Reinhard Exel

Bio: Reinhard Exel is an academic researcher from Austrian Academy of Sciences. The author has contributed to research in topics: Clock synchronization & Timestamping. The author has an hindex of 11, co-authored 17 publications receiving 316 citations.

Limits of synchronization accuracy using hardware support in IEEE 1588

Abstract: Clock synchronization protocols for packet-oriented networks, like IEEE 1588, depend on time stamps drawn from a local clock at distinct points in time. Due to the fact that software-generated time stamps suffer from jitter caused by non-deterministic execution times, many implementations for high precision clock synchronization rely on hardware support. This allows time readings for packets with very low jitter close to the physical layer. Nevertheless, approaches using hardware support have to carefully consider influences on synchronization accuracy when it comes to the range of nanoseconds. Among others, limits come from the update interval, oscillator stability, or hardware clock frequency. This paper enlightens the limits for such implementations based on an analysis of the influences of the main factors for jitter. The conclusions give hints for efficiently optimizing current implementations.

Clock synchronization in IEEE 802.11 wireless LANs using physical layer timestamps

Abstract: Packet-based clock synchronization protocols, such as IEEE 1588, depend on the quality of the timestamps taken at the reception and transmission of packets. As software-based timestamping generates large non-deterministic delays, Ethernet synchronization implementations have moved the timestamping closer to the physical layer. However, most wireless synchronization approaches are restricted to software timestamping due to the lack of hardware timestamping features. This paper1 presents a physical layer timestamping approach for IEEE 802.11b, which is able to generate timestamps with sub-100 picosecond accuracy. When synchronizing the clocks of two WLAN devices with the proposed approach, the measurements show that the system can reach synchronization accuracies below 1 ns even with standard crystal oscillators.

Localisation of Wireless LAN Nodes Using Accurate TDoA Measurements

Abstract: Time based localisation methods like GPS are widely used for outdoor navigation, whereas indoor navigation is typically performed only on a cell-basis or based on the Received Signal Strength Indicator. Since RSSI is not able to fulfil all current requirements, Time of Arrival and Time Difference of Arrival based approaches have recently gained focus. As time based localisation has high demands on the quality of the timestamps, we propose a special receiver architecture for IEEE802.11b capable of a timestamping accuracy in the sub-nanosecond range. The receiver's operation is demonstrated by an FPGA based wireless physical layer device implementation. Experimental results show an improvement in terms of accuracy by a factor of 100 up to 1000 over COTS wireless LAN hardware. This enables accurate localisation in wireless LANs just by adding of timing receivers.

A novel, high-precision timestamping platform for wireless networks

Abstract: The introduction of wireless networks in the factory floor offers many advantages. Besides a new flexibility for automation, also features like the localisation of wireless devices ease the use of this technology. However, for the application on the factory floor real-time guarantees have to be given, which can be ensured by schemes like TDMA, which is based on implicit or explicit clock synchronization. This is typically supported by a high accurate timestamping of incoming packets for the reduction of jitter effects introduced by the protocol layers. This paper1 introduces an open platform, which supports research on receiver and timestamper design. The receiver is implemented in a flexible fashion, in order to support simultaneous multi-channel monitoring as well as easy reconfiguration for technologies other than IEEE 802.11b/g for efficient deployment in automation systems.

Compensation of asymmetrical latency for ethernet clock synchronization

Abstract: Clock synchronization has become an indispensable service in most distributed systems as it allows to sort events on a common time scale and coordinate collaborative actions.With the demand for even higher synchronization accuracy, new challenges and barriers have to be tackled to fulfill these requirements. One of them, the inevitable signal propagation time between the devices, is compensated in many state-of-the-art synchronization protocols by round-trip measurements, neglecting any form of delay asymmetry of the communication link. In this paper, we analyze the impact of asymmetry in networks based on the physical layer of copper-based Ethernet and compare different approaches on how to mitigate the impact of asymmetry. We propose a non-invasive system performing asymmetry measurements on a link basis and show that such a system can integrate into existing synchronization solutions.

White rabbit: Sub-nanosecond timing distribution over ethernet

Abstract: White Rabbit (WR) is the project name for a ambiguous project that uses Ethernet as both, deterministic (synchronous) data transfer and timing network. The presented design aims for a general purpose, fieldbus like transmission system, which provides deterministic data and timing to approximately 1000 timing stations. The main advantage over conventional systems is the highly accurate timing (sub-nanosecond range) without restrictions on the traffic schedule and an upper bound for the delivery time of high priority messages. In addition, WR also automatically compensates for transmission delays in the fibre links, which are in the range of 10 km length. It takes advantage of the latest developments on synchronous Ethernet and IEEE 1588 to enable the distribution of accurate timing information to the nodes saving noticeable amounts of bandwidth.

Robust Convex Approximation Methods for TDOA-Based Localization Under NLOS Conditions

Abstract: In this paper, we develop a novel robust optimization approach to source localization using time-difference-of-arrival (TDOA) measurements that are collected under non-line-of-sight (NLOS) conditions. A key feature of our approach is that it does not require knowledge of the distribution or statistics of the NLOS errors, which are often difficult to obtain in practice. Instead, it only assumes that the NLOS errors have bounded supports. Based on this assumption, we formulate the TDOA-based source localization problem as a robust least squares (RLS) problem, in which a location estimate that is robust against the NLOS errors is sought. Since the RLS problem is non-convex, we propose two efficiently implementable convex relaxation-based approximation methods to tackle it. We then conduct a thorough theoretical analysis of the approximation quality and computational complexity of these two methods. In particular, we establish conditions under which they will yield a unique localization of the source. Simulation results on both synthetic and real data show that the performance of our approach under various NLOS settings is very stable and is significantly better than that of several existing non-robust approaches.

A Survey of Clock Synchronization Over Packet-Switched Networks

Abstract: Clock synchronization is a prerequisite for the realization of emerging applications in various domains such as industrial automation and the intelligent power grid. This paper surveys the standardized protocols and technologies for providing synchronization of devices connected by packet-switched networks. A review of synchronization impairments and the state-of-the-art mechanisms to improve the synchronization accuracy is then presented. Providing microsecond to sub-microsecond synchronization accuracy under the presence of asymmetric delays in a cost-effective manner is a challenging problem, and still an open issue in many application scenarios. Further, security is of significant importance for systems where timing is critical. The security threats and solutions to protect exchanged synchronization messages are also discussed.

Method and system for user interface for interactive devices using a mobile device

Abstract: A software application and system that enables point-and-click interaction with a TV screen. The application determines geocode positioning information for a handheld device, and uses that data to create a virtual pointer for a television display or interactive device. Some embodiments utilize motion sensing and touchscreen input for gesture recognition interacting with video content or interactive device. Motion sensing can be coupled with positioning or localization techniques the user to calibrate the location of the interactive devices and the user location to establish and maintain virtual pointer connection relationships. The system may utilize wireless network infrastructure and cloud-based calculation and storage of position and orientation values to enable the handheld device in the TV viewing area to replace or surpass the functionality of the traditional TV remote control, and also interface directly with visual feedback on the TV screen.

Clock Synchronization Over IEEE 802.11—A Survey of Methodologies and Protocols

Abstract: Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment toward real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.