# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 2 Preliminaries and State of the Art
### Indoor Positioning Technologies  
Indoor positioning is made possible via many technologies, each with their own advantages and disadvantages. Several overview papers are available, with some having over 300 references [6], reviewing the vast amount of positioning research from different points of view. The subjects vary from system requirements such as robustness [7] and accuracy for mobile targets [8] to dedicated positioning methods such as fingerprinting [9]. This section mainly discusses the current SOTA of acoustic and hybrid RF-acoustic systems, and compares them with the capabilities of UWB on the one hand, which achieves best in class accuracy in RF-based technologies, and RFID on the other hand, providing energy-neutral operation. An overview of innovative indoor positioning systems selected from the perspective of this paper is presented in Table 1. In addition to the accuracy, the low-power character of the mobile node is also examined. Most research does not consider strict energy constraints, which is nevertheless required to allow operation purely based on harvested energy. A checkmark () is given when a systems has a power consumption below 100 uW. It should be noted that a quantitative comparison of reported results is not straightforward. Different room sizes and materials in experiments highly influence the reflections that waves encounter in a venue, and consequently, the typical accuracy and precision achieved in the positioning. In this paper, we will further assess precision by means of the CDF of the distance error. RFID is currently the most established and deployed technology for positioning energy-neutral devices. Hereby, the RF signal is used for activating and positioning the device at the same time. A backscattering mechanism is used, altering the characteristics of an incident RF wave based on some unique parameter(s) of the passive device. This technology offers an elegant and low-complexity approach. However, the accuracy of RFID localization systems is defined by the limited read range, and false or missed detections occur due to multipath [10]. In [11], an overview is given of current RFID positioning systems, dividing them in three classes based on the underlying positioning technology. For RSSI-based systems, the accuracy is very limited, in the order of meters for larger distances, and the reliability is strongly affected by multipath effects and tag orientation [12; 13]. For smaller distances, sub-decimeter can be achieved with machine learning algorithms using extensive data sets [11]. PoA methods typically show a better localization accuracy, but currently suffer from two main issues: ambiguity due to phase periodicity and phase offset [14]. The most promising RF-based technologies that meet many quality metrics for indoor positioning are typically based on UWB signaling. In UWB localization, the system usually relies on the ToF of sub-nanosecond pulses to calculate the distance between the transmitter and the target [15]. Since UWB uses a very large frequency bandwidth (>500 MHz), a high resolution in time and thus distance can be obtained, resulting in a positioning accuracy in the order of tens of centimeters [16]. In addition, the high bandwidth enables great resistance against multipath fading and consequently makes UWB highly attractive for indoor environments. The average power consumption of commercial UWB systems depends on the device mode, but even in idle mode, this value, which is heavily influenced by the design, is larger than \(5\,\mathrm{mW}\)[17]. There is still room for lower power design optimization for this technology. In order to coexist with narrowband system architectures, the spectral power density in UWB is limited to \(-41.3\,\mathrm{dBm}/\mathrm{MHz}\) by regulations. Unfortunately, this is insufficient to remotely power UWB tags at useful distances [15; 18]. In combination with the higher power consumption, the latter makes the design of an energy-neutral UWB tag at the current time of writing practically impossible.