TDOA & TWR

Sequitur ecosystem has the unique feature of combining different architectures to accommodate the largest class of user needs for localization projects.

It exploits proprietary Time-Difference-of-Arrival (TDOA) and Two-Way-Ranging (TWR) schemes, that can be also combined for specific use cases.

What is Two-Way-Ranging (TWR)

TWR is a localization scheme using the measurement of the distance from a tag with respect to several anchors in order to compute the tag position. Every distance corresponds to a circle around the anchor. The interception of the circles identifies the tag’s position (trilateration).

In Sequitur, TWR positioning is driven by the tag; then, it measures its distance wrt anchors, and it estimates its own location.

Distance is computed from the round-trip-time, i.e., the time needed by a signal to go and come back between a couple of radio node.

A minimum of 3 anchors is needed for 2D positioning (4 anchors for 3D). Specific techniques are implemented in the Sequitur localization engine to cope with multiple measurements and errors.

TWR

Key benefits

Precise. More robust of TDOA for 3D localization and outside the perimeter of anchors
In the absence of 3 anchors positioning cannot be guaranteed, but the explicit tag-anchor distance information is still available

Main drawbacks

The number of tags is limited, since every tag must talk with all the anchors in the neighborhood (channel occupation)
Energy-hungry. It cannot work with small, battery-powered tags

What is Time-Difference-of-Arrival (TDOA)

TDOA is a localization scheme using the measurement of the difference of travelling time involving a tag and a couple of anchors. It can be operated according to a centralized or decentralized architecture.

For the centralized architecture, a tag transmits a signal, and a couple of anchors measure the difference in time related to the reception of the same signals at the 2 different locations. This time difference is related to the tag position.

For the decentralized architecture, a couple of anchors transmit signals (e.g. at the same time instant), and a tag measures the time elapsed among the reception of the 2 received signals. This time difference is, again, related to the tag position. GNSS systems operate according to this principle.

Centralized TDOA

Key benefits

Small and simple tags (they only transmit a short blink and can sleep for the rest of the time)
Very low energy-consumption
Very large number of tags wrt TWR

Main drawbacks

Position computed by an external unit and not available at tag side
Need of very precise synchronization
The number of tags in the same area is not unlimited

Decentralized TDOA

Key benefits

Unlimited tags localized simultaneously
Position available at tag side, which can be used for navigation
Possibility of integration with IMU sensors for improved accuracy and reliability

Main drawbacks

Computational power needed at tag side
Tag form factor not as small as centralized TDOA
Need of very precise synchronization

TWR

TWR is usually adopted when a few tags are present in each area. It guarantees robustness and accuracy, and it is more robust than other schemes also outside the perimeter of anchors, where TDOA usually fails. Moreover, if in particular conditions a reduced set of anchors is available, the anchor-tag distance is still reported (e.g., presence detection or gate control without the need of exact positioning).

Centralized TDOA

Centralized TDOA is the key-player in asset and people tracking (e.g., sport tracking and warehouses). Tags are very small and they do not need power supply. The position is needed only at server side, since it is not used for control and navigation.

Decentralized TDOA

Decentralized TDOA is the key-player in control and navigation. The position is available at tag side, so it can be plugged directly to the robot controller. No limits in the number of tags are present, so it is the true fully scalable solution. Neither servers nor connection among anchors are needed, so the system can be deployed very quickly and easily.

Depending on the project specifications, Sequitur can be used in every of the previous listed configurations, allowing the maximum of flexibility.

Moreover, the different architectures can be also combined in the same network, ensuring a plurality of behaviors in the same or in different environments, with a common set of anchors and with a complete transparency to the end-user.