# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 4 Experiments Demonstrating Opportunities and Challenges in Hybrid RF-Acoustic Positioning
### Experimental Environment and Measurement Setup  
The experiments are performed in a multi-functional measurement infrastructure named Techtile, depicted in Figure 9b. This is a 8 \(\times\) 4 \(\times\) 2.4 m modular structure built for distributed sensing and communication technologies [58] with 140 tiles covering the walls, ceiling and floor, each equipped with a power, radio, processing and communication module. A DAQ-setup with 384 single-ended, synchronized, 16-bit, analog input channels enables the fast sampling (3.3 MS/s) of sensors and actuators. This offers a true-to-life, modular testbed for both acoustic and RF-based research.  
The acoustic characteristics of this room are defined by the RT60 value, the time it takes for the room to attenuate an impulse signal with 60 dB and linked critical distance. With an RT60 value of 0.41 s above 8 kHz for a volume of 77 m\({}^{3}\)[59], this room has a critical distance of only 0.75 m. At this distance, the direct and reverberated sound signals can exhibit equal SPL, making this an acoustically harsh environment to conduct measurements in. Additionally, SPL measurements with an 60 Hz of 80 kHz frequency and a 10 s time window show different sources contributing to the ambient noise. In the audible domain, peaks over the 60 dB can be detected close to the server and networking rack, whilst a 40.9 dB peak is measured in the ultrasonic domain, coming from switched power supplies.  
Figure 8: (**a**) Spectrum of the received backscattered signal, showing the two symmetric sidebands with the RF carrier frequency at the center; and (**b**) 1 ms of samples from the received backscattered signal in the time domain before the cross-correlation (\(\Delta x_{a}\) = 200 cm).  
Figure 9a depicts the top view of the experimental setup in the Techtile environment. The following elements can be distinguished:  
* RF source: a single frequency carrier sine wave of 868 MHz with an output power of 12 dBm is sent out by R&S SMC100a signal generator. A directional patch antenna with a 6.15 dBi gain is attached to this setup, contributing to a total output power of 18.15 dBm, well below the 27 dBm EIRP upper limit imposed by the ETSI regulations. The antenna is mounted on a pole, at the same height as the antenna of the backscatter device.
* Processing unit for audio transmission (@anchor node): The acoustic signal is generated on a Raspberry Pi 4 running a Debian-based operating system. A C library PiGPIO is used for sending out the chirp with a frequency between 40 kHz and 20 kHz  
Figure 9: (**a**) Top view of the measurement setup in the Techtile environment; and (**b**) actual measurement infrastructure.  
by binary switching a GPIO pin for 30 ms. The audio signal is amplified with an off-the-shelf amplifier with a frequency range over 45 kHz. The audio signals are transmitted by a Foster FT17H ultrasonic tweeter with a HPBW below 30\({}^{\circ}\) in the xy-plane at 25 kHz. Similar to the RF source, the speaker is mounted on a pole and directed towards the MEMS microphone of the backscatter device.
* Backscatter device (@mobile device): Consists of both RF and audio components and was detailed in Section 3.4.
* Processing unit with SDR (@anchor node): The same Raspberry Pi 4 is used as processing unit running Python-based GNU radio in parallel with custom C++ timing and an acoustic signal-generating code. The RF backscattered waves were received by a dipole antenna connected to an Ettus B210 USRP SDR, and converted, filtered, and handled by signal-processing blocks in GNU radio. These FM-demodulated signals are stored in the hard drive for further distance range calculations. Interrupt-based precise timing between the start-up of the acoustic chirp and the RF wake-up time is performed by the software as well, with a maximum measured offset of 10 us.