Bluetooth LE is widely used in various fields ranging from healthcare, consumer, audio, and industrial devices, and is also the most common and stable technology for implementing device location solutions. This short-range wireless technology can be used to detect and report the presence of other devices in the vicinity, estimate the distance between devices, or calculate the direction in which other devices are located.

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

In this article, we will look at channel sounding technology and how developers can implement novel distance measurement applications using SoCs and development tools from Nordic Semiconductor.

■ Channel Sounding Technology Overview

Bluetooth LE has continued to develop positioning and location services for devices. 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.

Since Beacon emerged as a major application of Bluetooth LE, the 'TX Power' value of the Bluetooth specification has been used as the reference output value corresponding to a distance of 1 meter from a Beacon or other transmitting device. Since RF signals are attenuated at a rate inversely proportional to the square of the distance from the transmitter, knowing the TX Power value can be used to roughly estimate the distance between beacons or other transmitters and receivers (e.g., close, connected, or out of range, etc.) 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 an incoming signal using phase information measured by the Bluetooth LE controller. This direction finding function is defined in two ways: Angle of Arrival (AoA) and Angle of Departure (AoD).

Channel sounding has several key elements that enable simpler and more reliable distance measurement applications. First, the technology ensures interoperability through standardization. Second, it can be added to advanced products or supported by very simple devices with minimal software footprint and no additional hardware cost. Third, it has low power consumption, similar to basic Bluetooth LE-based data transmission. Finally, channel sounding supports a variety of hardware and software configuration options depending on accuracy, latency, security, and power consumption.

The Bluetooth specification for channel sounding defines new radio (PHY) capabilities and controller capabilities, procedures for collecting raw measurement data, and security measures. However, the conversion of this raw data into distance measurements using proprietary algorithms is performed at the application level and is not within the scope of the specification. Therefore, the measurement and algorithm configuration can be adjusted depending on the balance between accuracy, latency, security, and power consumption required by the application.

Channel sounding is implemented using fairly complex technologies, but in particular, it is based on Phase-Based Ranging (PBR) or Round-Trip Timing (RTT). PBR measures the distance by measuring the phase shift of the signal transmitted from an initiator device over several frequencies and reflected back by a reflector device. The conversion of this raw data into distance information is done at the application level by dedicated algorithms. Meanwhile, the RTT technique measures the distance based on the time it takes for a wireless packet to travel back and forth between the initiator and the reflector. RTT can be used as a distance bounding technique that strengthens stability by cross-validating the PBR measurement value, and the algorithm used for calculating the distance is relatively simpler than PBR. (See the box article 'Precise distance measurement principle of channel sounding')

■ New application support

As with all Bluetooth LE enhancements, channel sounding technology will enable a range of new applications that were previously unimaginable, some of which are already happening. One example is improved tag solutions. While today’s tag solutions work well, they can be difficult to detect when activated by vibration and sound when covered by cushions or blankets. Channel sounding can overcome the limitations of audible or vibration alarms by providing proximity alerts, such as “hot or cold,” based on distance measurement, even at long ranges.

Another application that can benefit from channel sounding technology is smart locks. In addition to improving the detection of people trying to operate the lock, it can provide powerful features to protect smart locks from man-in-the-middle attacks or relay attacks. This technology can also contribute to improving the functionality of home appliances. The user experience can be greatly improved through situational awareness based on the presence or distance of the user. For example, the physical contextual awareness that channel sounding provides can be usefully leveraged to link multiple devices, while also improving safety by ensuring that control functions only work when the user is near the device.

Bluetooth LE is already widely used in asset tracking and location services. Channel sounding can improve the accuracy, 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 Series and nRF54H Series, support Bluetooth 6.0 and channel sounding. Nordic provides development support for the nRF54 Series via its scalable, unified software development kit, the nRF Connect SDK.


While Bluetooth channel sounding provides an excellent foundation technology for PBR and RTT-based proximity detection and ranging applications, Nordic also supports another option in its own proprietary way. Nordic Distance Toolbox (NDT) provides advanced distance measurement and proximity detection capabilities based on PBR and RTT for developers who want to implement these capabilities 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 that can precisely calculate the distance between NDT-enabled devices, providing much higher accuracy than existing solutions that only use RSSI. The nRF Connect SDK also includes sample code that demonstrates NDT functionality.

According to Fortune Business Insights, the asset tracking market is expected 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. The advent of Bluetooth channel sounding technology gives developers a powerful new tool to implement competitive applications in fast-growing markets such as asset tracking and security.

Principles of precision distance measurement in channel sounding Phase-based ranging (PBR) measures distance using an initiator and a reflector. The initiator transmits a signal of a fixed frequency, which is reflected back by the reflector. At this time, the phase difference between the transmitted and received signals 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 by using the difference between the phase difference and the frequency of the signal (Figure 1). Round-trip timing (RTT)-based distance measurement measures the time it takes for a packet to return from the initiator to the reflector, multiply this round-trip time by the speed of light (C), and then divide by 2 to calculate the distance. At this time, 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 exact distance (Figure 2). ▲Box article use Figure 1. The distance between the initiator and the reflector can be measured using the phase difference between two different frequency signals. ▲Box article use Figure 2. The distance between two devices can be calculated using the ToF (Time-of-Flight) of the wireless packets transmitted between the initiator and the reflector.

※ Contributor
Ha Byung-woo, CEO, Nordic Semiconductor