# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 2 Preliminaries and State of the Art
### Indoor Positioning Technologies
Extensive research was conducted on acoustic localization systems [19; 20; 21]. As acoustic signals have a relative low propagation speed (\(343\,\mathrm{m}/\mathrm{s}\)), centimeter positioning accuracy can be achieved based on ToF measurements without the need for high-resolution clocks and high-speed and hence power-hungry electronics [22; 23; 24; 25; 26; 27; 28; 29]. Figure 2 depicts a typical hybrid RF-acoustic positioning set-up with at least three fixed nodes for 2D positioning and four for 3D positioning with a known position in the system which we call anchors, and also are referred to as beacons in the literature. Possibly more anchor nodes are desired to obtain better coverage and precision. The mobile node operating in energy-neutral mode can be implemented as a tag. Distance measurements from the anchors to the mobile node, hereafter called ranging, are at the basis of determining the position of the mobile node. When using TDoA measurements [24; 30; 31; 32; 33; 34; 35], no synchronization is needed between the mobile node and anchor node, which is an additional advantage compared to the ToF system. Nevertheless, the anchor nodes must still be mutually synchronized. Usually, ultrasonic signals are used in acoustic localization systems given that these are not audible to humans. Acoustic-based positioning systems were developed, for example, the LOCATE-US system [19; 30], and more R&D is being conducted on custom designs, for example for ultrasonic transducers [33; 34], to provide low-power functionality since these ultrasonic transducers can require a lot of power. The emergence of MEMS technology is highly interesting due to their ultra-low power character. Smartphones are often used as mobile nodes in acoustic localization systems since they are widespread and have a built-in speaker, microphone, and available computing power [31; 32]. However, these are not low-power devices and are not designed to deal with ultrasonic sound signals as the audio components are usually limited to the audible region given their primary functions. Moreover, RF-based positioning for smartphones is more straightforward, given the presence of appropriate antennas. Figure 2: Overview of a hybrid RF-acoustic indoor positioning setup  
A disadvantage of acoustic localization in contrast to, e.g., UWB, is that acoustic signals are susceptible to room acoustics (reverberation) and current low-power acoustic electronics have limited bandwidth compared to RF. Reverberation in acoustic signaling is the main source of self-interference, resulting in ambiguity in acoustic indoor positioning systems. When tests are performed in an ideal free-space environment, e.g., through simulation [23], then high accuracy can be achieved, which shows that acoustic systems can be an interesting technology for positioning. In highly reverberating situations, optimizations to improve accuracy such as Doppler compensation [36], likelihood expressions [24], or additional anchor nodes to increase spatial diversity [30; 37] are required. In some acoustic localization architectures, the acoustic channel is combined with an RF channel to form a hybrid system. Early hybrid systems adopted the RF signal for both synchronization and communication. A previous overview paper from [PERSON] et al. [38] bundles these first generation RF-acoustic positioning systems. The papers discussed in this work [22; 25; 26; 27; 28; 29; 35; 39] focus more on recent energy-efficient systems. Although the energy efficiency of wireless acoustic sensor nodes has significantly progressed by the developments in MEMS and CMUT microphones and low-energy computing, current hybrid RF-acoustic solutions have not achieved fully energy-neutral operation for long-range (\(>\)10 m), centimeter-accurate and multiple access operation, with RF communication as the main limiting factor to achieve this [40]. Active low-power wireless standards such as Ant+ [25; 26] achieve a long range but are still power hungry, taking up a large part of the energy budget in energy-constrained devices.