Angle-of-Arrival (AoA), as well as Angle-of-Departure (AoD) localization are general principles for performing localization based on angular measurements. In recent years, these principles have been gaining interest over more traditional techniques in the field of indoor localization and positioning. In contrast to Time-of-Arrival (ToA) and Time Difference of Arrival (TDoA)-based localization, AoA may do so without requiring a high number of anchor nodes or effective synchronization between them, with the trade-off of higher hardware and algorithmic complexity. Research on angular-based localization encompasses a wide range of technologies, each with its own advantages and disadvantages for dealing with the generally more complex propagation channels of indoor environments, as opposed to those in outdoor settings. In this contribution, we perform a systematic survey and meta analysis of the field of angularbased indoor localization systems, covering the technologies: Acoustic Positioning, Wi-Fi, Bluetooth, Ultra-Wideband (UWB), Light-based Positioning, Positioning in Cellular Networks (like 5G), mmWave, Terahertz-band Localization, Radio Frequency Identification (RFID), and RADAR. The research and literature on algorithms are examined as well. Furthermore, a new set of design considerations for angular-based localization systems is proposed, as well as several trade-offs between technologies, are discussed, and future research directions presented. Since angular-based localization is expected to play a key role in the future of indoor localization, this contribution provides a structure for researchers, as well as practitioners, to find a unified perspective on the topic.

The demand for a replacement of global navigation satellite systems in indoor environments has led researchers to investigate into the field of indoor localization. In this contribution, we propose RAILS, a three-dimensional Real-Time Angle of Arrival Ultrasonic Indoor Localization System. The proposed system is capable of accurately and precisely locating a source in three-dimensions using angle of arrival measurements. Our acoustic angle of arrival technique relies on a constellation of spatially distributed arrays of microphones and on a source emitting inaudible ultrasonic chirp signals. Real-time features are enabled by introducing a localization chirp detector constantly listening to the medium and triggering the localization algorithms only when signals of interest are approaching. The core of the three-dimensional (3-D) localization method are the time delay estimates, aiming to compute the 3-D direction vectors. In order to overcome the challenges introduced by the acoustic indoor channel and to provide accuracy and robustness in the delay estimates, we have designed a time delay estimator. Furthermore, an ad-hoc 3-D positioning algorithm based on 3-D vectors intersection has been developed to furnish reliable 3-D source positions. The performance of our proposed ultrasonic indoor localization system has been evaluated through static single-source real-world experiments in a warehouse-like scenario, covering an area of 40 m ² with a maximum source-receiver range of 10 m. The system achieved an outstanding average of 10 cm 3-D positioning accuracy, with an average standard deviation of 1 cm over all the measurements. Furthermore, dynamic single-source experiments have been carried out demonstrating that our system is capable not only of locating a source, but also to track it.

In this contribution, we propose a reliable, longrange, communication system based on chirp spread spectrum techniques for aerial ultrasonic channels, to be employed for user identification in ultrasonic indoor localization systems. We adopt binary orthogonal keying, where the binary information, modulates the shape of the chirp instantaneous frequency, mapping zeros onto down-chirps and ones onto up-chirps. In order to reduce the demodulation computational costs, we exploit the band-pass sampling theorem and develop an ad-hoc algorithm to under-sample the received signal. The incoming packet detection and frame synchronization are guaranteed by employing a characteristic chirp signal, typically utilized for localization purposes, as a pilot for the transmitted identification packet. A simple, yet efficient, pilot detection scheme enables live features by constantly listening to the channel, triggering the demodulator only when the pilot is detected. The performance of our system has been evaluated in low power and ultra-low power real-world experiments in a warehouse-like environment. It is demonstrated that our system is capable of error-free communication up to a range of 22 m in the low power setup, where the transmitter had a transmission power of 80 mW, corresponding to a sound pressure level of approximately 37 dBSPL at 1 m distance. In the ultra-low power mode, with a transmission power to 15 mW, corresponding to a sound pressure level of approximately 29 dBSPL at 1 m, the system achieved 99% success rate up to 16 m. Moreover, the capability of the system to be robust against the Doppler effect and in a multi-user environment has been tested via simulations.

Hydrogen-based technologies provide a potential route to more climate-friendly mobility in the automotive and aviation industries. High-pressure tanks consisting of carbon-fiber-reinforced polymers (CFRPs) are exploited for the storage of compressed hydrogen and have to be monitored for safe and long-term operation. Since neither wired sensors nor wireless radio technology can be used inside these tanks, acoustic communication through the hull of the tank has been the subject of research in recent years. In this paper, we present for the first time a passive wireless sensor technology exploiting an ultrasonic communication channel through an electrically conductive transmission medium with an analog resonant sensor featuring a high quality factor. The instrumentation system comprised a readout unit outside and a passive sensor node inside the tank, coupled with geometrically opposing electromechanical transducers. The readout unit wirelessly excited a resonant sensor, whose temperature-dependent resonance frequency was extracted from the backscattered signal. This paper provides a description of the underlying passive sensor technology and characterizes the electric impedances and acoustic transmission as an electrical 2-Port to design a functional measurement setup. We demonstrated a wireless temperature measurement through a 10 mm CFRP plate in its full operable temperature range from −40 to 110 °C with a resolution of less than 1 mK.

Ultrasonic applications such as non-destructive testing, biomedical imaging or range measurements are currently based on piezoelectric bulk transducers. Yet, these kinds of transducers with their mm to cm dimensions are rather impractical in fields in which both frequencies in the kHz region and small-feature sizes are required. This fact mainly relates to the inverse relationship between the resonance frequency constant and the transducers’ dimension, yielding a higher frequency and attenuation with a decreasing size. Piezoelectric micromachined ultrasonic transducers (PMUTs), in comparison, incorporate a small-scale µm design while preserving the operating frequency in the desired kHz range. This contribution presents the detailed manufacturing of such a PMUT with a multiple ring electrode‑based structure to additionally adjust the sound pressure fields. The PMUT will be characterized by its deflection in air along with the characterization of the piezoelectric material lead zirconate titanate (PZT) itself. The measurements showed a maximum polarization of 21.8 µC/cm² at 50 kV/cm, the PMUT’s displacement of 30.50 nm/V in air when all electrodes are driven, and an adjustable deflection via different electrode excitations without the need for additional hardware. The ring design also offered the possibility to emit two distinct frequencies simultaneously. These results demonstrate the potential of the designs for small-feature-size applications as they are in high demand for implantable devices, miniaturized ultrasonic-based communication or drug delivery.

Angle-of-Arrival (AoA) estimation technology, with its potential advantages, emerges as an intriguing choice for indoor localization. Notably, it holds the promise of reducing installation costs. In contrast to Time-of- Flight (ToF)/Time-Difference-of-Arrival (TDoA) based systems, AoA-based approaches require a reduced number of nodes for effective localization. This characteristic establishes a trade-off between installation costs and the complexity of hardware and software. Moreover, the appeal of acoustic localization is further heightened by its capacity to provide cost-effective hardware solutions while maintaining a high degree of accuracy. Consequently, acoustic AoA estimation technology stands out as a feasible and compelling option in the field of indoor localization. Sparse arrays additionally have the ability to estimate the Directionof- Arrival (DoA) of more sources than available sensors by placing sensors in a specific geometry. In this contribution, we introduce a measurement platform designed to evaluate various sparse array geometries experimentally. The acoustic microphone array comprises 64 microphones arranged in an 8×8 grid, following an Uniform Rectangular Array (URA) configuration, with a grid spacing of 8.255 mm. This configuration achieves a spatial Nyquist frequency of approximately 20.8 kHz in the acoustic domain at room temperature. Notably, the array exhibits a mean spherical error of $1.26^{\circ }$ when excluding higher elevation angles. The platform allows for masking sensors to simulate sparse array configurations. We assess four array geometries through simulations and experimental data, identifying the Open- Box and Nested array geometries as robust candidates. Additionally, we demonstrate the array's capability to concurrently estimate the directions of three emitting sources using experimental data, employing waveforms consisting of orthogonal codes.