# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 5 Conclusions and Road Ahead  
The accurate positioning of energy-neutral devices could significantly extend current battery-driven location-based applications. A comprehensive overview of the active research in low-power indoor positioning systems shows that there are plenty of open  
Figure 13: CDF plot of the influence of the angle between the RF source and backscatter with \(\Delta x_{a}=250\) cm and \(\Delta x_{rf}=190\) cm.  
research challenges and opportunities to progress towards the ambitious goal of the accurate and precise 3D positioning of energy-neutral devices. Our assessment shows that disruptive methods are needed, putting the energy neutrality of the mobile node as a fundamental objective when designing. A key example preaching this design method can be found in RFID. However, its solid identification capabilities cannot be straightforwardly scaled up to positioning. The results presented in this paper demonstrate the very promising potential of the combination of slower propagating acoustic waves with low-power backscatter RF communication technology to enable this sought-after feature in location based services. The measurements of the proposed hybrid RF-acoustic ranging system show that this technology is mature enough to be exploited for 3D positioning. With an energy consumption below 5 \(\upmu\)J per distance measurement, energy harvesting methods should be able to provide enough power for standalone operation. The 10 cm median accuracy error could be reduced by implementing better peak selection methods, adapting omnidirectional backscatter antennas, and selecting optimal impedances for the load modulation of the antenna.  
The road ahead requires first of all further R&D to develop the full 3D positioning solution for possibly a large amount of energy-neutral devices. This requires (I) the implementation of the RF wake-up necessary for synchronization purposes; (II) the development of adequate algorithms to determine accurate 3D positions based on the ranging measurements; and (III) the extension of these ranging methods to provide multiple access capabilities. Furthermore, solutions to increase precision and reliability in challenging reflective indoor environments are to be researched. For example, simultaneous localization and mapping techniques (SLAMs) could help combat bad outliers. Furthermore, physics-inspired learning methods taking into account high-level information on the surroundings, e.g., position of walls, are also promising.  
Conceptualization: B.C., L.V.d.P. and L.D.S.; methodology: B.C.; software: B.C.; validation: B.C.; formal analysis: B.C.; investigation: B.C.; resources: B.C., C.B. and D.D.; data curation: B.C.; writing--original draft preparation: B.C., C.B., D.D. and L.V.d.P.; writing--review and editing: L.D.S.; visualization: B.C., C.B. and D.D.; supervision: B.C., L.V.d.P. and L.D.S.; project administration: B.C., L.V.d.P. and L.D.S.; funding acquisition: L.V.d.P. and L.D.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not Applicable, the study does not report any data.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} DAQ & data acquisition system \\ PoE & power-over-Ethernet \\ USRP & universal software radio peripheral \\ MEMS & micro-electro-mechanical system \\ ADC & analog-to-digital converter \\ DAC & digital-to-analog converter \\ FS & full scale \\ RFPT & radio frequency power transmission \\ SoC & system on chip \\ SDR & software-defined radio \\ LIDAR & laser imaging detection and ranging \\ DDS & direct digital synthesis \\ RT & reverberation time \\ RTA & real-time analyzer \\ SPL & sound pressure level \\ \end{tabular}