Alexander Ens

Acoustic Self-Calibrating System for Indoor Smart Phone Tracking

This paper presents an acoustic indoor localization system for commercial smart phones that emit high pitched acoustic signals beyond the audible range. The acoustic signals with an identifier code modulated on the signal are detected by self-built receivers which are placed at the ceiling or on walls in a room. The receivers are connected in a Wi-Fi network, such that they synchronize their clocks and exchange the time differences of arrival (TDoA) of the received chirps. The location of the smart phone is calculated by TDoA multilateration. The precise time measuring of sound enables high precision localization in indoor areas. Our approach enables applications that require high accuracy, such as finding products in a supermarket or guiding blind people through complicated buildings. We have evaluated our system in real-world experiments using different algorithms for calibration-free localization and different types of sound signals. The adaptive GOGO-CFAR threshold enables a detection of 48% of the chirp pulses even at a distance of 30 m. In addition, we have compared the trajectory of a pedestrian carrying a smart phone to reference positions of an optic system. Consequently, the localization error is observed to be less than 30 cm.

Unsynchronized ultrasound system for TDOA localization

Indoor localization based on time difference of arrival (TDOA) has been recently a promising field of study. We consider the previously unsolved problem of locating a moving target receiver by using unsynchronized stationary beacons without requirement of manual calibration. Thus, the received signals and their time of arrival (TOA) have to be assigned to a beacon. Besides, in order to automatically calibrate the system it is required to estimate the time offsets between the senders, their positions and the initial receiver position. We present an approach to estimate all the variables of the scenario using the gradient descent and the Gauss-Newton method, two local optimization algorithms which use the derivative of a system of hyperbolic error equations. Besides, we present an ultrasound transmission system approach which fulfils the requirements of this scenario, being robust against multipath and estimating the reception time with high accuracy. In order to avoid interference by echoes the packet size is reduced by using two frequencies in Orthogonal Frequency Division Multiplex (OFDM). Further, the transmission system enables distinction of the beacons, as the ultrasound signals are used both for localization and for information transmission. The simulations show the local optimization algorithms are capable of estimating the positions of the beacons, receivers and offsets. They require only a rough knowledge of the sender positions. Further, real experiments show that the timestamps are measured with a standard deviation of only 1.19 μs for a SNR of 10 dB, which corresponds to standard deviation of about 0.4 mm for the distance measurement.

Acoustic indoor-localization system for smart phones

In this paper we present our acoustic indoor-localization system ASSIST (Acoustic Self-calibrating System for Indoor Smart phone Tracking) running on embedded ARM CPUs. The developed system uses acoustic signals beyond the audible range to localize COTS (commercial off-the-shelf) smart phones. The sound receivers installed with the infrastructure are connected using a Wi-Fi network. Furthermore, the sound receivers synchronize the clocks and exchange the time differences of arrival of the received sound signals from the Wi-Fi network. In this way, using an iterative multilateration algorithm, the locations of smart phones are calculated. We present our developed novel receiver hardware. The integrated single-board computer based on ARM replaces the external computer units to synchronize the receivers with the network and calculates the position via a TDoA (Time Difference of Arrival) algorithm. By powering the receivers via power-over-Ethernet the installation effort is minimized. The integrated inertial sensors of the smart phone can be used to calculate the position in a non-line of sight condition. Therefore different methods are available to support the position calculation for ASSIST.

Multicarrier airborne ultrasound transmission with piezoelectric transducers

In decentralized localization systems, the received signal has to be assigned to the sender. Therefore, longrange airborne ultrasound communication enables the transmission of an identifier of the sender within the ultrasound signal to the receiver. Further, in areas with high electromagnetic noise or electromagnetic free areas, ultrasound communication is an alternative. Using code division multiple access (CDMA) to transmit data is ineffective in rooms due to high echo amplitudes. Further, piezoelectric transducers generate a narrow-band ultrasound signal, which limits the data rate. This work shows the use of multiple carrier frequencies in orthogonal frequency division multiplex (OFDM) and differential quadrature phase shift keying modulation with narrowband piezoelectric devices to achieve a packet length of 2.1 ms. Moreover, the adapted channel coding increases data rate by correcting transmission errors. As a result, a 2-carrier ultrasound transmission system on an embedded system achieves a data rate of approximately 5.7 kBaud. Within the presented work, a transmission range up to 18 m with a packet error rate (PER) of 13% at 10-V supply voltage is reported. In addition, the transmission works up to 22 m with a PER of 85%. Moreover, this paper shows the accuracy of the frame synchronization over the distance. Consequently, the system achieves a standard deviation of 14 μs for ranges up to 10 m.

Mehrträger-Ultraschall-Kommunikation für die Innenraum-Lokalisierung

Zusammenfassung Unsynchronisierte Ultraschall-Lokalisierungssysteme ermöglichen dezentrale Positionsbestimmungen, unabhängig von Steuerleitungen oder -signalen. Somit benötigt das gesendete Signal zusätzliche Informationen über den Sender. Auf diese Weise ist eine kostengünstige Messung der Verschiebung mehrerer Objekte in zwei Dimensionen möglich. Störungen durch Echos werden durch die Aufteilung der Daten im orthogonalen Frequenz-Multiplex (OFDM) auf das Ultraschall-Signal reduziert. Bei der Verwendung zweier Trägerfrequenzen zur Entfernungsmessung wird eine Standardabweichung von 0,4 mm erreicht. Die Kommunikation mit 5,7 kBaud ist bis zu einer Reichweite von 18 m möglich.