Ultra-wideband and obstacles
One of the great advantages of using ultra-wideband compared to optical solutions like laser or camera tracking, or sound-based positioning solutions like ultrasound, is that ultra-wideband is a radio technology and can easily penetrate objects. Pozyx relies on the transmission of an ID and timestamp over UWB for positioning, and not merely on signal strength like many other Bluetooth/Beacon-based solutions. This means that in most cases, there is only a small loss in accuracy and range, even when the signal is heavily obstructed. But there are limitations.
In this article, we will explain what line-of-sight “LOS” means in the context of UWB positioning and how you should interpret it.
Clear LOS means that the antennae of all devices involved can “see” each other. If Pozyx devices had faces, then in clear LOS they would be able to stare into each other’s eyes. Clear LOS is the ideal scenario and will give you both the best range and accuracy.
For wireless radio technology, the effect of obstacles on the radio performance depends a lot on the material of the obstacle and its radio transparency. In general, signals will partially bounce off of obstacles causing reflections, and will partially penetrate the object. Depending on the type of material, the signal will pass through the object with little effect, or will be completely absorbed. When there are obstacles, we generally call this non-line-of-sight (NLOS).
Most materials can be split up into two categories: insulators and conductors. Because all radio waves are electromagnetic waves, these material categories have a big impact on a signal. Of the conductor class, most common elements are metals, with the well-known exception of saltwater.
Materials like wood, plastics, glass, cardboard, insulating foam, fabrics, fibers, brick, etc. are all great insulators and are very transparent to radio waves. This means that if one of these materials obstructs the path between two Pozyx devices, the impact will generally be negligible. Does that mean the signal will go through a pile of logs or bricks without any impact whatsoever? – No, there are limits. But it does mean that a plastic casing, none-load-bearing wall, table, etc. will not affect your setup much.
Metals are the most common conductors. Conductors will reflect most of the radio waves and give the part that does go through a harder time passing. This gives you two negative properties:
- The signal will have less power and thus reduced range.
- The signal will spend extra time trying to get through the material, but since Pozyx positioning relies on calculating the time of flight, this will reduce accuracy.
You should not interpret this as “any amount of metal will ruin the signal”. Placing a tag on the back of a phone or tablet, near a battery, inside a car, on a robot, inside a soda can, etc. will still work fine, albeit with a few cm reductions of accuracy. Thin sheets of metal will have a bigger impact but can also be penetrated. Concrete walls with plumbing, wiring, and metal supports structures, on the other hand, will cripple the signal. The signal will get through but your accuracy will be reduced and your range after the obstruction will be very short.
When metals are very close to the antenna, they can alter the antenna’s properties in an unpredictable way. Because of this, it is always recommended to keep a minimum distance of 20 cm from metals.
Similar to metals, liquids absorb radio waves. The saltier the liquid, the more it absorbs the radio waves resulting in reduced range. Furthermore, radio waves travel slower in liquids (about 30%). Because Pozyx relies on calculating the time of flight of the signal, this will reduce the accuracy of several centimeters.
How big the negative influence of the liquid obstruction is on the signal, is directly related to its volume. Holding your hand between two devices will have almost no impact. Placing a large person or barrel of water between two devices will significantly reduce the range and result in range measurements which appear to be further away.
NLOS and reflections
As we saw in the previous article about UWB, reflections have little impact on the accuracy of ranging because it is only the line-of-sight signal that is used for calculations. However, in the case of severe non-line-of-sight, it may be that there is no line-of-sight signal, and all that is received is the reflection. Because the reflection travels a much larger distance, this can result in ranging errors of several meters! Yikes! Fortunately, there exist several ways to deal with these large NLOS errors using NLOS detection and mitigation techniques. At Pozyx we use several advanced techniques to eliminate the adverse effect of NLOS. This Pozyx ‘special sauce’ is available in all Pozyx systems and allows for operation in challenging environments such as warehouses or retail stores.