Introduction to Bluetooth® Channel Sounding

However, Bluetooth Low Energy has also been establishing itself as a reliable technology for device positioning applications. There was the first generation with presence detection with Bluetooth beacons (Received Signal Strength Indicator - RSSI), followed by Bluetooth Direction Finding with Angle of Arrival (AoA) in 2019.

More recently the new Bluetooth® Core 6.0 (released in September 2024), included a new feature enabling a secure and more accurate ranging technique known as "Channel Sounding".

Today we can imagine a world where our Bluetooth devices can accurately understand their distance from each other (between 0.3-1m), enhancing security and convenience in ways that it was never possible before with Bluetooth technology. This is the promise of Bluetooth Channel Sounding.

Bluetooth technology positioning techniques evolution

Key Features:

Bluetooth Channel Sounding provides true distance awareness between two Bluetooth devices. This feature enhances convenience, safety, accuracy and security of Bluetooth connected devices.

• True Distance Awareness: Allows devices to measure the precise distance between them, enhancing applications such as 'Find My' solutions and digital key solutions.
• High Accuracy: Utilizes Phase-Based Ranging (PBR) and Round-Trip Time (RTT) methods to achieve 0.3-1m level accuracy. The range estimation accuracy can be further improved by using up to 4 antenna paths.
• Security: Incorporates robust security measures such as combining PBR and RTT methods to cross-check results to detect possible inconsistencies and protect against relay attacks and spoofing.
   

How It Works:

The Bluetooth specification outlines the necessary radio features, stack features, security measures, and measurement procedures to collect raw data. A dedicated algorithm at the application level then converts the raw data into distance.

It measures the phase difference between radio signals and/or the roundtrip time of the radio signal to estimate the distance between two Bluetooth devices.

There are two methods available that provide precise and secure distance measurements between two Bluetooth devices, offering flexibility in accuracy, security, range latency and complexity: Phase-Based Ranging (PBR) and Round-Trip Time (RTT).

Both techniques can be used independently or in conjunction with each other, with RTT acting as a secure distance bounding technique to cross-check the PBR measurement that provides a highly accurate distance measurement.

Phase-Based Ranging (PBR) 

measures the phase differences of signals sent across multiple frequencies to calculate distances.

1. Signal sent from one device to another and back again
2. The received signal’s phase is recorded
3. Procedure is repeated across multiple frequencies 
   (up to 72 channels available for transmission)
4. Distance is calculated based on the phase difference between the transmitted and received signals at these frequencies

Round-Trip Time (RTT) 

uses CS_SYNC packets to measure the time it takes for signals to travel back and forth, providing a secure distance measurement. It can be used standalone for coarser ranging over longer distances.

1. Scrambled packet (called CS_Sync) sent from one device to another and back again
2. The time needed to send a packet back and forth between two devices is measured
3. Turnaround time at reflector is subtracted
4. The distance is calculated, knowing the round-trip time and the speed of light

The Initiator is the device that initiates the 2-way ranging to calculate its distance from another device, while the Reflector is the device responding to the initiator. The distance calculation is only performed in the Initiator and the Reflector doesn’t receive the information as part of the procedure.

It is worth noting that the calculation of the distance measurements is outside the scope of the Bluetooth Channel Sounding specification and is done on application level.

Comparing different Bluetooth positioning technologies and UWB

Advantages: Enhanced accuracy and security in Bluetooth enabled devices, making it a valuable addition to the Bluetooth technology.

• High Accuracy: It provides 0.3-1m level accuracy in distance measurements, which is significantly better than all previous Bluetooth ranging techniques (without infrastructure).
• Enhanced Security: Adds an extra layer of protection with the Round-Trip Time (RTT) method reducing /removing/mitigating the risk of signal manipulation and relay attacks by checking for mismatches with the Phase-Based Ranging (PBR) distance measurement.
• Improved User Experience: It improves the performance of location-based services, such as Find My solutions, making it easier and faster to track and locate lost items, people or pets.
• Energy Efficiency: It is designed to be low-cost and power-efficient, making it suitable for a wide range of applications requiring sub-meter positioning accuracy.
• Versatility: It is applicable to many segments of the consumer and industrial markets, opening up new areas of applications.
• Ubiquity: Bluetooth technology is already embedded in all major consumer devices and applications, paving the way for easy and accurate distance measurements for newer generations of products.

Zoom in on Channel Sounding

It offers standardized security features of Bluetooth Channel Sounding

• Deterministic Random Bit Generator (DRBG) during security startup data exchange for random bit patterns in transmission
• Normalized Attack Detector Metric (NADM) to assess the likelihood of an attack on a communication channel
• Cross-checks the results of both methods (PBR and RTT) to detect inconsistencies in distance estimation.

Single antenna vs multiple antennas

• The Bluetooth Channel Sounding standard works with either a single or multiple antennas on both sides of the connection. However, using multiple antennas improves robustness and accuracy in the distance measurement.
• Devices may include multiple antennas for use during phase-based ranging exchanges. The maximum total number of antennas for use during PBR exchanges is 4.

Applications

Customer case example:

Customers are interested in including a Bluetooth Channel Sounding module in their tools to be able to locate them accurately in their facility. This ensures tools are where they should be, reduces time spent locating them, and improves efficiency.

In addition to meeting customer needs with Channel Sounding, proximity alerts can be triggered when tools approach hazardous areas, informing users of potential dangers. This feature can also help prevent theft by alerting users if tools move outside a designated area, allowing for quick actions such as locking the tools and setting off an alarm to deter the thief.

Demo: Channel Sounding for industrial applications with NORA-B2

The Bluetooth Channel Sounding feature can be utilized effectively in industrial environments. To demonstrate this, we have created a model factory building to illustrate a common use case: access control. In this demonstration, the EVK-NORA-B206 acts as the Channel Sounding Initiator, enabling the automatic opening of a factory building door when a tag is brought within proximity (approximately 80 cm). This advanced functionality not only enhances operational efficiency but also showcases the potential of Bluetooth technology in facilitating smart automation.

Frequently Asked Questions