What is Channel Sounding?

Channel Sounding (formerly HADM) enables two Bluetooth devices to measure the distance between them with centimeter-level accuracy using phase-based ranging and round-trip time measurements across multiple frequency channels. By hopping across many channels, it builds a wideband channel estimate, achieving accuracy similar to UWB but using the existing 2.4 GHz BLE radio.

Channel Sounding achieves approximately 10 cm accuracy in line-of-sight conditions by combining phase-based and time-based measurements across 40+ channels. This is significantly better than RSSI (3-5 m) and comparable to UWB (10-30 cm). The multi-channel approach mitigates multipath effects that limit single-frequency ranging.

Digital car keys (secure unlock when phone is within 1 m). Smart locks (unlock only when user is at the door, not behind a wall). Asset tracking with precise zone detection. Indoor navigation with room-level accuracy. Anti-theft (detect when a tagged item moves beyond a perimeter). All with relay attack protection built into the protocol.

Wireless Protocols

Bluetooth 6.0

Bluetooth 6.0 (released September 2024) introduced Channel Sounding for centimeter-level distance measurement using phase-based ranging across multiple BLE channels. By combining round-trip time and phase difference measurements across 40+ frequencies, it achieves ~10 cm accuracy comparable to UWB, enabling secure digital keys, smart locks, and precise asset tracking. Additional features include Decision-Based Advertising Filtering, Monitoring Advertisers, and ISOAL enhancements.

Category: Wireless Protocols

Released: September 2024

Accuracy: ~10 cm

Understanding Bluetooth 6.0

Channel Sounding works by exchanging tones across multiple BLE channels. Each tone measurement yields a phase value. By combining phase measurements across a wide frequency span (the full 80 MHz BLE band), the system synthesizes an effective wideband measurement, achieving fine range resolution. Time-of-flight measurements provide coarse distance; phase-based measurements refine it.

Security is built-in: the protocol includes random nonces and authenticated exchanges that prevent relay attacks, where an attacker amplifies and retransmits signals to make a distant device appear close. This is critical for digital car keys and access control.

Channel Sounding Ranging

Phase-based distance:
d = c·Δφ / (4π·Δf)
Δf = frequency step between channels

Resolution:
Δd = c / (2·BW_eff)
80 MHz BW: Δd ≈ 1.9 m (resolved by phase)

Ranging Technology Comparison

Technology	Accuracy	Anti-Relay	Cost	Ecosystem
BLE RSSI	3-5 m	No	Lowest	Universal
BLE 5.1 AoA	0.5-1 m	No	Medium	Growing
BLE 6.0 CS	~10 cm	Yes	Low	Emerging
UWB	10-30 cm	Yes	High	Limited

Common Questions

Frequently Asked Questions

Channel Sounding?

Phase-based + ToF ranging across 40+ BLE channels. ~10 cm accuracy. Multi-channel synthesis overcomes single-frequency multipath limits.

Accuracy?

~10 cm LOS, comparable to UWB. Much better than RSSI (3-5 m). Built-in relay attack protection for secure access.

Use cases?

Digital car keys, smart locks, asset tracking, indoor navigation. Anti-relay security prevents amplification attacks.

Related Terms

← Bluetooth 5.4 Bluetooth Audio →

Secure Ranging

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