Bluetooth Channel Sounding for Accurate Distance Measurement

Ranging is the process of determining the distance from one location or position to another. Bluetooth-based ranging is currently used in many applications wherein the distance between the two Bluetooth devices needs to be accurately determined.

Bluetooth Core Specification v6.0 has introduced a new capability called Bluetooth Channel Sounding (CS), which facilitates secure centimeter-level, real-time distance measurements between two Bluetooth devices. This breakthrough opens the way to a new class of autonomous, location-aware proximity-based systems.

Traditional Bluetooth systems use received signal strength indication (RSSI), a relative measurement that is mostly defined by each chip manufacturer for localization. RSSI is a method with limited accuracy. It typically achieves accuracies of one to four meters and has known security vulnerabilities. In contrast, Bluetooth CS uses Phase-Based Ranging (PBR) for accuracy, combined with round-trip time (RTT) for end-to-end security.

CS requires Bluetooth Low Energy (BLE) transmitters (beacons) and receivers that perform PBR by exchanging tones, enabling a more accurate distance estimate through measuring the phase shift in the tones. An initiator device sends cryptographically scrambled packets to a reflector device, which returns the packets. This process is repeated across multiple frequencies. The distance between the devices is calculated based on the phase differences between the transmitted and received signals over those frequencies.

Bluetooth CS incorporates RTT as a countermeasure against sophisticated man-in-the-middle relay attacks that involve attackers secretly intercepting and altering communications between two parties who believe they are directly connected, enabling real-time theft of login credentials or financial data.

Bluetooth technology is already embedded in all major consumer platform devices, including smartphones, tablets and laptops. With Bluetooth CS, developers will now be able to add true distance awareness to Bluetooth-connected devices. RSSI can be used to determine presence, proximity and course ranging when the devices are farther apart and Bluetooth CS will be used to increase accuracy as the two devices approach each other.

What is the Expected Accuracy?

The Bluetooth CS feature was designed to achieve centimeter-level accuracy, which means the accuracy of a distance measurement should be plus or minus tens of centimeters of the actual distance. It uses up to 72 channels in the 2.4GHz spectrum and supports up to four antenna paths between devices to minimize multipath effects and further enhance accuracy.

The maximum range of Bluetooth CS depends on several factors, with one of the most significant being the transmission power of the Bluetooth devices involved. If both devices can transmit at the maximum allowed power level, accurate measurements at distances up to 150 meters can be achieved.

Bluetooth CS introduces an entirely new protocol stack, including a Physical Layer (PHY), which may require a BLE integrated circuit upgrade to enable support for the feature.

Applications

Bluetooth Human-Machine Interface (HMI) solutions can enhance personnel safety by only allowing usage from a safe distance and Bluetooth device networks, such as those used for wireless lighting control, can self-optimize to enhance system performance.

Other applications include remote keyless entry to a vehicle and smart locks in homes. It can be used for zonal detection through ranging to authenticate approaching users and unlock the vehicle. It employs Bluetooth ranging by using the established connection between data channels, specifically between the key fob or phone and the vehicle. You can remotely operate smart home locks through a smart device and permit access only when an authorized device is within a designated proximity to the lock.

BCS enables precise, real-time tracking of assets or people. This application involves storing goods, tracking their movement, controlling inventory and managing various operations and logistics within a warehouse.

Bluetooth mice, keyboards and game controllers could automatically switch between active and inactive states based on their distance from a smartphone, tablet or laptop. Bluetooth HMI solutions can enhance personnel safety by only allowing usage from a safe distance. And Bluetooth CS can elevate the performance of Find My applications by delivering centimeter-level accuracy.

Available Examples

NXP’s new channel-sounding-certified KW47 and MCX W72 wireless MCUs will help automakers with distance measurement, bringing an additional ranging solution for car access and autonomous systems and will be utilized across a broader spectrum of applications. Also currently available is Silicon Labs xG24-DK2606A EFR32xG24 Channel Sounding Dev Kit, designed to support the development and prototyping of products that use channel sounding for distance estimation over a Bluetooth LE connection. When the kit is connected to a PC running Simplicity Studio, a channel-sounding visualizer tool allows customers to effortlessly view distance measurements in real time.

The TI SimpleLink 2.4GHz CC27xx family of MCUs enables improved ranging technology for channel sounding, introduced in the Bluetooth Core Specification version 6.0. These devices are said to be the industry's first with an integrated algorithm processing unit (APU) to help engineers perform channel sounding functions with high accuracy and low latency. And the Nordic Semiconductor nRF Toolbox mobile application for Android now includes a channel sounding mode, allowing compatible phones to interact with Bluetooth CS reflectors, showing the distance between them on the screen.