2.1 TDoA

The following figure illustrates a typical TDoA application used for Real-Time-Location-Services (RTLS) in the industry that uses several Anchor nodes and Tag nodes. The Anchor nodes are fixed and the position is known, whereas the position of the mobile Tag nodes must be determined. In this application, the distances from the Tag node to the Anchor nodes must be determined, which is derived from the Time-of-Arrival (ToA) measurement and the propagation speed of light in free air.

Figure 2-1. TDoA Example Setup with Wired Clock Distribution and Synchronization

The ToA at the Anchor nodes with known positions is captured with timestamps to calculate the Tag position, which requires a synchronization of the clocks between the Anchor nodes to calculate the TDoA between the Anchor nodes.

The synchronization of the timestamp clock at the Anchor nodes is achieved using wired connections with synchronization pulses to synchronize the Anchor clocks (see Figure 2-1).

It is also possible to achieve synchronization with wireless communication using a reference Tag with a known position (see Figure 2-2). By measuring the ToA from the reference Tag in regular time intervals, the deviations of the timestamps at the Anchor nodes are determined. These deviations are, then, used in the TDoA calculation to correct the measured timestamps from the target Tag node.

Figure 2-2. TDoA Example Setup with Wireless Synchronization and Reference Tag

The following figure illustrates the timestamp capturing at the transmitter (Tag) TXTi and the receiver (Anchor) RXAi. The TDoA software application can display these values and the calculated differences of the timestamps in the PC terminal window. Both nodes are not synchronized and run at their own clock's frequency (fCLKT and fCLKA); therefore, the deviations between the subsequent ToFi can be observed over time. This software is only for demonstration purposes, as there is only one Anchor node and no coordinator that performs the TDoA and multilateration calculations.

Figure 2-3. TDoA Demo Software Application
To perform the localization of the Tag node, the received data telegrams and timestamps from the Anchor nodes have to be evaluated at the coordinator, to compensate the clock drift between the Anchor nodes and to calculate the Tag position. This requires a hardware setup beyond the UWB demo application kit. For more details on how to perform the calculation and how to set up such a system, refer to the J. Tiemann Publication from TU Dortmund in Germany (see Reference Documentation).

The localization method shown in Figure 2-2 requires that the Tag node transmit a UWB telegram, which is captured by the Anchor nodes with their timestamps. In literature, a reverse method is described, known as Down Link TDoA, where each Anchor node transmits a data telegram, which is captured by the Tag nodes. This requires the calculation of the position at the Tag node and can be used typically in indoor environments only due to UWB regulation requirements and restrictions.