Signal strength was never a distance sensor. Bluetooth Channel Sounding, added in Core 6.0, measures true physical distance to centimetre scale by reading the phase of the radio across the band and timing the round trip — a measurement that is both far more accurate than signal strength and, crucially, hard to spoof.
| Element | What it does | What it gives |
|---|---|---|
| Phase-based ranging | Reads phase shift across many 2.4 GHz channels | Fine distance from the phase-versus-frequency slope |
| Round-trip time | Times a signal to the far device and back | An absolute distance and a security reference |
| Combined | PBR precision anchored by RTT’s absolute range | Unambiguous distance at centimetre scale |
| Initiator and reflector | A connected pair exchanges tones both ways | The measurement transaction itself |
| Multi-frequency stepping | Sweeps across the 2.4 GHz band | Resolves multipath and range ambiguity |
| Two-way, secure | Both ends take part in the timing | Distance bounding that resists spoof and relay |
For years, proximity in Bluetooth meant reading the received signal strength and mapping it to a distance with a path-loss model. It never worked well. Signal strength falls off with distance in theory, but in the real world it is dominated by antenna orientation, the human body, and multipath — reflections that arrive out of phase and make a near device look far or a far device look near. Worse, it is trivially spoofed: amplify the signal and a device three rooms away looks like it is in your hand, which is exactly the relay attack that has plagued keyless entry.
Channel Sounding replaces the guess with a measurement. Phase-based ranging reads the phase of the carrier at many frequencies across the 2.4 GHz band; because phase rotates with distance at a rate that depends on frequency, the slope of phase against frequency is a direct, fine measure of range. Round-trip time complements it by timing a signal to the far device and back, which gives an absolute distance and, because both ends participate in the exchange, a reference an attacker cannot fake by simply making the signal louder. Together they resolve to an unambiguous distance at centimetre scale, with the multi-frequency sweep handling the multipath that defeats signal strength.
The measurement runs between a connected pair — one device the initiator, the other the reflector — so it fits naturally into a normal Bluetooth connection rather than needing new radios. That is the strategic point: Channel Sounding turns the Bluetooth already in a phone, a car key, a tag or an earbud into a secure ranging sensor, which is why it matters for digital car keys and access control, for finding devices, and for indoor positioning. Distance is only half of a position, though — range says how far, not in which direction — so for a full location fix it pairs with angle-of-arrival, which supplies the bearing that Channel Sounding does not.
Because it measures the distance rather than inferring it. Signal strength reads how loud a signal arrives and maps that to a guess, which antenna orientation, the body and reflections all corrupt. Channel Sounding reads the phase of the radio across the band and times the round trip, which are direct physical measures of range and resolve to centimetre scale where signal strength manages only metres.
Phase-based ranging is precise but periodic — phase wraps, so on its own it can be ambiguous about which cycle you are in. Round-trip time gives an absolute, if coarser, distance that anchors the phase measurement to the right range, and because it is a two-way exchange it also provides the security reference that makes the result hard to spoof. One supplies precision, the other supplies absolute range and trust.
Channel Sounding rides on the Bluetooth already in the device, so it is the low-cost, ubiquitous way to get secure distance; ultra-wideband is a dedicated ranging radio that is more precise but needs its own hardware; angle-of-arrival gives direction rather than distance. For secure proximity and range on existing Bluetooth, Channel Sounding fits; for the highest precision, UWB; and for a full indoor position, range and angle are combined.
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