Methods, apparatus and device-readable mediums for detecting changes in distance between wireless devices
In one aspect, a method for detecting changes in distance between a first wireless device and a second wireless device is provided. The method comprises determining a variation of a parameter over time, based on measurements of first timing signals transmitted between the first and second wireless devices. The variation of the parameter depends on a distance between the wireless devices, and comprises periodic step transitions as a result of relative clock drift between the wireless devices. The method further comprises predicting a time at which a periodic step transition is expected to occur based on the variation of the parameter, and determining a value of the parameter based on measurements of second timing signals. In response to a determination that the determined value of the parameter differs from an expected value of the parameter, it is determined that the distance between the wireless devices has changed.
1. A method of detecting changes in a distance between a first wireless device and a second wireless device, the method comprising:
determining a variation of a parameter over time, based on measurements of first timing signals transmitted between the first wireless device and the second wireless device, the variation of the parameter depending on the distance between the first wireless device and the second wireless device, and the variation of the parameter including periodic step transitions that are a result of relative clock drift between the first wireless device and the second wireless device;
based on the variation of the parameter, predicting a time at which one of the periodic step transitions is expected to occur;
determining a value of the parameter based on measurements of second timing signals transmitted between the first wireless device and the second wireless device, the second timing signals being transmitted subsequent to the first timing signals and close to the predicted time; and
responsive to determining that the determined value of the parameter differs from an expected value of the parameter, determining that the distance between the first wireless device and the second wireless device has changed.
2. The method according to claim 1 , further comprising determining the expected value of the parameter based on the variation of the parameter over time.
3. The method according to claim 1 , wherein determining the value of the parameter based on measurements of second timing signals comprises:
determining a first value of the parameter based on measurements made prior to the predicted time; and
determining a second value of the parameter based on measurements made after the predicted time.
4. The method according to claim 3 , wherein determining that the determined value of the parameter differs from the expected value of the parameter comprises determining one or more of the following:
the first value of the parameter differs from a first expected value of the parameter; and
the second value of the parameter differs from a second expected value of the parameter.
5. The method according to claim 1 , further comprising:
prior to determining the variation of the parameter over time, determining a first time-averaged distance between the first wireless device and the second wireless device; and
responsive to determining that the distance between the first wireless device and the second wireless device has changed, determining a second time-averaged distance between the first and second wireless devices based on measurements of third timing signals transmitted between the first wireless device and the second wireless device.
6. The method according to claim 1 , wherein:
a resolution of the parameter is limited by a sampling frequency of the timing signal measurements; and
the periodic step transitions are also a result of the limited resolution of the parameter.
7. The method according to claim 1 , wherein:
the parameter is a measured distance between the first wireless device and the second wireless device; and
the variation of the measured distance is a square wave.
8. The method according to claim 7 , wherein:
the square wave varies between first and second values, and
the expected value of the parameter at the predicted time comprises one of the first and second values.
9. The method according to claim 1 , wherein the parameter is a measured clock difference value between the first wireless device and the second wireless device.
10. The method according to claim 9 , wherein the measured clock difference value varies according to a step function.
11. The method according to claim 1 , wherein the first timing signals and the second timing signals are fine time measurement (FTM) signals.
12. A wireless device configured to detect changes in a distance between the wireless device and a further wireless device, the wireless device being one of a first wireless device and a second wireless device, the further wireless device being the other of the first wireless device and the second wireless device, the wireless device comprising:
processing circuitry and a device-readable medium storing executable instructions that, when executed by the processing circuitry, cause the wireless device to:
determine a variation of a parameter over time, based on measurements of first timing signals transmitted between the first wireless device and the second wireless device, the variation of the parameter depending on the distance between the first wireless device and the second wireless device, and the variation of the parameter including periodic step transitions that are a result of relative clock drift between the first wireless device and the second wireless device;
based on the variation of the parameter, predict a time at which one of the periodic step transitions is expected to occur;
determine a value of the parameter based on measurements of second timing signals transmitted between the first wireless device and the second wireless device, the second timing signals being transmitted subsequent to the first timing signals and close to the predicted time; and
responsive to determining that the determined value of the parameter differs from an expected value of the parameter, determine that the distance between the first wireless device and the second wireless device has changed.
13. The wireless device according to claim 12 , wherein execution of the instructions further causes the wireless device to determine the expected value of the parameter based on the variation in the parameter over time.
14. The wireless device according to claim 12 , wherein execution of the instructions causes the wireless device to determine the value of the parameter based on measurements of second timing signals based on:
determining a first value of the parameter based on measurements made prior to the predicted time; and
determining a second value of the parameter based on measurements made after the predicted time.
15. The wireless device according to claim 14 , wherein execution of the instructions causes the wireless device to determine that the determined value of the parameter differs from the expected value of the parameter based on determining one or more of the following:
the first value of the parameter differs from a first expected value of the parameter; and
the second value of the parameter differs from a second expected value of the parameter.
16. The wireless device according to claim 12 , wherein execution of the instructions further causes the wireless device to:
prior to determining the variation of the parameter over time, determine a first time-averaged distance between the first wireless device and the second wireless device; and
responsive to determining that the distance between the first wireless device and the second wireless device has changed, determine a second time-averaged distance between the first and second wireless devices based on measurements of third timing signals transmitted between the first wireless device and the second wireless device.
17. The wireless device according to claim 12 , wherein:
a resolution of the parameter is limited by a sampling frequency of the timing signal measurements; and
the periodic step transitions are also a result of the limited resolution of the parameter.
18. The wireless device according to claim 12 , wherein:
the parameter is a measured distance between the first wireless device and the second wireless device; and
the variation of the measured distance is a square wave.
19. The wireless device according to claim 18 , wherein:
the square wave varies between first and second values, and
the expected value of the parameter at the predicted time comprises one of the first and second values.
20. The wireless device according to claim 12 , wherein the parameter is a measured clock difference value between the first wireless device and the second wireless device.
21. The wireless device according to claim 20 , wherein the measured clock difference value varies according to a step function.
22. The wireless device according to claim 12 , wherein the first timing signals and the second timing signals are fine time measurement (FTM) signals.
23. A non-transitory, device-readable medium storing computer-executable instructions that, when executed by processing circuitry of a wireless device configured to detect changes in a distance between the wireless device and a further wireless device, the wireless device being one of the first wireless device and the second wireless device, and the further wireless device being the other of the first wireless device and the second wireless device, configure the wireless device to:
determine a variation of a parameter over time, based on measurements of first timing signals transmitted between the first wireless device and the second wireless device, wherein: the variation of the parameter depends depending on the distance between the first wireless device and the second wireless device, and the variation of the parameter includes including periodic step transitions that are a result of relative clock drift between the first wireless device and the second wireless device;
based on the determined variation of the parameter, predict a time at which one of the periodic step transitions is expected to occur;
determine a value of the parameter based on measurements of second timing signals transmitted between the first wireless device and the second wireless device, wherein the second timing signals being transmitted subsequent to the first timing signals and close to the predicted time; and
responsive to determining that the determined value of the parameter differs from an expected value of the parameter, determine that the distance between the first wireless device and the second wireless device has changed.