# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 2 Preliminaries and State of the Art
### Interacting with Ultra-Low Power and Energy-Neutral Devices  
A sought-after feature in many applications is interaction with energy-neutral devices. An energy-neutral device is defined in this paper as a passive or active device with a guaranteed continuity of use through a WPT or energy harvesting link that offers sufficient energy (see Figure 1). In other words, the device experiences no net negative effect on its potential energy resources from its harvesting (\(E_{in}\)) and consumption operations (\(E_{cons}\)). An active device contains an internal energy source such as a battery, (super)capacitor, etc., while a passive device contains no internal energy source and depends entirely on the WPT or energy harvesting link. Since the energy used is equal to or less than the energy received, the device can be defined as an energy-neutral device. An energy-neutral device is classified under low-power devices, given its low power consumption. A low-power device may have a net negative value on the incoming and consumed energy and is therefore an active device with the need for energy storage.  
Energy-neutral operation hence creates the opportunity to eliminate the need for a battery in a device which in turn reduces the cost and maintenance and enables a small form factor. Furthermore, the localization possibility of the device could theoretically be extended to an unlimited lifetime since the harvesting of the necessary energy is inherently present. Energy can be harvested from a variety of sources and through different techniques [1], and recently, solutions were also specifically designed for small IoT devices [2; 3]. In view of the objective to position size-constrained devices indoors, solar energy is typically not a viable option. Coupled inductive power transfer is a very efficient solution for wireless charging in close proximity of devices reaching > 90% efficiency, however, the coupling deteriorates rapidly over distances larger than a few centimeters. Harvesting mechanical energy can  
Figure 1: Classification of low-energy devices illustrating the definition of energy neutrality for both active and passive devices.  
only be relied on when devices are repeatedly and significantly moving, and the form factor of the device allows the integration of a rather bulky harvester. APT is considered for reaching devices where inductive or RF transfer is prohibited, e.g., through human skin for biomedical implants or when metallic surfaces obstruct RF waves [4]. However, the power density is low (<0.1 nW cm\({}^{-2}\)) [4] and the conversion to electrical energy requires large transducers with rather low efficiencies. In recent years, RFPT has received significant interest as radio waves offer the advantage of being ubiquitous, and energy can hence be obtained over a large area, even in inaccessible places. RF power provides a density varying between 0.2 nW cm\({}^{-2}\) and 1 uW cm\({}^{-2}\)[5]. Practically, this results in a received power in the range of uW at the size-constrained sensor or actuator node. This makes the harvesting of RF signals highly suitable for the remote powering of small devices from a distance of several meters in indoor environments. Nonetheless, since the total amount of energy that can be transferred is very limited, innovative approaches for the positioning of these devices need to be developed.