Bluetooth Channel Sounding - What is it, how does it work, and how well will it work

Bluetooth Low Energy has established itself as one of the most successful communication technologies over the 15 years of its existence. Arguably it was the technology that launched the IOT revolution. However, whilst its success in the domain of low power data transfer is undisputed, attempts to use it as a locating technology have had a mixed record. The new Bluetooth Channel Sounding specification promises to deliver more precise locating technology than the existing Bluetooth beacon and Bluetooth Angle of Arrival technologies.

Beacons – the first Bluetooth Location Technology

Beacons allowed a signal to be detected, and basic information supplied without a connection between the beacon and receiver. From a location perspective, the distance was determined by a simple signal strength measurement – the nearer you are to the beacon, the stronger the signal.

This simple method works to a certain extent, but is prone to many inaccuracies. Firstly, without knowing the precise output power of all the devices used to determine distance, it is not possible to get an absolute measure of distance. Secondly, even if you have a set of beacons of identical output power, the received signal strength (RSSI) will vary significantly depending on the precise orientation of the receiving device and the angle to the transmitter. Furthermore, 2.4Ghz radiation,as used by Bluetooth, is easily absorbed by a human body, for example. So any obstacles between transmitter and receiver will have a major impact on measurement. Lastly, the accuracy of the measurement drops off significantly with distance.

The result is that a measurement of +/- 1m is about as good as it gets, and at longer distances of say 25m+, it will be a lot worse than that.

Bluetooth Angle of Arrival – flatters to deceive

The next measurement technology was Bluetooth Angle of Arrival. It didn’t measure distance, but the angle of a transmitter with respect to an antenna array. It used the phase difference of signals detected across the antenna array to make the angular determination. The theoretical accuracy claimed was +/- 5-10 degrees. However, as this relied on measurement of subtle differences in signal phase, it was prone to environmental factors. In an “ideal” environment such as an anchor array mounted on the ceiling with clear line of sight downwards, it could work reasonably well. However, even something as simple as a suspended metal floor, common in offices to allow wiring underneath, could badly affect it. In an environment with lots of metal, for example a warehouse with metal shelving, the system could get confused and be near useless.

Channel Sounding – the latest innovation

One of the basic problems with existing Bluetooth location technology such as Bluetooth Beacons and Bluetooth Angle of Arrival  is that Bluetooth channels are narrow band. There are 39 channels, each with a bandwidth of 1Mhz, each separated from the adjacent one by a 1Mhz gap. A narrow band signal means that pulses are not well defined in time, and therefore determining information about phase or timing will have limited precision.

With Bluetooth Channel Sounding, the theory is that by using the entire spectrum over the 39 channels, and therefore a total bandwidth of approximately 80 Mhz, one can benefit from this larger bandwidth to improve the precision of distance measurement.

Conceptually, this makes sense. However in practice, one is still working within the limitations of 39 individual channels. What Bluetooth Channel Sounding offers is not so much a detailed method of distance measurement, but more a “toolbox” of techniques that can be combined to make a measurement.

Core Elements of Bluetooth Channel Sounding

The first part of this is a simple “time of flight” measurement – one device sends a signal to the other, which immediately sends a signal back. The round-trip time can be converted to a distance as RF signals travel at the speed of light, so the distance between the two devices can be calculated. To do this you must have devices that can receive and send back a signal with a rapid and precisely known “turnaround time”, which needs to be deducted from the round-trip time, and devices that can also measure the timing of a signal’s departure and arrival precisely. This is the method used by Ultra-Wide-Band technology.

This will only ever give a crude measurement, as the narrow band limitations of Bluetooth mean that it is not possible to determine the timing of a pulse with great precision.

To improve the accuracy of a measurement, a second step is used. Here a signal is sent out on two different channels consecutively, and the phase difference between the signals measured. The magnitude of this phase difference can give a determination of distance, as illustrated in the diagram below. Note that as “phase” is a cyclical measurement, a single measurement will have multiple possible distances that could correspond to such a phase difference. Therefore, multiple combinations of channels are used to define a single solution and improve accuracy. Adjacent channels will provide the least ambiguity whereas more widely separated channels will provide better accuracy.

To get the best estimation of distance, the time of flight and phase difference measurements can be combined to get an overall answer. However, the specification for Bluetooth Channel Sounding doesn’t specify the details of how to do this.  This will be up to the providers of software stacks to implement their best solutions.

Further precision can be obtained by using multiple antennas at one or both ends. This will reduce the effect of reflections and multipath effects. The specification permits up to 4 antenna paths, which can be made up either by a single antenna at one end and four at the other, or two at each end. However, a single antenna at each end is also permitted.

Will it work?

The claim is that a distance measurement of +/- 0.5m is possible. What is less clear, and we won’t really know until we have hardware and software stacks available, is how reliable this measurement will be and how immune or otherwise it will be to environmental factors.

As is clear above, there are a lot of options available to make a measurement. Accuracy should be improved by using as many of them as possible, but this will come at the cost of it taking more time, processing power, and power consumption to make the measurement. It will also involve more complex algorithms to combine the different elements to come up with a single figure, and the more complex a solution, the bigger the risk that it generates anomalous results.

Finally, the effects of environmental factors are not eliminated. Channel Sounding relies on phase-based measurements, and these proved rather susceptible to environmental factors like reflections with Angle of Arrival solutions. Multiple antennas and measurements across different channel combinations should help, but combining the data presents additional challenges.

Comparison with Ultra-Wide Band (UWB)

Ultra-Wide Band technology offers a much simpler technique, based on a time of flight measurement. The key difference is that the wide-band transmission means a pulse can be sharply defined in time, and reflections thus easily detected. It has the virtue of simplicity, and only a simple statistical average needs to be used to improve precision. 10cm accuracy is realistic with UWB. It is largely independent of the separation of devices. It also offers high security, in that the measurement is very hard to fake.

Conclusion

Bluetooth Channel Sounding is undoubtedly a clever technique to overcome the limitations of Bluetooth’s narrow band channel structure. It may well be good enough for many use cases. It is also a connection-based protocol, so can incorporate security features for use in digital key technologies. It is cheap and low power, and comes “for free” with Bluetooth, which is present already in many devices.

We won’t know the real limitations until working devices and software are ready. We only have the specification today. Its complexity  is a concern. Ultra-Wide Band is a more natural technology for distance measurement. The basic physics means that it offers a much simpler and robust solution. However, it lacks the ubiquity of Bluetooth today, and add costs to any solution.

As is often the case in technology, there isn’t one clear “winner”, but trade-offs of cost, complexity, precision, and the best choice will be dependent on what you are trying to do and the constraints around your solution.

A version of this article was published in EE Times in December 2024.

For more information on Bluetooth Low Energy products supporting channel sounding, please go here ISP2454 Bluetooth Low Energy 6.0 Module. For more information on UWB products, please go here Built-in Antennas UWB and Bluetooth Low Energy ISP3080 Modules