How does it differ from AoA/AoD?

AoA/AoD (BLE 5.1) measures angle to determine direction. Channel Sounding (BLE 6.0) measures distance directly via phase and round-trip time. AoA requires antenna arrays at the infrastructure. Channel Sounding works between any two standard BLE 6.0 devices without arrays. They are complementary: AoA for angle, CS for distance.

How is relay attack protection implemented?

Channel Sounding includes cryptographic nonces exchanged during each measurement procedure. A relay attacker who amplifies and retransmits the signal adds measurable propagation delay (even at light speed, a 10 m relay adds 33 ns). The protocol detects timing anomalies that exceed the expected round-trip time, flagging relay attempts.

Channel Sounding vs UWB?

Both achieve ~10 cm accuracy with relay protection. CS uses existing 2.4 GHz BLE radio (lower cost, massive ecosystem). UWB uses dedicated 6-8 GHz radio (additional chip, limited ecosystem). CS operates on narrowband channels synthesized into wideband; UWB uses native 500 MHz+ bandwidth. CS may replace UWB for many consumer applications.

Wireless Protocols

Bluetooth Channel Sounding

Bluetooth Channel Sounding (CS), formerly known as HADM (High Accuracy Distance Measurement), is a BLE 6.0 feature that measures distance between two devices with ~10 cm accuracy. CS exchanges RF tones across multiple BLE channels, measuring both round-trip time (coarse distance) and phase differences (fine distance). Combining measurements across 40+ channels synthesizes wideband resolution from narrowband signals. Built-in cryptographic nonces protect against relay attacks.

Category: Wireless Protocols

Accuracy: ~10 cm

Anti-Relay: Yes

Understanding Channel Sounding

CS defines three measurement modes: Mode 0 (RTT only, ~1 m accuracy), Mode 2 (phase-based only, fine resolution), and Mode 3 (combined RTT + phase, ~10 cm). Mode 3 uses RTT for coarse estimation and phase for refinement. The initiator and reflector exchange tones on a sequence of channels, each providing one phase data point.

The multi-channel approach is equivalent to a stepped-frequency radar: individual narrowband measurements at different frequencies are combined to achieve the resolution of a wideband signal. The effective bandwidth equals the span of frequencies used (up to 80 MHz across the BLE band).

CS Measurement Modes

Mode 0 (RTT): d = c·t_RT/2
Resolution: ~1 m (limited by clock)

Mode 2 (Phase): d = c·Δφ/(4π·Δf)
Ambiguity: c/(2·Δf) per channel pair

Mode 3 (Combined):
RTT resolves ambiguity, phase refines: ~10 cm

CS vs Other Ranging Methods

Method	Accuracy	Radio	Anti-Relay	Cost
BLE RSSI	3-5 m	BLE	No	Lowest
BLE AoA	0.5-1 m	BLE 5.1+	No	Medium
BLE CS	~10 cm	BLE 6.0	Yes	Low
UWB (802.15.4z)	10-30 cm	6-8 GHz	Yes	High

Common Questions

Frequently Asked Questions

CS vs AoA/AoD?

AoA measures angle (needs arrays). CS measures distance (standard radio). Complementary: AoA for direction, CS for range.

Relay protection?

Cryptographic nonces per measurement. Relay adds detectable propagation delay (33 ns per 10 m). Protocol flags timing anomalies.

CS vs UWB?

Both ~10 cm with anti-relay. CS uses existing BLE radio (low cost, big ecosystem). UWB needs separate 6-8 GHz chip (higher cost).

Related Terms

← Bluetooth Beacon Bluetooth Classic →

Secure Ranging

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