TDoA2 VS TDoA3

This page describe the basic differences between TDoA2 and TDoA3.

When we designed TDoA2 the main goal was to be able to position multiple Crazyflies simultaneously. To solve the problem we had to go from Two way ranging to a TDoA solution. As the name TDoA2 implies, there was a first iteration that has been scrapped and TDoA2 was the first official version. After some time we found limitations in the TDoA2 that we wanted to solve and this resulted in TDoA3.

The main basic difference between TDoA2 and TDoA3 is that TDoA2 uses scheduled transmission times while TDoA3 is randomized. In TDoA2 there are 8 transmission time slots, one for each anchor, when the anchors transmit in a round-robbin fashion. Anchor 0 is used as a master and the other anchors synchronizes their transmission schedules to the transmissions of the master. The strict schedule ensures that no two anchors transmit at the same time and that we do not get packet collisions. In TDoA3 transmission times are randomized and the idea is to accept and handle a limited number of packet collisions.

The implications of these fundamentally different design principles are:

In TDoA2 anchor 0 is the master that the other anchors synchronize to. If an anchor does not get packets from anchor 0 it stops transmitting which means that anchor 0 is a single point of failure; if Anchor 0 dies the entire system stops. In TDoA3 on the other hand, each anchor transmits at random times and is not dependant on other anchors for synchronization.

Since the anchors use anchor 0 as a master in TDoA2, all anchors must be in range of anchor 0 which sets a limitation on the maximum size of the system. It also limits how the anchors can be placed, where anchor 0 typically has to be placed in the center of a large system. In TDoA3 this is not a requirement, and anchors can be placed more freely. The system can be expanded over larger spaces and also in multiple rooms or floors.

Since TDoA2 uses a time slotted scheme with 8 slots, this also sets the maximum number of anchors that can be supported. In essence it can cover a box shaped space, for instance a room. In TDoA3 there is no such limitation and more anchors can be added to the system as long as they use different ids. More anchors can be used to cover larger spaces, spaces that are not box shaped or that spans multiple rooms.

TDoa2 avoids packet collisions by using a schedule while TDoA3 relies on the ability to handle them. When two anchors transmit at the same time there are a few possible outcomes on the receiver side:

1. One of the packets is picked by the receiver and the other one is discarded
2. Same as 1. but the precision of the receive time is degraded
3. Both packets are discarded

The end result is that the precision of TDoA2 is slightly better than TDoA3.

In TDoA2 there are log variables set up for TDoA values between anchor 0-1 (in tdoa.d0-1), 1-2 (in tdoa.d1-2), 2-3, 3-4, 4-5, 5-6, 6-7 and 7-0 since these are the most common TDoA pairs due to the round-robbin schedule. In TDoA3 there is only one variable for TDoA values (tdoa3.tdoa). The anchors used for the value is set by the parameters tdoa3.logId and tdoa3,logOthrId. So setting tdoa3.logId=7 and tdoa3,logOthrId=23 would populate the log variable tdoa3.tdoa with the TDoA value between anchor 7 and 23.

TDoA2 is fully supported in the python client. There is basic support for TDoA3, but with limitations. For instance, anchor positions are set by transmitting settings through the Crazyflie via the UWB radio, and it is possible to design a TDoA3 system that is larger than what can be reached from one position.