For a long time, many product developers have struggled to create proximity-based services that are accurate and resilient against security threats. Bluetooth Channel Sounding may be the new solution. A new feature introduced as part of the Bluetooth Core Specification 6.0, Channel Sounding, helps engineers measure the distance between two devices with sub-meter precision. 

The foremost leader in the global Bluetooth market, Nordic Semiconductor has long been a pioneer in the Channel Sounding movement. With the Nordic nRF54L Series, teams can easily integrate the technology into their next generation of connected edge devices.

Channel Sounding Basics

Designers can typically take two approaches when implementing products or applications with Channel Sounding. 

The first is Phase-Based Ranging (PBR), which measures the phase of a radio wave to get highly accurate results. On Nordic platforms, PBR works with two devices, an initiator and a reflector. The initiator starts by sending a tone on a specific frequency to the reflector. The reflector then captures In-phase and Quadrature (IQ) data –which represents the phase and amplitude of the signal–  and actively reflects the signal back to the initiator. 

With PBR, engineers can calculate the exact distance between two devices by analyzing the phase differences of signals across up to 72 frequencies. Using multiple channels is required to resolve the phase ambiguity, as a single frequency has ambiguity already after approximately 12.5 centimeters. This multi-channel approach also reduces the risk of multipath interference, which occurs when signals bounce off walls and objects and interfere with one another, and commonly degrades performance in crowded environments.

The second Channel Sounding technology is Round-Trip Timing (RTT), which measures the Time of Flight of the signal as it travels at the speed of light. While PBR is more accurate than RTT, engineers often use both to achieve results that are more secure against malicious adversaries. With a combined approach, teams can build systems that are both precise and inherently secure against external interference.

Overcoming Traditional Ranging Constraints

While engineers have historically used the Received Signal Strength Indicator (RSSI) for proximity detection, this approach has been fraught with issues. 

RSSI is fundamentally limited because it uses signal amplitude as the primary vector to calculate the distance between two points. Under this scheme, if there are physical obstructions between two devices, the signal strength can fluctuate wildly and lead to distance errors. 

Channel Sounding, on the other hand, overcomes these amplitude-related errors by looking at the signal's phase. Phase-based measurements are more reliable because phase is a geometric property of the signal that doesn’t fluctuate as the signal attenuates. In this way, Channel Sounding enables consistent distance measurement, even when objects or people obstruct the line of sight between the initiator and the reflector.

Channel Sounding is also a more cost-effective solution than competing ranging technologies such as Ultra-Wideband (UWB). While UWB offers high precision, it increases a designer’s Bill of Materials cost because it requires specialized chips and complex antenna arrays. Additionally, UWB consumes a lot of power because it operates only with high-rate pulsed signals. Channel Sounding, offers teams similar precision using existing Bluetooth radios with minimal hardware changes, making the production of high-volume consumer goods much more cost-effective.

Channel Sounding Evolution: Hardened Security for Sensitive Applications

Since Channel Sounding’s original release in 2024, the technology has already matured significantly.

For example, the latest Bluetooth Core specifications include security enhancements that address vulnerabilities in legacy ranging technologies. Specifically, conventional RSSI systems are susceptible to man-in-the-middle attacks, in which an unauthorized actor relays a signal to trick a device, such as a smart lock, into believing the owner is nearby. In contrast, Channel Sounding provides a physical-layer defense against these attacks because the signals' phase and timing are difficult to imitate without detection.

In practical terms, teams designing keyless entry systems can now securely add a distance-based security model using Nordic platforms. Specifically, they can design the system to ensure that a door unlocks only when the authorized device is within a specific meter range. The combination of PBR and RTT technologies means that any attempt to spoof the distance will lead to a data mismatch and keep the system locked. This level of hardware-verified security can be a game-changer for devices in which physical access control is the primary function.

Channel Sounding Evolution: Consumer Adoption with Smartphone Integration

Today, the broad adoption of Channel Sounding heavily depends on its compatibility with popular smartphones. Luckily, there are a few positive developments in this area. 

For example, Google launched Channel Sounding support in the Pixel 10 and also enabled it on the Pixel 9. Market trends indicate that other major manufacturers will likely follow Google’s lead in their upcoming releases. As a result, the industry will likely soon see the widespread deployment of accessories and tags that use Channel Sounding to interact with new devices. Having Channel Sounding accessible in consumers’ phones will enable any product, such as a smart ring, remote control, smart appliance or a computer mouse to benefit from knowing its distance to the user, and vice versa.

Nordic makes development easier for product developers with an open-source Android application that natively supports Channel Sounding. With the open-source code, engineers can spend less time building and troubleshooting the initial functional prototype and more time perfecting the application logic.

Nordic and Channel Sounding

Designing Channel Sounding applications requires specialized Bluetooth hardware. As a long-time leader in the Bluetooth market, Nordic Semiconductor answers this call with something like the nRF54L Series of ultra-low power wireless SoCs. Combining a multiprotocol 2.4 GHz radio with a high-performance 128 MHz Arm Cortex-M33 processor, the nRF54L Series features built-in support for Channel Sounding to help engineers get started more quickly. 

Nordic designed the nRF54L Series to optimize the ranging process. For example, the radio has a transmit power of up to +8 dBm and a receiver sensitivity of -98 dBm at 1 Mbps Bluetooth Low Energy, guaranteeing a reliable connection even at the edges of the measurement range. The chip also has a global real-time clock and high-resolution timers that help the system compute the precise time-of-flight information needed for RTT.

By combining these Channel Sounding features with an ultra-low-power Bluetooth LE radio, a powerful compute core and advanced security features, the nRF54L Series is a comprehensive solution for product designers. With it, designers can incorporate Channel Sounding into their products without the usual design tradeoffs between power consumption, cost, and physical board size. 

Optimized Power and High-Volume Asset Tracking

For high-volume applications like asset tracking, the reflector must be power efficient. Typically, the distance computation happens on the initiator device, which can be an nRF54L Series device with a power-efficient compute core, instead of the reflector. This means a small tracking tag can act as a simple reflector without needing to spend processing power to calculate complex phase corrections. As a result, designers can build tags with smaller power sources and longer battery lives.

Similarly, in industrial settings like hospitals, nRF54L-based systems can perform trilateration to help staff find assets in real time. Trilateration determines a specific location by measuring the distances between a target asset and three or more known points. By deploying multiple nRF54L-based beacons at fixed locations, systems can calculate an asset’s 2D or 3D position with high confidence even if it’s moving. In a hospital environment, such features can help medical staff immediately find equipment. In warehouses, the same features can help managers optimize the flow of goods with pinpoint accuracy.

Implementation with the Nordic nRF54L Series

Bluetooth Channel Sounding uniquely unlocks precise, secure, and cost-effective ranging. As the technology matures and companies continue to build it into mass-market consumer devices, Channel Sounding will soon become a dominant ranging technology in both consumer and industrial sectors.

With specialized hardware like the nRF54L series and comprehensive SDK tooling, Nordic offers the industry the resources it needs for widespread adoption. Together, we can build the future of intelligent, connected devices.