The ability to measure distance can be immensely useful across many applications. For example, it adds security to digital keys by ensuring that a door will only unlock when a key or authorized user is within a specified distance.

Until recently, Bluetooth was only able to provide a rough estimation of the distance between two devices. For higher accuracy, also known as “fine ranging,” design engineers have had to turn to other technologies, such as ultra-wideband (UWB).

With Bluetooth now near-ubiquitous in electronic products, the ability to accurately and securely measure distance using only the device’s built-in Bluetooth radio would greatly benefit system designers by simplifying design and reducing costs.

Bluetooth Channel Sounding (CS) enables this capability through enhancements at the link layer, host controller interface and most importantly at the physical layer (PHY) for fine ranging accuracy and secure distance measurement. Let’s look at some of the traditional shortcomings of Bluetooth, how Bluetooth CS works and explore the importance of test for Bluetooth Low Energy devices based on this new distance measurement feature.

Limitations of Bluetooth for Distance Estimation

For more than a decade, Bluetooth devices have used Received Signal Strength Indicator (RSSI) as a way of estimating distance – the farther apart two devices are, the weaker the signal. However, RSSI is only accurate to within a distance of about three to five meters. This is insufficient for applications such as secure access control, precise real-time positioning, and asset tracking, which require accuracy of less than one meter.

The other challenge with RSSI has been that the signal itself is relatively insecure. This has enabled so-called “relay attacks” on cars, where the signal from a user’s key fob is captured and then replayed from another device nearer the vehicle, enabling it to be unlocked and stolen. While additional security checks and encryption can help, using Bluetooth in this way has proven to be markedly less secure than UWB.

Enter Bluetooth Channel Sounding

To overcome these challenges, the industry has turned to Bluetooth CS as a means of improving accuracy to sub-meter distances (~50 cm), while ensuring a much more secure signal than previous Bluetooth Low Energy implementations. Channel Sounding was officially ratified as part of the Bluetooth 6.0 specification in September 2024.

Bluetooth CS uses two techniques to accurately measure distance: phase-based ranging (PBR) and time of flight (ToF). 

The principle of PBR is that the phase of RF signals, such as Bluetooth, is proportional to the frequency of the carrier and the distance travelled. In Bluetooth CS, the first device, or “initiator,” sends a signal to the second device, or “reflector.” The reflector then sends a similar signal back to the initiator.

The two devices measure the received signals’ phases relative to their local oscillators. Measurements are performed at different frequency hopping channels and combined for final distance estimation.

Figure 1: Phase-Based Ranging (PBR)

So why do we need to also measure the time of flight? The reason is that phase cycles “roll over” every 2π radians, making it difficult to distinguish between distances that differ by whole multiples of the signal’s wavelength. For example, Bluetooth signals with a wavelength of 150m will roll over at a one-way distance of 75m, so PBR is unable to distinguish between two devices at distances of xm and x+75m. By delaying the signal, hackers can make a device appear closer than it is.

To solve this, Bluetooth CS uses ToF to refine the PBR measurements. The two devices exchange packets to measure the propagation channel’s round-trip time (RTT), which provides a second measurement that is used to verify the PBR distance.

Figure 2: Time of Flight (ToF)

Bluetooth Channel Sounding and UWB Compared

UWB is popular as the most accurate and secure fine-ranging technology, using ToF measurements to calculate distance. UWB has a wide, 500-MHz bandwidth, which means that the signal pulses are extremely short – just a few nanoseconds – resulting in accuracy better than 10 centimeters.

Bluetooth, on the other hand, has the advantage that it is widely available in almost all electronic devices, such as smartphones. It also consumes less power than UWB. Having said that, each technology is here to stay and will address different applications. UWB, because of its strong security feature, is being rapidly adopted as a digital access in car key fobs, warehouse asset tracking, personal item finding and is being considered for use in automotive child presence detection. Bluetooth CS, on the other hand, could be used in retail for hyper-personalized marketing, within manufacturing sites or in healthcare for patient monitoring. In fact, there are devices where including both technologies is useful. A hybrid set-up, for example, can use Bluetooth CS for low-power coarse ranging over long distances while relying on UWB for fine-positioning.

LitePoint Test Solutions for Bluetooth Channel Sounding

LitePoint’s IQxel family of testers supports physical layer measurements for Bluetooth CS. From a PHY perspective, the focus revolves largely around phase, frequency and power measurements, including but not limited to phase stability, SNR output control, RF mask testing, modulation characteristics and frequency offset on both the transmitter and receiver side. Although this may sound straightforward, each of these test cases must be run across different symbol rates and channel sounding modes to validate device performance as both the initiator and reflector. Therefore, having a tool that provides a simple interface to seamlessly automate these test cases and run them in loops is extremely helpful. 

LitePoint offers the IQfact+ software tool, which becomes especially useful as we move beyond R&D tooling and transition into module development and end-product manufacturing. In these stages IQfact+ leverages LitePoint tester resources to enable multi-device testing, thus optimizing and streamlining the production process.  

Conclusion

With the new channel sounding feature, the accuracy of Bluetooth distance measurements will increase dramatically. This will enable fine-ranging applications such as security, device location and asset tracking, all while operating at low power and without the complexity and cost of adding another technology just for distance measurement.

As with any new technology, the ability to robustly and accurately test Bluetooth CS is essential. LitePoint’s unique strengths in testing Bluetooth have enabled us to add support for Bluetooth CS to our popular IQxel-MW7G range of test solutions. Visit our product page to find out more.