Bluetooth Low Energy (BLE) is a common method used for communication between two (or more) devices. Everyday examples include having your smart phone connect to your car’s entertainment system or a wireless external headset.
Besides providing communication between devices, BLE can also be used for positioning by determine the approximate distance between the devices. Positioning can be helpful in some applications like social distancing devices that need to determine when two things (i.e. people, devices, tags) are in close proximity.
A somewhat similar method that has been around for decades, but just recently started making its way into everyday devices is Ultra-Wide Band (UWB). For example, most of the smart phone makers like Apple and Samsung have started incorporating UWB technology into their devices.
At ARIN Technologies, all of our products like ARINAlert™ and the new SocialDistancer™ digital PPE devices rely on UWB and here is why.
UWB and BLE are similar in that they are both low-power radio systems. However, the biggest differentiator is the bandwidth.
Typical bandwidth of UWB is 500MHz compared to 20MHz for BLE. This massive bandwidth differential, along with other characteristics, makes UWB more suitable for positioning.
It [BLE] was not made for tracing, but at least it can give an indication [of] which other phones have been in close range.Sven Mattisson, Bluetooth designer and engineer, Ericsson AB, 1995
It [BLE] was not made for tracing, but at least it can give an indication [of] which other phones have been in close range.
UWB uses a technique called Time-of-Flight (ToF) to calculate distances. It is similar to echo location used by bats. On the other hand, BLE uses Received Signal Strength (RSS) for calculating position.
As an example, consider six people, each with a PPE device that can communicate with each other. Each person is anywhere from one to five meters from the center (0 meters) person as illustrated below.
The ToF method of computing distance is through timestamps and does not require the devices (tags) to be in Line of Sight (LOS) with one another. The normal distribution curve for distance calculated using ToF between different devices at different distances is shown in Figure 1. As you can clearly see, each distance is very distinguishable and within an accuracy of 10 centimeters.
Figure 1
As mentioned above, BLE uses RSS to measure distances between devices. The normal distribution curve for RSS indication is shown in Fig.2 and indicates the unreliable nature of BLE to differentiate devices even when they are 1 meter apart.
Figure 2
Another interesting point about Figure 2 is that the tags are in LOS with each other. While the yellow curve can be clearly distinguished, the others are not.
For this example, the device represented by yellow curve is placed right on top of the receiver while the other six devices are spread 1 meter apart (or anywhere from one to six meters from the receiver). As shown, you cannot distinguish whether a distance is 1 meter or 6 meters because there is no clear relation between RSS and distance outside of an ideal environment. As such, this is the reason why BLE can only provide proximity and not precision.
In Non-Line of Sight (non-LOS) situations, the UWB radio signal will be attenuated, but the signal strength measurement is not used for range calculation. As such, the timestamps required by the ToF technique remains accurate in calculating the distance with an accuracy of 10 centimeters. We have tested our UWB devices in non-LOS situations where there are obstacles (e.g. concrete blocks, humans, etc.) and the distance calculations have always been accurate.
To see ARINAlert at work in a non-LOS scenario, please view our video.
In contrast, BLE’s reliance on RSS for distance calculations can be negatively influenced by external environmental factors. The signal can be diminished, reflected, or even amplified, thus resulting in unreliable calculations. For instance, if the person is simply turned away from the receiver, the distance calculated by RSS is greater than the true distance and thus inaccurate.
Here are two articles that might be of interest.
FiRa Consortium – How UWB Works
ElectronicDesign. – What’s The Difference Between Measuring Location By UWB, Wi-Fi, and Bluetooth?
UWB with the ToF technique is the more accurate method of measuring distances when compared to BLE with the RSS technique. This is critical for social distancing and contact tracing applications, where accuracy and precision can make or break the effectiveness of a safety system.
