Published : February 2, 2026
What is Channel Sounding?
Explaining the Latest Bluetooth Distance Measurement Feature
Channel Sounding, a new feature in Bluetooth Low Energy (Bluetooth LE), enables high accuracy distance measurement that far surpasses traditional methods based on Received Signal Strength Indicator (RSSI). This feature is attracting significant attention because it is expected to make possible not only great accuracy distance measurement below one meter, which was previously difficult, but also ranging over wide areas.
This article provides a comprehensive explanation of the basic principles of Channel Sounding, key points for indoor and outdoor ranging, and anticipated future use cases.
Basic Principles of Channel Sounding
Channel Sounding uses a technique that measures the propagation characteristics for each frequency channel to accurately determine the distance between devices. This method is part of the high accuracy measurement process newly introduced in the Bluetooth Core 6.0, and it is notable for its ability to obtain multifaceted information beyond just RSSI. Specifically, it sequentially measures multiple channels allocated in the 2.4 GHz band and calculates distance with high accuracy using time and phase information of the propagating radio waves. This provides precise distance results. While the measurement with this technique can be performed with a single antenna, using multiple antennas or combining algorithms that analyze and correct for multipath effects caused by obstacles or walls can further improve accuracy.
Fundamentals of Channel Sounding
Channel sounding combines two main methods for distance measurement: Phase-Based Ranging (PBR) and Round-Trip Time (RTT). Channel Sounding operates between two devices: the initiator (a device sending a signal and measuring the distance) and the reflector (a device responding to the initiator). These devices repeatedly send and receive signals using multiple frequency channels (up to 72 channels) within the Bluetooth 2.4 GHz band.
Phase-Based Ranging (PBR) Measurement Method
PBR calculates distance by utilizing the phase differences between the transmitted and received signals.
1. Signal Transmission: The initiator sends an unmodulated tone at a specific frequency.
2. Phase Measurement and Response: The reflector receives this signal, measures its phase, and then sends back an unmodulated tone to the initiator.
3. Phase Difference Calculation: The initiator compares the phases of the sent signal and the returned signal from the reflector and calculates the difference.
4. Frequency Change and Repetition: This process is repeated at multiple frequencies. Plotting the phase differences across frequencies yields a “phase difference vs. frequency” curve.
5. Distance Calculation: Based on the principle that the phase difference is a function of propagation distance and frequency, the distance between the two devices is calculated with high accuracy from this curve.
To improve measurement accuracy in multipath environments (where reflected signals are present), PBR can use Inverse Fast Fourier Transform (IFFT). The phase difference data for each frequency represents the channel’s response (including both amplitude and phase) in the frequency domain. Applying IFFT to this data reconstructs the time-domain waveform of the signal, generating the “channel impulse response,” which shows how the signal changes over time as it passes through the channel. Peaks in the impulse response correspond to the arrival times of direct and reflected waves. The earliest peak corresponds to the direct wave, which is the signal that traveled the shortest path from initiator to reflector. The time at which this direct wave peak appears is the signal’s propagation time. Multiplying this propagation time by the speed of radio waves yields the precise distance between the two devices.
Round-Trip Time (RTT) Measurement Method
RTT measures distance by timing how long it takes for a packet sent from the initiator to reach the reflector and for the response packet to return to the initiator.
1. Exchange of Synchronization Packets: The initiator and reflector exchange special synchronization packets called CS_SYNC.
2. Measurement of Round-Trip Time: The initiator measures the time from sending a packet to receiving the response from the reflector --- this is the “round-trip time.”
3. Distance Calculation: By subtracting the processing time at the reflector (turnaround time) from the measured round-trip time, the one-way travel time (Time of Flight: ToF) is calculated. Multiplying this ToF by the speed of radio waves gives the distance.
Because RTT is a time-based measurement, it is difficult to manipulate the signal, reducing the risk of man-in-the-middle or relay attacks.
- KAGA FEI offers wireless modules that support Channel Sounding and Bluetooth Core 6 and are qualified by Bluetooth SIG. Detailed specifications for each module are available on our website.
- ・Channel Sounding Compatible Modules:
EC4L15BA1 / EC4L10BA1 / EC4L05BA1 / ES4L15BA1 - ・Press Releases:
- KAGA FEI Develops EC4L15BA1 Bluetooth Low Energy Module Balancing Low Power Consumption with High Processing Capability
- Develops Ultra-small Bluetooth Low Energy Module Compatible with Bluetooth 6.0 (ES4L15BA1 announced)
- Expands Wireless Module Lineup with Bluetooth 6 Support (EC4L10BA1 / EC4L05BA1 announced)
Distance Measurement Indoors and Outdoors
In real-world environments, radio wave propagation characteristics and interference factors affect ranging. Since indoor and outdoor environments differ, the way radio waves propagate also differs. To achieve high accuracy with channel sounding-based ranging, it is essential to understand these differences and optimize measurement strategies accordingly.
Key Points for Indoor Ranging
Indoors, radio waves reflect off various objects such as partitions, steel furniture, floors, ceilings, and even people, reaching the reflector via multiple paths. As a result, multiple arrival times and phase information are mixed in the received signal. By analyzing the impulse response using IFFT, direct and reflected waves can be separated, and the arrival time of the earliest direct wave can be identified. This allows for accurate distance calculation by eliminating the influence of reflected waves. However, in very complex multipath environments, the direct wave may be weak and buried among reflected waves, making accurate peak detection difficult.
Key Points for Outdoor Ranging
Outdoors, there are generally fewer obstacles and better line-of-sight, allowing signals to propagate over longer distances. However, if the distance is too great, signal strength weakens, making accurate ranging difficult. While there are no walls as indoors, large obstacles such as buildings, trees, hills, and vehicles may exist, and multipath effects from reflections off the ground or water surfaces must be considered. As with indoor environments, it is important to use signal processing techniques such as IFFT to separate direct and reflected waves.
Use Cases Enabled by Channel Sounding
Accurate distance information enables a wide range of applications. By precisely determining the distance between Bluetooth devices, it becomes possible to build new services and systems that enhance security and convenience.
Enhanced Digital Keys and Access Control
Traditional keyless entry systems have relatively large errors in distance information, making them prone to malfunctions near doors and vulnerable to relay attacks. Introducing high accuracy ranging with Channel Sounding enables accurate distance determination, allowing the system to correctly recognize whether the user is actually in front of the door. This greatly reduces unauthorized access and enables safe and smooth access control.
Loss Prevention Tags and Asset Tracking Solutions
Accurately tracking people and objects both indoors and outdoors is a major advantage for inventory management and loss prevention. By leveraging Channel Sounding, it is possible to measure distances accurately even in environments prone to multipath effects, such as between shelves or behind walls, by integrating information from multiple channels. This improves asset management efficiency in large facilities and factories, leading to cost savings and reduced work time.
Indoor Positioning and IoT Sensor Applications
In smart homes and factory automation, real-time location information of objects is essential. Channel Sounding can provide sub-meter accuracy, enabling advanced control and automation when combined with sensor data. Furthermore, since it is compatible with low-power Bluetooth devices, it is suitable for building large-scale IoT environments with many sensors.
Summary
Channel Sounding enables high accuracy distance measurement both indoors and outdoors, surpassing traditional RSSI-based methods. This feature is expected to have wide-ranging applications, from security to smart homes. As more related devices and evaluation kits become available, the spread of high accuracy ranging solutions will bring significant benefits to many industries and consumers.
If you have any questions or concerns about Bluetooth, please feel free to contact us.