Improving Accuracy with a New Channel Sounding Dev Kit

When Bluetooth Channel Sounding was adopted as part of Core Specification 6.0, the Bluetooth Special Interest Group (SIG) communicated a target accuracy of 0.5 m in most scenarios. This accuracy target created a challenge because in most scenarios it’s difficult or even impossible to reliably achieve 0.5 m of accuracy without the use of multiple antennas. Device to device relative orientation, human body, and environmental obstructions, as well as multipath interference all conspire to limit the reliability of single antenna ranging with Bluetooth Channel Sounding.

This challenge was made more formidable by the fact that Bluetooth Channel Sounding is a bi-directional ranging process, with both initiator and reflector in turn transmitting and receiving tones and packets. Both ends are fairly symmetrical in their capabilities, especially compared to Bluetooth Direction Finding, which defines multiple asymmetries of capability and antenna requirements between two endpoints. Due to the symmetrical nature of Bluetooth Channel Sounding, both communication endpoints must understand each other's antenna capabilities and switching patterns to accurately capture data, analyze it, and determine relative distance.

Like any new feature, the goal of Bluetooth Channel Sounding is interoperability. Standardizing a solution so that it ‘just works’ from phone to phone and device to device provides product designers with the assurance they need to deploy worldwide. The only way to achieve this ‘just works’ functionality with multiple antennas is to standardize antenna usage along with the rest of the Bluetooth Channel Sounding procedure.

Though other aspects of this technology are considered leading edge and groundbreaking, the flexible though controlled standardization of antenna switching for Bluetooth Channel Sounding is no less critical. This feature ensures Bluetooth Channel Sounding delivers on its promise of robust accuracy with maximized interoperability.

Why You Should Design Products with Multiple Antennas

Two wireless devices most effectively communicate when their antennas are tuned to a frequency and the boards have clear line of sight and/or sufficient power to be received through all the materials and interference sources in the environment. Walls and barriers, including humans, floors and ceilings, and other RF signals can all have detrimental impact on two antennas’ ability to communicate.

Additionally, relative antenna orientation can cause issues. Device A’s antenna orientation relative to Device B’s antenna isn’t something that can be controlled or predicted in environments with mobile, battery-powered IoT devices. Unfortunately, some orientations will create frequency-dependent nulls and severe amplitude reductions. If the two IoT devices are positioned at just the right angle and distance from each other, they’ll no longer be able to communicate.

In asynchronous connection-oriented logical (ACL) transport communication over Bluetooth Low Energy (LE) connections, these nulls tend to cause packet retries and potentially dropped connections. In Bluetooth Channel Sounding, the impact can be more nuanced, where IQ data from PBR measurements will have distorted phase information, which introduces error in distance estimation algorithms.

Devices with multiple antenna orientations can exchange phase-based data for the same channel from different antenna orientations, increasing the likelihood of capturing undistorted IQ data describing the channel.

Undistorted IQ data leads to highly accurate distance estimation and a better end solution.

Designs should always consider adding a second antenna to a Bluetooth Channel Sounding design. As can be seen in our antenna guidelines document, adding a second antenna is possible even in constrained form factors. The EFR32xG24 Bluetooth Channel Sounding Dev Board provides a best practices-based design example supporting two antennas in a form factor of less than 33 x 33 mm. This form factor is itself bigger than a final product would be, as the Bluetooth Channel Sounding Dev Board offers a full debug circuit on-board.

We’ll talk more about the board later, but first let’s discuss how Core Specification 6.0 actually standardized the use of multiple antennas.

How Channel Sounding Antenna Switching Works

There are three fundamental ways that antenna switching has been standardized in the Bluetooth Core Specification 6.0.

• The capabilities exchange
• Mode 2 phase-based ranging operation
• IQ data structure exchange

During capabilities exchange, the peripheral in the connection responds to a request indicating the maximum number of antennas it can use, and the number of antenna paths it can support. An antenna path is exactly what it sounds like: the path Board A’s antenna takes to communicate with Board B’s antenna. Some devices may not be able to support multiple antenna paths because of board design or memory limitations.

Before a procedure starts, a configuration gets selected by the controller and communicated to the reflector so that the number of antenna paths and the antenna configuration are understood symmetrically.

During a Bluetooth Channel Sounding step with mode 2 phase-based ranging is being executed, the initiator transmits tones across the channel from each antenna path, in a pattern understood by the reflector. The reflector then transmits tones on the same channel across the same sequence of antenna paths.

A Bluetooth Channel Sounding algorithm running on the initiator saves all the antenna path-specific IQ data for that channel, but before it can derive a distance, it also needs the reflector’s corresponding IQ data.

During data retrieval from reflector to initiator, which happens over the LE ACL connection, IQ data needs to be transmitted with a pre-defined data structure. This data structure is also defined as part of the Ranging Profile, which was adopted a few months after the core specification updated.

Silicon Labs supports all of the functionality described above with our Bluetooth Channel Sounding solution running on an EFR32 xG24. Our Bluetooth stack has been qualified to be compliant with 6.0 requirements, and our implementation of the Ranging Profile is qualification-ready. All of this functionality can be evaluated using BRD2606.

But What About Designs That Have Only a Single Antenna?

Not all designs can follow every design best practice. Board or cost constraints can force a design to use only a single antenna. The reliability of the distance estimation algorithm might be diminished, but even so, those devices are still qualifiable for Bluetooth Channel Sounding. Operating with a single antenna path, where Device A and Device B both only have one antenna path is still a valid design choice.

The test below was run in an office environment, which creates reflections and multipath interference. Tests were conducted with BRD2606, but only exercising one antenna per board, resulting in a single antenna path.

In these eight test runs, two boards were 11 meters apart, and one board was rotated such that the antennas on the boards were either co-polarized or cross-polarized.