Bluetooth LE is widely used in a wide range of fields, from healthcare, consumer electronics, and audio to industrial devices, and has established itself as the most common and reliable technology for implementing device location solutions. This short-range wireless technology can be used to detect and report the presence of other nearby devices, estimate the distance between devices, or calculate the direction in which other devices are located.

Bluetooth Channel Sounding, newly added in the latest Bluetooth Core Specification update (included in Bluetooth 6.0 details), enables safer and more precise distance measurement between two Bluetooth devices, heralding new possibilities for various innovative wireless proximity sensing and distance measurement applications. In particular, channel sounding is expected to make significant progress in terms of standardization and interoperability as it is widely adopted in various battery-based Bluetooth LE products, including mobile phones.

In this article, we will examine channel sounding technology and explore how developers can implement new distance measurement applications using Nordic Semiconductor's SoCs and development tools.

■ Overview of Channel Sounding Technology

Bluetooth LE has continuously advanced device positioning and location services technology. For example, 'Find Me Profile' was included as part of the Bluetooth Core specification when Bluetooth LE was first released and is considered one of the key features of Bluetooth 4.0.

Subsequently, as beacons emerged as a major application of Bluetooth LE, the 'TX Power' value of the Bluetooth specification was used as a reference output value corresponding to a distance of 1 meter from a beacon or other transmitting device. Since RF signals attenuate at a rate inversely proportional to the square of the distance depending on the distance from the transmitter, knowing the TX Power value allows one to roughly estimate the distance between beacons or between other transmitters and receivers (e.g., close, connected, or out of range) using the measured RSSI (Received Signal Strength Indicator).

In 2019, Bluetooth Direction Finding was introduced as part of the Bluetooth 5.1 specification. This technology allows applications to precisely calculate the direction of a received signal using phase information measured by a Bluetooth LE controller. This direction finding function is defined in two ways: AoA (Angle of Arrival) and AoD (Angle of Departure).

Channel sounding possesses several key elements that enable the implementation of simpler yet reliable distance measurement applications. First, this technology ensures interoperability through standardization. Second, it can be added to high-end products or supported by very simple devices with minimal software space and without additional hardware costs. Third, power consumption is as low as that of basic Bluetooth LE-based data transmission. Finally, channel sounding supports various hardware and software configuration options based on accuracy, latency, security, and power consumption.

The Bluetooth specification for channel sounding is defined by new radio (PHY) and controller functions, as well as procedures and security measures required for collecting raw measurement data. However, the process of converting this raw data into distance measurements using dedicated algorithms is performed at the application level and is not included within the scope of the specification. Therefore, the measurement and algorithm configurations can be adjusted according to the balance between accuracy, latency, security, and power consumption required by the application.

Channel sounding is implemented through quite complex techniques, but in particular, it is based on Phase-Based Ranging (PBR) or Round-Trip Timing (RTT). PBR measures distance through the phase shift of a signal reflected back from a reflector device by a signal transmitted from an initiator device across multiple frequencies. The task of converting this raw data into distance information is performed at the application level using dedicated algorithms. Meanwhile, the RTT technique measures distance based on the time it takes for a wireless packet to travel back and forth between an initiator and a reflector. RTT can be utilized as a Distance Bounding Technique to enhance stability by cross-verifying PBR measurements, and the algorithm used for distance calculation is relatively simpler than that of PBR. (Refer to the box article 'Principles of Precise Distance Measurement in Channel Sounding')

■ Support for new applications

As with all Bluetooth LE improvements, Channel Sounding technology will enable a variety of new applications that were previously unimaginable, some of which are already becoming a reality. One example is enhanced tag solutions. While current tag solutions operate without major issues, it can be difficult to perceive the vibrations and sounds generated upon activation when the device is covered by a cushion or blanket. Channel Sounding overcomes the limitations of sound or vibration alerts by providing proximity alerts—such as "getting closer" or "getting further away (Hot-Cold)"—based on distance measurements, even over long distances.

Another application that can benefit from channel sounding technology is smart locks. It can not only enhance the detection of individuals attempting to operate the lock but also provide robust protection against Man-in-the-Middle attacks and relay attacks. Furthermore, this technology can contribute to improving the functionality of home appliances. It can significantly enhance the user experience through context awareness based on user presence or distance information. For example, the physical situational awareness information provided by channel sounding can be usefully utilized to link multiple devices, and safety can be enhanced by ensuring that control functions operate only when the user is near the device.

Bluetooth LE is already widely used in asset tracking and location services. Channel sounding can improve the precision, reliability, and convenience of these applications without adding significant complexity or cost to existing solutions.

■ Channel Sounding Development Support

Nordic’s fourth-generation wireless SoCs, the nRF54L and nRF54H series, support Bluetooth 6.0 and channel sounding. Nordic provides development support for the nRF54 series through its scalable integrated software development kit, the nRF Connect SDK.


While Bluetooth channel sounding provides an excellent foundational technology for PBR and RTT-based proximity sensing and distance measurement applications, Nordic also supports another proprietary option. The Nordic Distance Toolbox (NDT) provides advanced distance measurement and proximity sensing capabilities based on PBR and RTT for developers who wish to implement these functions in applications outside the Bluetooth ecosystem using Nordic’s nRF52 and nRF53 series SoCs.

The software libraries provided in the nRF Connect SDK include algorithms capable of precisely calculating the distance between NDT-enabled devices, offering significantly higher accuracy compared to existing solutions that rely solely on RSSI. Additionally, the nRF Connect SDK includes sample code to demonstrate NDT capabilities.

According to Fortune Business Insights, the asset tracking market is projected to grow from $21.25 billion in 2023 to $59.64 billion in 2032. In addition, advanced distance measurement and proximity sensing capabilities are expected to be utilized in many other applications. With the emergence of Bluetooth channel sounding technology, developers have gained a new, powerful tool to implement competitive applications in rapidly growing markets such as asset tracking and security.

Principle of precise distance measurement in channel sounding Phase-based Ranging (PBR) measures distance using an initiator and a reflector. The initiator transmits a signal of a predetermined frequency, and this signal is reflected back by the reflector. At this point, the phase difference between the transmitted signal and the received signal is measured. This process is repeated using signals of different frequencies. Then, the distance between the initiator and the reflector can be calculated using a dedicated algorithm that utilizes the difference between the signal's phase and frequency (Figure 1). Round-trip Timing (RTT)-based distance measurement calculates the distance by measuring the time it takes for a packet to travel from the initiator through the reflector and back, multiplying this round-trip time by the speed of light (C), and then dividing by 2. In this process, the time it takes for the reflector to receive and retransmit the packet is subtracted from the total Time-of-Flight (ToF) to calculate the accurate distance (Figure 2). The distance between the initiator and the reflector can be measured using the phase difference between two different frequency signals. The distance between two devices can be calculated using the Time-of-Flight (ToF) of wireless packets transmitted between the initiator and the reflector.

※ Contributor
Ha Byeong-woo, General Manager, Nordic Semiconductor