# Error Investigation on Wi-Fi RTT in Commercial Consumer Devices
## 4 Experimental Results and Analysis
### Range and Position-Fixed Environment  
The samples collected in LOS in RFPF are shown in Figure 7. The Wi-Fi RTT distance usually has a negative bias to the ground truth distance. The mean of each set of Wi-Fi RTT distance at different ground truth distances was then calculated. It can be seen from Figure 8a-c that the slopes of the Wi-Fi RTT distance measured by all three smartphones are almost one. This means that the Wi-Fi RTT distance changes linearly with the ground  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **LOS** & **Wood Blocker** & **Glass Blocker** & **Metal Blocker** \\ \hline Google Pixel 2 & \(-\)74.3364 & \(-\)80.2417 & \(-\)79.5688 & \(-\)84.3012 \\ Xiaomi Redmi & \(-\)68.2389 & \(-\)72.9978 & \(-\)71.9982 & \(-\)79.3582 \\ K20 & \(-\)73.0837 & \(-\)82.3104 & \(-\)77.9049 & \(-\)83.4068 \\ LG V50 & \(-\)73.0837 & \(-\)82.3104 & \(-\)77.9049 & \(-\)83.4068 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Preliminary experimental results of the average RSSI (dBm).  
Figure 6: Schematic of the RVPV environment: (**a**) LOS condition; (**b**) NLOS condition.  
truth distance. Hence, a positive offset was applied to each curve to compensate for such a negative bias. The offsets for Google Pixel 2, Xiaomi Redmi K20, and LG V50S were 0.45 m, 1.85 m, and 0.24 m, respectively. The RMSE between the Wi-Fi RTT distance and the ground truth distance at each distance was calculated. As it can be seen from Figure 8d-f, the RMSE of the compensated Wi-Fi RTT distance is very close to 0 m regardless of the change of the ground truth. The offsets were applied to all the measurements in the following experiments to better visualize and analyze the errors caused by the motion and NLOS effects.  
Figure 8: Wi-Fi RTT distance and error in RFPF and LOS condition: (**a**) Wi-Fi RTT distance from Google Pixel 2; (**b**) Wi-Fi RTT distance from Xiaomi Redmi K20; (**c**) Wi-Fi RTT distance from LG V50S; (**d**) RMSE of Wi-Fi RTT distance from Google Pixel 2; (**e**) RMSE of Wi-Fi RTT distance from Xiaomi Redmi K20; (**f**) RMSE of Wi-Fi RTT distance from LG V50S.  
Figure 7: Raw Wi-Fi RTT distance measured by the three smartphones in RFPF and LOS condition: (**a**) Google Pixel 2; (**b**) Xiaomi Redmi K20; (**c**) LG V50S.  
Next, we investigated the NLOS effects on Wi-Fi RTT distance. As illustrated in the experimental settings, we set different materials of blockers, including wood, glass, and metal. We can observe from Figure 9a-c that the Wi-Fi RTT distance still varies linearly in NLOS but has a different bias with different blockers. One fact that leads to the bias could be the diffraction effects at the edges of the finite blocker. Since the blocker was set to block the primary Fresnel zone, the signal can diffract around the blocker, which causes losses. The reflections from the blockers and the ground should also be considered.  
Moreover, we can observe from Figure 9d,e that the RMSE remained stable in LOS condition, while the RMSE increased as the ground truth went further in the NLOS condition. Moreover, for all smartphones, the metal blocker impeded the transmission of the RTT signal the most among the three materials. The glass blocker showed moderate effects on Wi-Fi RTT distance, while the Wi-Fi RTT distance blocked by wood showed the lowest RMSE. This could be caused by the skin effects. The Wi-Fi RTT signals propagate as electromagnetic waves in the air at approximately the speed of light. The skin effects mean when Wi-Fi signals carried by electromagnetic waves pass through some blockers or media, some will stay on the surface of the blockers, and the rest can pass through and keep propagating to the destination. Materials with better conductivity would attenuate more Wi-Fi signals.