# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 4 Experiments Demonstrating Opportunities and Challenges in Hybrid RF-Acoustic Positioning
### Conducted Experiments
#### 4.2.2 Ranging-RF  
A second parameter to test is the influence of the distance between the RF-source and the antenna of the backscatter device. In the first set of acoustic measurements, this distance was kept above the far field distance and below 2 m. In the next measurements, we gradually increase the distance \(\Delta x_{rf}\) while keeping the distance between the acoustic transmitter and receiver constant. The influence of the inter-RF distance can be seen in two selected CDF plots, for \(\Delta x_{a}\) 300 cm and 400 cm, respectively, in Figure 12a,b. The left plot shows that for smaller values of \(\Delta x_{rf}\), some interference at a fixed frequency occurs. This results in a more prominent peak than the correct distance peak after correlation. This interference was only noted at this distance, and its influence diminishes with the increasing distance. This diminution potentially highlights the influence of the RF wave's incoming angle (\(\alpha\)), as this angle decreases when the RF source linearly moves away from the backscatter device in parallel with the backscatter-audio source axis. Out of the  
Figure 11: Example of a correlation plot between the received RF samples and the original transmitted chirp for an interdistance of \(\Delta x_{a}=5\) m. The red line is the actual distance, whilst the orange line is the selected maximum peak for the used distance estimation. Figure 12: (a) CDF plot of the measured distance error for \(\Delta x_{a}=300\) cm and different \(\Delta x_{rf}\); and (b) the fixed acoustic distance is here \(\Delta x_{a}\)= 400 cm. second CDF figure, it can be understood that for a well-performing ranging measurement, there is no influence from the interference or the distance, as some larger distances of \(\Delta x_{a}\) show better results. However, as not all lines follow the same course, the interference effect must come from somewhere else. As in this measurement set, the angle of arrival from the incoming RF-wave (\(\alpha\)) was kept as consistent as possible but was not actively monitored. To assess this influence of the angle, a last set of measurements was performed. Here, the backscatter device is kept at a constant distance of 250 cm from the acoustic source. The RF-source antenna moves around this device on a 190 cm radius, with the incoming angle (\(\alpha\)) going from 0\({}^{\circ}\) to 180\({}^{\circ}\) in steps of 27.5\({}^{\circ}\). With this increasing angle, the ranging performance drops drastically, as can be seen in Figure 13 and confirms our previously hypothesis stated. For angles of 27.5\({}^{\circ}\) and above, the curves behave similarly to how they do in Figure 12a. Again, some fixed frequency interference in the received backscatter signal can be observed, with a larger influence on the cross-correlation and ranging measurements, resulting in the known plateaus. Only at 90\({}^{\circ}\)does the shape of the plot differ and incline more uniformly. The source antenna is herein perpendicular to the PCB dipole antenna. From a backscatter perspective, only a small surface of this latter dipole antenna can be seen by the source antenna, leading to poor absorption and reflection capabilities. This can be attributed to the directivity of the half-wave dipole backscatter antenna and is normally embedded in the extended radar range equation from which the incident power density \(W_{i}\) is derived. Half-wave dipoles have a three-dimensional toroid shape. In this measurement setup, the radiation peaks when the transmission antenna faces the backscatter dipole (\(\alpha=0^{\circ}\)) and goes towards zero when it is positioned in perpendicular to the antenna (\(\alpha=90^{\circ}\)). The cause of the deviations in the previous ranging measurements depicted in Figures 10a and 12a can thus be explained by the incoming angle, which in most cases, is best kept low. With this, it can be seen that it is better to position the RF side along the acoustic source rather than opposite it.