can be obtained by the product \(P_{coverage}=P_{downlink}P_{uplink}\). In all the experiments, we consider vehicle as a point traveling at constant velocity \(\
u\) along a straight line. The height of the vehicle antenna from the ground is assumed to be equal to 1.3 m and the effective radiated power transmitted by the reader is set to 2 W, according to ETSI regulation EN 302-208. The sensitivity of the reader and the RFID tags are set to \(-80\) dBm (\(10^{-8}\) mW) and \(-10\) dBm (\(0.1\) mW) respectively, which are realistic values for passive RFID technologies [43]. Figures 7-12 show in red line \(P_{coverage}\) for different design parameters and RFID tags located at several heights of the traffic sign (1, 1.25, 1.75, and 2 m for each subfigure) from the ground. For comparison, this probability is also shown in black line for the case in which the tag and the vehicle antenna are at the same height (1.3 m). In Figure 7, the distance from the traffic sign to the road is assumed to be \(d=2\) m, the speed of the vehicle is set to 50 Km/h and the frequency of the wave equals 866.5 MHz. Furthermore, we suppose that there is no significant multipath interference, apart from the wave reflected at the road (thus we set \(\sigma=0.1\)\(r_{rms}\)), and assume a vertical polarization, which optimizes the cover area, as describedin [32]. Note that Figure 7 has two x-axes: 'time taken to reach the traffic sign' (starting at \(t_{0}=-0.5\) s and finishing at \(t=0\)) at the bottom, and 'linear distance between the vehicle and the sign' (with \(r=d\) when \(t=0\)) at the top. These axes will be used in Figures 8-12 as well. Figure 7 shows, for example, that \(P_{coverage}\) starts to be close to one for about 350 ms in the case of the tag and the antenna are faced (tag height at 1.3 m), and for about 250 ms in the case of the tag is at 1 m from the ground. Results for \(h=1.25\) m are very similar to \(h=1.3\), whereas \(h=1.75\) m presents more accused notches or blind zones due to the multi-path effect. This effect has less influence at higher heights (\(h=2\) m), where the results are more similar to the ideal case (tag and antenna faced). In general, regardless of height, there is at least 250 ms with maximum \(P_{coverage}\) before reaching the traffic sign. Since the usually employed RFID protocol EPC Global 1G2 ISO 18000-6C provides a data rate of 40 Kbps (i.e., 400 tags/s, so it requires about 25 ms for identifying a tag), we can ensure that the complete radio link will be reliable for the whole transaction time, and that the tag can be read about 10 times during the passage of the vehicle. Therefore, even considering a high density of tags (such as the case of an urban environment, where the facilities to be inventoried are often close to each other), this obtained performance allows to read several tags at the same cover area (around 5 m in the simulations). Additionally, due to the possible presence of several tags close to the reader, the time required for managing possible collisions has to be considered. This time leads to an additional reading time (25 ms) for each given collision [44], i.e., basically the reading process has to be repeated.