Tdoa based positioning with calculation of correction factors for compensating the clock offsets of unsynchronized network stations

Abstract

The present invention presents a method, arrangement and computer program product for clocking exploiting the relative behavior of clocks of individual receiving stations as well as a corresponding modeling to derive a time difference of arrival of a signal from a user device which can be used to correct the time difference of arrival based on the modeled clock behavior and leads to a correct clocking of received user signals without the need of synchronization of the clocks in the various receiving stations. This principle is applicable to a plurality of pairs of receiving stations and beacon signals transmitted amongst them and allows for a correct location estimation of a user device.

1 .- 17 . (canceled) 
     
     
         18 . A method of determining a position of a first device relative to a second device, a third device and a fourth device, the second, third and fourth devices being in a fixed positional relationship and having second, third and fourth local clocks respectively, the method comprising:
 estimating respective offsets of the second clock and the third clock relative to the fourth clock as a function of time;   transmitting a signal from the first device;   receiving the signal transmitted from the first device at the second and third devices;   determining respective reception times of the signal at the second and third devices using the second and third local clocks respectively; and   estimating the position of the first device using the respective reception times of the signal at the second and third devices and the respective offsets of the second clock and the third clock as a function of time.   
     
     
         19 . The method of  claim 18 , wherein estimating the position of the first device using the respective reception times of the signal at the second and third devices and the respective offsets of the second clock and the third clock as a function of time comprises estimating the position of the first device using the respective reception times of the signal at the second and third devices and respective offsets of the second clock and the third clock at times bracketing the respective reception times of the signal at the second and third devices. 
     
     
         20 . The method of  claim 18  wherein estimating the position of the first device using the respective reception times of the signal at the second and third devices and the respective offsets of the second clock and the third clock as a function of time comprises using time difference of arrival (TDOA). 
     
     
         21 . The method of  claim 18 , wherein the first device is a mobile wireless communication terminal and the second and third devices are wireless access points deployed at fixed locations. 
     
     
         22 . The method of  claim 18 , wherein the fourth device is a beacon deployed at a fixed location. 
     
     
         23 . The method of  claim 18 , wherein estimating respective offsets of the second clock and the third clock relative to the fourth clock as a function of time comprises:
 transmitting a sequence of individually identifiable signals from the fourth device;   receiving the sequence of individually identifiable signals at each of the second device and the third device;   determining respective reception times of each of the individually identifiable signals at the second device using the second clock;   determining respective reception times of each of the individually identifiable signals at the third device using the third clock; and   estimating the respective offsets of the second clock and the third clock relative to the fourth clock from the reception times of the individually identifiable signals at the second and third devices.   
     
     
         24 . The method of  claim 23 , wherein the sequence of individually identifiable signals comprises a sequence of beacon signals. 
     
     
         25 . The method of  claim 24 , wherein each beacon signal comprises an identifier which is unique to that beacon signal. 
     
     
         26 . The method of  claim 25 , wherein each beacon signal is a frame, and the identifier which is unique to that beacon signal is a frame number. 
     
     
         27 . The method of  claim 26 , wherein each frame is a frame compliant with IEEE 802.11. 
     
     
         28 . The method of  claim 23 , wherein:
 transmitting the sequence of individually identifiable signals from the fourth device comprises transmitting at least a first beacon signal and a second beacon signal from the fourth device;   determining the respective reception times of each of the individually identifiable signals at the second device comprises determining reception times for the first and second beacon signals at the second device using the second clock;   determining respective reception times of each of the individually identifiable signals at the third device comprises determining reception times for the first and second beacon signals at the third device using the third clock; and   estimating the respective offsets of the second clock and the third clock relative to the fourth clock from the reception times of the individually identifiable signals at the second and third devices comprises estimating the respective offsets based on the reception times of the first and second beacon signals at the second device determined using the second clock and the reception times of the first and second beacon signals at the third device determined using the third clock.   
     
     
         29 . The method of  claim 28 , wherein estimating the position of the first device using the respective reception times of the signal at the second and third devices and the respective offsets of the second clock and the third clock as a function of time comprises estimating the position of the first device using the respective reception times of the signal at the second and third devices and the reception times of the first and second beacon signals at the second device determined using the second clock and the reception times of the first and second beacon signals at the third device determined using the third clock. 
     
     
         30 . The method of  claim 23 , wherein:
 transmitting the sequence of individually identifiable signals from the fourth device comprises transmitting more than two successive beacon signals from the fourth device;   determining the respective reception times of each of the individually identifiable signals at the second device comprises determining reception times for each of the beacon signals at the second device using the second clock;   determining respective reception times of each of the individually identifiable signals at the third device comprises determining reception times for each of the beacon signals at the third device using the third clock; and   estimating the respective offsets of the second clock and the third clock relative to the fourth clock from the reception times of the individually identifiable signals at the second and third devices comprises estimating the respective offsets based on the reception times of the first and second beacon signals at the second device determined using the second clock and the reception times of the beacon signals at the third device determined using the third clock.   
     
     
         31 . The method of  claim 30 , wherein estimating the position of the first device using the respective reception times of the signal at the second and third devices and the respective offsets of the second clock and the third clock as a function of time comprises estimating the position of the first device using the respective reception times of the signal at the second and third devices and the reception times of the beacon signals at the second device determined using the second clock and the reception times of the beacon signals at the third device determined using the third clock. 
     
     
         32 . The method of  claim 30 , wherein estimating the respective offsets based on the reception times of the first and second beacon signals at the second device determined using the second clock and the reception times of the beacon signals at the third device determined using the third clock comprises modelling a function that relates the reception times of the beacon signals at the second device determined using the second clock to the reception times of the beacon signals at the third device using the third clock. 
     
     
         33 . The method of  claim 32 , wherein modelling a function that relates the reception times of the beacon signals at the second device determined using the second clock to the reception times of the beacon signals at the third device using the third clock comprises interpolating the function between the determined times of the beacon signals. 
     
     
         34 . The method of  claim 33 , wherein interpolating the function between the determined times of the beacon signals comprises modelling the function as a polynomial. 
     
     
         35 . The method of  claim 34 , wherein modelling the function as a polynomial comprises using least squares fitting.