## 3 Results of the Experimental Validation
### Operation Performances in the Presence of Other Microwave Signals  
The fourth experiment was conducted with a person moving in a public hall. In cases where all WiFi devices and IMU devices do not use the same frequencies, as is the case here, and also meet regulatory requirements, both services should work well independently. To further verify this, Wi-Fi reception strength (RSSI) for channels in the 2.4 GHz and 5 GHz Wi-Fi bands was recorded in parallel with WSA packet losses. The possible relationship between packet loss and interference was the main concern. The packet loss shown in the top graph in Figure 7 has different colours for five observed IMUs. Glitches that jump from 0 to 1 at different times represent lost packets. The percentage is given between the two corresponding lines that have the same colour. The middle graph in Figure 7 shows the measured RSSI for channels in 2.4 GHz WiFi, the bottom diagram shows the same information for the 5 GHz WiFi band. Sampling was performed in 100 ms using the computer's standard Wi-Fi network-communication interface.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Packet Loss [\%]**} \\ \multicolumn{3}{c}{**(n = 90,000 Samples, Duration = 900 s)**} \\ \cline{2-6}
**Environment** & **Pelvis** & **Right** & **Left** & **Left** & **Right** \\
**IMU** & **Thigh** & **Thigh** & **Thigh** & **Shank** & **Shank** \\
**IMU** & **Tlow** & **Tlow** & **IMU** & **IMU** & **IMU** \\ \hline
**Treadmill** & 0.0133 & 0.0044 & 0.0111 & 0.0056 & 0.0044 \\ \hline
**Public hall** & 0.0244 & 0.0078 & 0.0078 & 0.0178 & 0.0222 \\ \hline
**City streets** & 0.5555 & 0.0155 & 0.0233 & 0.0178 & 0.0466 \\ \hline
**Lab test** & 0.0083 & 0.0083 & 0.0100 & 0.0133 & 0.0083 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Packet loss is measured between the five IMU units and the master unit while a person is walking on a treadmill, in a public hall, in an urban street environment, and in a laboratory test.  
Similar to the above experiments, the subject was equipped with a complete set of WSA, including five IMU units, as shown in Figure 6. The session lasted 10 min, and the sampling rate was 100 Hz for each IMU and each variable. The same laptop PC with a Wi-Fi interface was in the backpack. The same software was used for the data acquisition and post-processing.  
Figure 7 shows the traces of packet loss measured for the IMU units attached to the pelvis, right/left thigh, and right/left lower leg. Glitches appearing on the green, purple, yellow, orange, and blue traces represent single events of a single packet loss. Most traces are at the 0 mark, which represents no packet loss. Traces with jumps to the value 1 represent single packet losses. The middle graph shows the RSSI signal levels for different 2.4-GHz Wi-Fi band channels. The channels are identified by different colours. An index for the colour coding of the channels is not provided, as this would make Figure 7 even more complex without adding relevant information. The bottom diagram shows the RSSI signal levels for different channels in the 5-GHz Wi-Fi band. For the same reason as for the 2.5-GHz band, the colour traces are not coded.