# Error Investigation on Wi-Fi RTT in Commercial Consumer Devices
## 3 Wi-Fi RTT Distance Error Measurements
### Experimental Sites  
Our study mainly investigated the measurement error caused by blockers (with different materials), phones, communication distance, and motion. All the experiments were in open outdoor space to evaluate the single source of interference or some of their combinations on the accuracy of Wi-Fi-RTT distance measurements. In general, we had three different experimental sites:  
* Range and position-fixed environment (RFPF): considering a sampling period of \(t\) s, the smartphone was fixed at a certain position at a distance \(d\) m to the AP.
* Range-fixed and position-varied environment (RFPV): considering a sampling period of \(t\) s, the smartphone was held by a pedestrian (volunteer) while the distance between the pedestrian and the AP remained at \(d\) m to the AP.
* Range and position-varied environment (RPVV): considering a sampling period of \(t\) s, the smartphone was held by a pedestrian (volunteer) while the distance between the pedestrian and the AP varied between \(d\) m to \(d^{\prime}\) m.  
All the experimental sites were set outdoors, away from other objects (except the ground). The samples were collected in both LOS and NLOS conditions by setting different blocks in each site. The LOS between the transmitter and receiver antenna is the central direct transmission path line of an ellipsoidal-shaped region in space. Such a long ellipsoidal-shaped region is also called the Fresnel zone. To ensure the LOS condition, the  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Model** & *** Chipset** & **OS** \\ \hline Google Pixel 2 & Snapdragon 835 & Android 10 (upgrade to Android 11) \\ Xiaomi Redmi K20 & Snapdragon 730 & Android 10 \\ LG V50 & Snapdragon 855 & Android 10 (upgrade to Android 12) \\ \hline \hline \end{tabular} \({}^{*}\)Performance: Snapdragon 855 \(>\) Snapdragon 835 \(>\) Snapdragon 730.  
\end{table}
Table 1: Specifications of the selected smartphones.  
primary (first) Fresnel zone at the radius of \(R\) should be ideally 60-80% clear of obstacles. The radius \(R\) can be calculated as follows:  
\[R\approx\sqrt{(d_{1}d_{2})/(d1+d2)*\lambda} \tag{3}\]  
where \(d_{1}\) and \(d_{2}\) represent the distance from the transmitter to the blocker and the distance from the blocker to the receiver, and \(\lambda\) denotes the wavelength. As \(d_{1}\) and \(d_{2}\) are in the range of 1 m to 5 m and 1 m to 9 m, and \(d_{1}+d_{2}\) is less than 10 m in this study (will be illustrated later), the maximum \(R\) can be calculated to about 0.39 m when \(d_{1}=d_{2}=5\) m. So, the AP was placed 0.74 m above the ground to reduce the influence from the ground.  
To obtain the NLOS condition, the ideal radius of the blocker should yield the maximum \(R\) of 0.39 m. As shown in Figure 2, the blocker was supported by a wooden frame and placed 0.4 m above the ground. The main body of the blocker was in the shape of a 1.2 m \(\times\) 0.8 m rectangle with a thickness of 2 mm. With such a design, the blocker can sufficiently block the first Fresnel zone. We swapped the main body of the blocker to different materials, including glass, wood and metal for each experiment in each site.  
The blockers' settings in each environment are shown in Figure 3. The detailed illustration of each environment is as follows.  
Figure 3: Blockers’ settings in all environments: (**a**) range and position-fixed environment; (**b**) range-fixed and position-varied environment; (**c**) range-varied and position-varied environment.  
Figure 2: Specification of the frame and the blocker (metal in this figure).