semiwiki.com, Sept. 11, 2024 –
I posted a blog on this topic a year ago. Now the Bluetooth Sig has (just) ratified the standard it is timely to provide a reminder on what this new capability can offer. Channel Sounding introduced in Bluetooth Core specification version 6.0 is a method to significantly increase the accuracy of Bluetooth-based distance measurements, from an accuracy of around 3-5 meters down to around 30-50 centimeters. Channel sounding opens new, more efficient and more secure options to support keyless entry, Find My Device, and other applications.
Why do we need a new standard for ranging?
Bluetooth is everywhere and already provides ranging support through RSSI (received signal strength indication). Does that suggest there is no need for a new standard and new devices? Can't we just use what we already have? RSSI is as simple as it gets, measuring the attenuation in signal strength from the transmitter. However attenuation depends on more than distance; obstacles and diffraction also contribute. Multiple beacons can help improve accuracy in say retail or hospital settings, but such solutions are obviously not helpful for personal applications like keyless car entry.
Angle of arrival (AoA) and angle of departure (AoD) measurements, introduced in Bluetooth 5.1, can help improve accuracy through trigonometric refinements, though they require multiple antennas on receiver or transmitters. And these methods are equally compromised by multipath propagation resulting from reflection from surfaces around the path between receiver and transmitter.
Bluetooth 6.0 Channel Sounding instead uses phase-based ranging. A device sends a sine wave to a peer device which then sends the same signal back to the original device. The phase difference between the initial signal and the received signal gives a quite precise measure of distance. Better yet, a reflection off an obstacle will travel a longer path than the direct return, exhibiting a bigger phase shift and making it easy to ignore in distance estimation.
On the other hand, UWB (ultra-wide band) is very accurate, able to deliver positions with accuracy down to a centimeter or below. The tradeoff is that UWB requires an additional and specific MAC, modem and radio (either integrated or in separate chipset), adding to a device bill of materials (given that the device will also need to support Bluetooth for other reasons). And UWB is more energy hungry than Bluetooth, draining a battery faster unless used only when absolutely needed.
Is there a best solution?
One idea might be to combine RSSI methods for approximate ranging with UWB for accuracy close-in. There are two problems here. First, earlier generation Bluetooth versions are vulnerable to relay attacks. In a raw ranging attempt an attacker can intercept the BT communication and relay it to another device, allowing them to open your car door if UWB is not used for added security.
The second problem is with power consumption under some conditions. Suppose your car is parked near your front door and your car keys are on a table just inside the door, within range of the car. It's not hard to imagine your key fob constantly trying to communicate with the car, triggering UWB ranging sessions and quickly draining the key fob battery.