Bluetooth Channel Sounding Brings Crucial Ranging to New Bluetooth Devices

Ever Forward – Bluetooth SIG Introduces Innovative Bluetooth 6 Specification

The Bluetooth SIG has relentlessly pushed forward the capabilities and feature sets of the Bluetooth specification since it splashed onto the scene as the go-to wireless audio solution of the new millennium. From wireless audio and device control through to the modern era of ultra-low-power data gathering and power-sipping pro audio, the Bluetooth specification continues to evolve and grow to meet modern wireless challenges and new opportunities for personal area networking. 

The Bluetooth 6 specification was released on August 27, 2024 and introduces a number of new features available to developers. They include Decision-Based Advertising Filtering to simplify advertisement scanning, Monitoring Advertisers to increase efficiency for duplicate advertising packets, ISOAL Enhancement to break up large data frames into smaller link-layer packets, and configurable Frame Space Update to define the delay between packets in a connection event of connected isochronous stream subevent. 

All of these features are useful for OEMs looking to finely adjust the functionality of their connection, increase device efficiency, and otherwise create a better user experience for their connected devices. However, the most dramatic new feature is Bluetooth Channel Sounding. With Bluetooth Channel Sounding, Bluetooth devices gain an enhanced ability to determine their range from other devices, and with this Bluetooth becomes much more useful for applications that depend on proximity or finding other devices in physical space. This is enabled by well-proven methods of ranging and security mechanisms that create a comprehensive and safe ranging capability. It also lets OEMs prioritize the aspects of ranging that matter most to their application, whether it’s security, accuracy, or latency that drives their primary use case. 

In this post, we’ll look closer at Bluetooth Channel Sounding, the underlying techniques and processes that make it special, and how manufacturers can leverage this feature into new ranging applications to delight their customers. 

A Brief History of Bluetooth Location Finding

For full details on the history of Bluetooth Location and the advancements that define Bluetooth Channel Sounding, the Bluetooth SIG has released a technical overview that covers each of these elements in greater technical detail. Please see the Bluetooth SIG’s Bluetooth Channel Sounding overview for an in-depth look at the history and present of Bluetooth locationing and positioning. What follows is a summary. 

In the Bluetooth 6 specification, Bluetooth Channel Sounding expands upon and improves upon a key Bluetooth use case that has been defined in the Bluetooth specification since Bluetooth LE was first introduced: The Find Me profile. This profile created a standardized approach for one device to act as a “Locator” and one as a “Target,” enabling one device to find the other by triggering a beep or an alert tone in the target device. But this functionality was nothing like a location-based solution. This early approach simply relied on generating an alert, but did nothing resembling direction-finding, ranging, or any of the features we would associate with “finding” in the sense we’ve come to expect from wireless devices. 

Future developments to the Bluetooth specification would eventually grant true ranging and location-finding tools as represented by projects like Apple’s iBeacon or Google’s Eddystone, which estimated distance between Bluetooth devices using received signal strength (RSSI), relying on measured signal attenuation. But these too were incomplete, as they lacked a means of determining the direction between two devices. Distance is one measure that’s critical to determining location, but without angle between two devices, positioning is impossible to estimate. 

With the Bluetooth 5.1 specification, AoA and AoD direction finding gave Bluetooth devices a few mechanisms to determine the angle between two devices, utilizing multiple antennas and the miniscule differences between the signal received at each to make intelligent judgments about the relative direction of the other device in a link. Subtle differences in the phase of the received signal created a useful picture of positioning, the first meaningful direction-sensing native to the Bluetooth specification. 

Now, Bluetooth Channel Sounding expands on this running history by creating the most accurate ranging measurements ever natively included in the Bluetooth Core Specification. 

Bluetooth Channel Sounding – How Does it Work? 

The full technical details of Bluetooth Channel Sounding are found in the Bluetooth SIG’s Bluetooth Channel Sounding overview, but the following is an attempt to simplify the mechanism for easier understanding. In short, this new feature relies on a few basic characteristics of every radio signal, which are ultimately waves with determinable frequency, amplitude, and measurable received phases. 

Put very simply, if we know the frequency of a transmitted wave, there is a lot we can determine: For one, we know the wavelength (the physical length of one cycle of that RF wave). We can use this information, in conjunction with the received phase of the wave on the other end, to start to narrow down the possible distances that the two devices may be separated by. For example, if we send a signal and it’s received at the “lowest” point of the phase of that wave, we know that the distance between the two devices is some multiple of the wavelength + the fractional wavelength that leads to the signal being received at that phase. Now, if this process is repeated again with a different frequency, we can complete a similar calculation, and importantly, find a distance that satisfies both measurements and that represents the most likely distance between the two devices.

We can then combine this with the measured difference between the send time and received time of a signal bounced from the sender to the receiver and back again. By doing so, we can further validate this expected distance against the measured wavelengths. Using the constant speed of light in addition to a fixed turnaround time between receiving the signal at device B and sending back to device A, we can estimate distance by the time over the air and validate the calculations made in the previous steps. We arrive at a much more precise distance than can be calculated by either of these methods alone. 

There are many additional factors, such as obstacles, signal scattering, interference and more, which make it impossible to determine this distance exactly. But where previous methods alone were only capable of achieving accuracy down to a margin of error of 3-5 meters, Bluetooth Channel Sounding drops that to as low as 10cm accuracy. 

Additionally, security mechanisms make this proximity measurement much harder to fake by a third party attacker. The combination of techniques described above provides information to the host device that a second device would have a very difficult time creating without being in the precise location it claims to be. This precision and security enables all kinds of range-based applications that rely on trust: door access control, indoor wayfinding, real-time location, logistics, and proximity detection are just a few.

Available Now in the Bluetooth 6 Specification

As with all developments in the Bluetooth Core Specification, it will be some time before silicon vendors begin to offer support for the features outlined in this new specification. After that, it will take time for this support to trickle through to module manufacturers and ultimately into the hands of developers. But the groundwork is now laid to provide the most accurate distance measurement ever supported by the Bluetooth SIG, and the full details and theory of operation are available to developers in the core specification itself. 

Ezurio is your connectivity expert, and we’ve always got our eyes on the horizon of the developing world of wireless. As a member of organizations like the Bluetooth SIG, the Wi-Fi Alliance, the LoRa Alliance, and more, we are committed to keeping our customers on the forefront of technological innovation and giving them the best path to integrating the latest tech in their unique designs. 

Our latest Bluetooth modules, the BL54 series, bring the latest in Nordic silicon to our Bluetooth portfolio with support for the nRF54H20 and nRF54L15 SoCs, with up to 2x ARM Cortex-M33 application processors and up to 2x RISC-V co-processors supporting Bluetooth LE 5.4 applications. Our Lyra 24 Series leverage Silicon Labs’ excellent EFR32BG24 SoC to bring our expertise in wireless to SiLabs’ software and design tools for Bluetooth 5.4 LE in SIP, SMT Module, or USB form factor. And our Vela IF820 represents a next-generation Bluetooth Classic and Bluetooth LE combo module that leverages Infineon’s AIROC CYYW20820 for a comprehensive Bluetooth offering in SIP, SMT Module, and USB form factor. 

To learn more about our Bluetooth portfolio, visit our website: 

https://www.ezurio.com/wireless-modules/bluetooth-modules 

For more details on the Bluetooth Core Specification v6, please visit the Bluetooth SIG at: 

https://www.bluetooth.com/blog/now-available-new-version-of-the-bluetooth-core-specification/