Highly accurate time sources, such as Cesium-based or GPS clocks, are the best candidates to provide a correct notion of time, but are too costly and impractical for implementation in every modern digital device. Ubiquitously, (crystal) oscillators are employed instead. To maintain a shared notion of time, the offsets and drift/jitter have to be compensated via the concept of synchronization, divided into time offset equalization and syntonization. Packet-based distribution protocols such as Network Time Protocol (NTP) and Precision Time Protocol (PTP) can provide time offset equalization, whilst synchronous Ethernet can deliver syntonization. To reach high synchronization accuracy, PTP and synchronous Ethernet can be combined into an improved protocol, referred to as White Rabbit. It enhances the one-way delay calculation through fine and coarse delay measurements. These improve PTP timestamps to reach sub-nanosecond synchronization accuracy.

The White Rabbit project has limitations though: it relies on extensive calibration parameter sets that are only available for limited devices and do not account for hardware variability. This is in contrast with the initial project objectives. Secondly, the update cycle of the hardware implementation is slow. Namely, the current White Rabbit switch is based on FPGA platforms from ten years ago. Thirdly, the switch contains costly components and requires full replacement of present network switches upon deployment.

This work opted to define a new switch design, which distributes the synchronization tasks over several smaller FPGAs to provide a low-cost implementation and improve applicability, whilst requiring minimal factory calibration and maintaining sub-nanosecond accuracy between devices.

The new switch concept is based on the smaller and widely applied White Rabbit node design. The proof of concept is based on two interconnected FPGAs forming a 1-port White Rabbit switch. Fine delay measurements, asymmetry estimations and fine delay compensations are applied to reach synchronization between the FPGAs. The Digital Dual Mixer Time Difference (DDMTD) is preferred for the fine delay measurements due to its superior resolution, area and invariance to large clock periods. A three-line asymmetry calculation is introduced and motivated to provide an improved one-way delay estimation. Lastly, the IDELAY element was selected to provide the fine delay compensation due to its process-voltage-temperature (PVT) invariance, negligible jitter addition and area demands.

Whilst both the fine delay measurement and fine delay compensation were thoroughly discussed by means of specifications and simulations, the three-line asymmetry calculation had to rely on a theoretical discussion. As such, measurements have shown that the transmission delay over an added bi-directional link is approximately equal when the clock signal moves from slave to master and vice versa. A large delay variability between other transmission lines is subsequently exposed. The variability leads to inaccurate one-way estimations with a traditional two-line approach. A three-line asymmetry calculation will be conclusively beneficial for the synchronization accuracy. The low-cost requirement of the distributed switch can be maintained through elimination of components in the slave and master node. The reduced switch lowers the hardware costs with approximately 25% by trading off cost-effectiveness with jitter filtering and consecutive performance.

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.