TDoA vs ToF/TWR in Ultra Wideband RTLS

ultra wideband

Ultra Wideband radio technology enables to accurately locate people, assets and machinery. Most companies use time-difference-of-arrival (TDoA) because on the surface, it seems like the better choice. However, Eliko’s RTLS uses an advanced version of two way ranging (TWR/ToF) positioning method. Learn how it affects installation costs, flexibility and robustness.

The Ultra Wideband systems are either based on multilateration using distance measurements or time differences between anchors as the basis for coordinate calculations. Similar to GNSS, it is possible to calculate the exact location of the tag in Ultra Wideband, if you have at least four reference points or anchors.

The difference between ToF and TDoA lies in what we actually measure and how we use them to calculate the position of the tag.

When using the time-of-flight method (ToF), the tag measures how long it takes for the radio signal to travel from the tag to the anchor and back to the tag. We can then calculate the distance from the tag to each anchor. Once we have the distances to at least four anchors, we can calculate the X, Y and Z coordinates of the tag.

When using the time-difference-of-arrival method (TDoA), the tag sends out data packets, sometimes called “blinks”. All the nearby anchors pick up this blink message but do not respond back. As the anchors are located at different distances from the tag, the message does not reach every anchor at the exact same moment in time. These time differences between the anchors are the basis for the calculation to determine the X, Y and Z coordinates of the tag. The key point here is that all the anchors must work in sync; otherwise, the results will be meaningless. In reality, it’s a real challenge to measure these really tiny time differences precisely every single time.

The characteristics of both Ultra Wideband methodologies translate into real-life advantages and disadvantages for an application.

ToF/TWR saves installation costs

As previously stated, one of the essential prerequisites of the TDoA method is keeping the anchors in sync. As we measure the flight times of radio wave propagation, the synchronization must be really precise. TDoA has special hardware for this, which makes the whole RTLS setup more expensive. There are special clock generation units needed, as well as approximately 20-40% more expensive anchors, which should be able to extract and use the synchronization signals. Moreover, TDoA anchor planning has more limitations, because you have to consider that each tracking cell has to have a master anchor. 

In ToF/TWR the distance measurements enable special configurations for positioning that reduce the hardware count and installation costs by up to 50% compared to TDoA-based systems.

For example, ToF enables to position with just one anchor when the object moves only in one dimension such as an overhead crane. Alternatively, with a single anchor, it is possible to do distance-based proximity. This means that the anchor can detect tags presence 360 degrees around it and do it much more reliably compared to RSSI-based solution. RSSI is based on signal strenght, it is what Bluetooth and Wi-Fi use for positioning. The only real advantage of TDoA is that it saves airtime, and thus enables to position more tags and save tag battery life compared to TWR. Overall, the TDoA method has a direct negative impact on the RTLS’ payback time.

TDoA has a single point of failure

The ToF/TWR method is based on 2-way communication between the tag and the anchor, the time of flight is converted to distance measurements between each anchor and tags. Distances are a very reliable and easy way to understand how the system is performing thus simplifying both the configuration and troubleshooting of any network. In TDoA, when a master anchor fails, there is clear packet loss, since, without clock synchronisations, there are no coordinates. It is also more difficult to understand the cause of more subtle problems in the system. Troubleshooting time differences is complex compared to looking at distance measurements.

To minimise the risk of measurement errors due to clock synchronization, the TDoA anchors are usually connected in a wired system. In wireless synchros, anchors need to have line-of-sight with each other. This all makes the installation less flexible. With a TDoA based RTLS, each installation needs specific calibration after the initial set-up. It is also important to know that any anchor or even cable replacement will trigger the need for a new calibration. There is no such requirement in ToF/TW.

Overall, ToF/TWR-based Ultra Wideband positioning is more robust and easy to scale compared to TDoA. Troubleshooting is straightforward and extending the tracking area is simple as there is no master-slave logic to the anchor network.

Eliko has a unique Active-Passive Two-Way Ranging RTLS that combines the best features from both the ToF and TDoA methods. By combining passive measurements with better flexibility, robustness and scalability of ToF, Eliko is able to offer better performance and lower costs in even difficult environments.


Learn more about the Eliko UWB RTLS

Written by

Sander Ulp