Wireless Clock Synchronisation for UWB Positioning

Abstract

An Indoor Positioning System (IPS) is being developed at TOPIC Embed- ded Systems to track equipment in hospitals. The system should prevent the loss of equipment en make procedures more efficient. The IPS will con- sist of anchors and tags. Anchors are the radios that form an infrastructure in the building to localise tags that are placed on equipment. Different localisation techniques and methods exist for indoor localisation, of which Ultra-wideband (UWB) is a very promising technology as it is robust to mul- tipath interference. Also the Time Difference of Arrival (TDoA) method has advantages over Two-way Ranging (TWR) in terms of energy consumption of a tag and the rate of supported location measurements. A fundamental requirement for Time Difference of Arrival (TDoA)-based localisation is that all anchors must be precisely synchronised as radio signals propagate trough air with the speed of light. Existing synchronisation solutions synchronise the anchors either trough wires or wirelessly. To keep the installation costs of the IPS to a minimum synchronisation should work without adding extra infrastructure to a building such as a clock distribution network. Therefore a wireless solution is required. The existing solutions have been evaluated on hardware equipped with precise clock sources that have tight tolerances. These clock sources are not available on the com- modity hardware (Decawave DWM1001) that is intended to be used by TOPIC. Also the existing synchronisation algorithms are not designed for large multi-hop networks. A new synchronisation algorithm based on a 3-state Kalman filter is devel- oped and evaluated with the existing solutions showing that linear interpola- tion performs the best in terms of the Mean Absolute Error (MAE),Ḣowever the linear-interpolation algorithm comes at the cost of a latency as the times- tamps become only available after the next synchronisation message. If the latency (order of seconds) cannot be tolerated, the developed 3-state Kalman filter is the best alternative. As TOPIC requires a latency of 5 min the linear-interpolation algorithm is integrated in a synchronisation scheme for multi-hop networks. This scheme has the additional ability to measure the propagation delay. The linear- interpolation algorithm is evaluated with practical experiments for single- hop and multi-hop synchronisation. The MAE of the synchronised clock is 229 ps for single hop and 258 ps for multi hop when using a synchronisation period of 1 s. This shows that that the synchronisation algorithm is very suitable for multi-hop networks. To determine the effect of the clock synchronisation on position accuracy an experiment has been conducted where anchors measure the TDoAs of a message sent by a tag. A standard multilateration algorithm [1] was usedto estimate the locations based on these measurements. The accuracy of an individual estimate was low, but by averaging subsequent measurements a mean error of 51 cm was achieved, meeting the requirement of 1 m accuracy.