Hans-Martin Tröger

A low-cost RSSI based localization system design for wildlife tracking

We present a low cost localization system for bats tracking. The system is based on an angle-of-arrival estimation using field strength difference measurements of two directional antennas. The receivers are constructed of low-cost commercial broadcast receivers and an embedded processing platform. A concept for an efficient 2-stage differential correlation is shown to address the dynamic range limitations of the low-cost hardware, the processing power limitations and frequency offsets. We show that the dynamic range can be improved by >16dB compared to a power detection method. The full system has been validated in a field trial in the rain forest of Panama.

Temporal wireless synchronization with compressed opportunistic signals

In this paper we introduce a wireless temporal synchronization scheme based on wideband signals of opportunity (SOO) such as DVB-T or LTE signals. Since these signals may not be decodable we show that it is necessary that one (reference) node broadcasts an excerpt of the SOO to all other nodes to provide a reference. However, the transmission of this reference signals requires a high bandwidth. Therefore, we propose to replace this transmission with a lower-bandwidth “compressed” version of this reference signal, using ideas from the field of compressed sensing (CS). We show that the high time resolution of the original wideband SOO can be maintained so that accurate temporal synchronization is possible. On the other hand, the compression leads to higher sidelobes in the correlation function which reduces the effective SNR. Therefore, the compression rate allows to control the trade-off between the required bandwidth and the SNR.

Partial OFDM demodulation for frequency syntonization of Wireless Sensor Nodes

Synchronization of a Wireless Sensor Network is a crucial task and is based on a precise syntonization of all clocks within the network. The synchronization precision is usually closely connected to the positioning accuracy in networks for the purpose of localization. This paper introduces a concept, how the clocks of low-complexity stationary receivers can be adjusted to the same frequency with the help of a television broadcast signal. Only parts of the signal information are used to achieve a manageable data rate for the embedded low-power processor. With a new algorithm the performance of the frequency estimation can be kept high compared to the use of the total signal energy, while the processing load can be reduced dramatically.

Compressed temporal synchronization with opportunistic signals

In this paper we propose a temporal synchronization method using opportunistic signals for wireless networks. Instead of broadcasting a dedicated broadband signal throughout the network for synchronization, high-bandwidth opportunistic signals already existing in the environment can be used. To avoid the necessity that all the nodes in the network are able to decode the opportunistic reference signal, a central node should receive the reference signal and then sends it again to all other nodes. Using ideas from the compressed sensing (CS) area, the high-bandwidth reference signal to be transmitted can be compressed efficiently before being sent to the other nodes, while still achieving an accurate correlation function compared to the low bandwidth reference signal scenario. In this way, only a low-bandwidth signal needs to be transmitted. Numerical results demonstrate that the proposed compressed technique achieves a similar performance at a much lower cost compared to using the high-bandwidth opportunistic signal directly.

Synchronization of wireless sensor networks utilizing broadcast signal time stamps

The synchronization of wireless sensor networks is a crucial task, especially for wide area networks. In case of many applications state-of-the-art techniques such as GPS or network based timing protocols are not suitable. In this paper we present a new concept that allows for the efficient synchronization of sensor networks using the time stamps of so-called Signals of Opportunity. We explain how these time stamps can be examined on the example of Digital Audio Broadcasting and show measurement results on the achievable performance.

Probabilistic multipath mitigation in RSSI-based direction-of-arrival estimation

Nowadays, wireless sensor networks (WSNs) have become omnipresent. Also location-based services become more and more popular. In this context, energy- and location-awareness are essential properties of modern sensor networks. Addressing low-power positioning, received signal strength indicator (RSSI)-based direction finding is a prospective approach for WSNs providing location-based services. However, like with all radio-based localization techniques, RSSI-based direction finding is prone to multipath propagation. In this paper, a probabilistic approach to multipath mitigation in RSSI-based direction estimation is presented. The presented approach is verified by Monte Carlo simulations. Simulations reveal the capability of zero-mean estimates for arrival angle in presence of multipath. Furthermore, the impact on the position estimation with recursive Bayesian filters is assessed. Trajectory estimation in a multipath setting shows promising results. Errors arising from multipath propagation can be minimized to a large extent.

Characterization of an opportunistic wireless syntonization using a low-cost test-bed

The most promising way of bringing localization services to GPS denied areas is the use of wireless sensor networks. The attainable accuracy for many methods of localization is highly depending on the synchronization between the sensor nodes. In this paper we describe a concept of adjusting the frequency of the local clocks with the help of signals of opportunity. It is explained, how broadcast signals can be used as a wireless reference signal for this syntonization process. We characterize the syntonization accuracy with the help of time domain analysis techniques (e.g. Allan Deviation). The measurements are carried out with a new low-cost test-bed using a software-defined radio hardware platform and the open source software GNU Radio. Using this measurement setup we characterize the wireless syntonization performance between two sensor nodes.

Time-domain OFDM carrier phase estimation for wireless sensor network syntonization

One of the most promising ways of bringing localization services to GPS denied areas is the use of wireless sensor networks. For time difference of arrival (TDOA) systems, the achievable accuracy highly depends on the synchronization accuracy between the sensor nodes. In this paper we describe a new algorithm that achieves an ultra-precise wireless frequency synchronization using OFDM signals. Especially for indoor environments, which typically suffer from multi-path propagation, significant improvements compared to state-of-the-art algorithms can be reached. An additional benefit of our algorithm is the low complexity, which allows for low-cost implementations. After the explanation of our concept and the algorithms, its simulated performance is evaluated.