In Nov 2018, Belgium IMEC demonstrated the first solution for secure and very accurate passive keyless car entry based on Bluetooth Low Energy (BLE) technology.
“This technology enables a seamless car access experience where a BLE device, such as a mobile phone, acts as a digital car key. Imec’s demonstrator builds on the research center’s secure and very high accuracy ranging algorithms implemented on BLE. Imec’s BLE ranging technology achieves an accuracy better than 30 centimeters – which is over 10 times better than current approaches, even those already using phase-based ranging techniques – and will be enriched with physical layer security features that create a secure distance measurement between two Bluetooth-enabled communication devices”
This phase based ranging technology is adopted by Bluetooth and is known as HADM, High Accuracy Distance Measurement.
In a nutshell, HADM calculates the distance between two points, A and B, from the single wave or single tone phase difference. Let’s say A sends a single tone of frequency f to B and the distance between A and B is L. Then point B sees the phase of the received single tone as
where c is light speed and is the initial phase difference between A’s LO and B’s LO. (LO is RF local oscillator)
Point B is required to bounce back a single tone of the same frequency and the received phase. Point A receives this single tone and sees the phase as
where refers to Round Trip (RT) total phase difference. Therefore we have
Above equation means value of repeats itself every half wavelength, c/f. BLE frequency is around 2.4Ghz so wavelength is about 12cm. Then phase repeats every 6cm is obviously not very useful. However, if we do the same operation with two frequencies, f1 and f2, we have
If is 1Mhz, then above equation can make value of repeat itself around every 80 meters. It is good enough for short range distance measurement.
To improve measurement accuracy and combat channel fading, more than two frequencies can be used
HADM operation flow can be
- A and B communicates and initiates HADM
- A sends single tone of frequency f to B for a pre-arranged time
- B does frequency and phase measurement
- B sends back single tone of same frequency and measured phase for a pre-arranged time
- A does frequency and phase measurement. A has RT phase difference of one frequency f.
Repeat 1-5 for multiple frequencies.
Normal BLE packets are used for channel control and management. BTW, to send and receive single tone in BLE is nothing new. Single tone is already adopted in BLE test mode.