TOF vs TDOA
ToF vs TDOA in Location Systems
The fundamental method used in most location systems, including GPS systems, is trilateration. That is, determining un-known point location based on its distance from other known 3 points (for 2D, in case of 3D location 4 anchor points are needed).
However, the engineering implementations of it has many variants. Within those, and within UWB specifically, there are two main architectures: ToF (Time of Flight) and TDOA (Time Difference of Arrival).
With ToF, a two-way ranging is done between the tag and the other 3 anchors (or other tags). This system is explained in depth in white paper.
TDOA works differently. The tag is broadcasting periodic signal, sometimes called blinking. The time difference of the signal reception between each two anchors defines hyperbolic curve (surface in 3D, curve in 2D) in which the transmitting tag should be. Each pair of anchors defines such a curve, and assuming no noise, the intersection of the 3 curves (there are 3 pairs) sets the point location.
The implications of TDOA implementation
Tag is transmitting only, it does not need to receive or perform meaningful processing – this is making the tag simpler, and more energy efficient. In UWB specifically, receiving operation is the most power-hungry operation. On the tag side, a single blink is enough to signal its location, as the processing is done on the anchors. This is compared with 3 ToF operations that requires both TX and RX on the tag side when working with ToF method.
BUT, and this is a big but, on the anchor side there are few major requirements:
- Need sun-nanosecond clock synchronization. Any miss in clock synchronization between two anchors is translated directly to miss in the tag location. Typically, this requirement is address by wiring the anchors, so there is a solid synchronization connection between them.
- Grid power connection – in practice the anchors need to be in listening mode all the time. RX is a power-hungry operation and keeping the receivers open constantly will drain battery device fast.
- Condense deployment – the tag signal needs to be received by at least 3 anchors.
This explains why the TDOA method, despite being very efficient on the tag side, is not being adopted widely. A unique dedicated infrastructure, all sensors are powered and meshed-wired have lead the traditional RTLS systems to be costly for purchase, and expensive to install and maintain. It would probably make sense in an Amazon warehouse or Boeing manufacturing facility. Currently, there is no real cost reduction path to make this type of solutions affordable for more markets.
The ToF method is more balanced and does not impose such restrictions on the anchors (infrastructure). There is no need to synchronize the anchors so setup and installation can be very fast.
From a battery life perspective, 24/7 tracking is power heavy on both method, though in TDOA the anchors power consumption is much higher than ToF.
The use case that best fit ToF method is “find on demand”, with no infrastructure or light infrastructure. Find on demand means that you do not constantly track an object location, you find it's location upon request. In a facility in which no infrastructure installed it is the only viable option.
The term light infrastructure refers to the option of setting up an RTLS system without wiring / powering anchors. With Pixie LoT Platform, it means that by using 4 Pixie Points you create a location aware facility just by attaching Points on few walls. No power, no wire. The life time can be weeks or months. Think on construction sites, infrastructure deployment projects (communication cables), a performing art show running for couple of weeks in town or even graduation show in school.