Positioning system and method with multipath mitigation
The present invention proposed a weighted Centroid Localisation (WCL) algorithm, which does the location estimation based on the known positions of the gateways and the measurements times of arrival (TOA) at the gateways. The algorithm computes the weight of the gateway based on their rank when the gateways are sorted by their TOA. Simulations have demonstrated the algorithm's robustness under different multipath/fading channel conditions and its good location performance.
1. A system comprising at least a mobile device and a plurality of gateways whose positions are known, wherein the gateways are operatively arranged receive a radio signal from the mobile device and to determine a time of arrival of said signal at the gateway, the system comprising a solver unit operatively arranged to compute the position of the mobile device based on said times of arrival, characterised in that the solver unit is arranged to assign to each gateway a weight factor dependent from its rank when the gateways are sorted by their received time of arrival, whereby those gateways whose time of arrival is earlier are assigned a heavier weight factor, the system being further arranged to compute the position of the mobile device by a weighted sum of the position of the gateways using said weight factors.
2. The system of claim 1 , the solver being arranged to compute the weight factors based on the rank and the time-difference of arrival of the signal at the gateways.
3. The system of claim 2 , the solver being arranged to divide the time-difference of arrival is by a scale factor dependent from the average distance between the gateways that received the signal.
4. The system of claim 1 , wherein the weight factors are always positive or zero, and/or they are zero when the time-difference of arrival exceeds a determined value.
5. The system of claim 1 , wherein the solver is arranged for computing the weight factors (w i ) of each gateway by:
= r 2 i ·max(0, k−t i /Δ)
where r i represents the rank of the gateway, t i its time-difference of arrival, Δ the average distance between the gateways that received the signal, and k is a predetermined value.
6. The system of claim 1 , operatively arranged to: compute also a range-based position estimation from said times of arrival; determine a first quality parameter expressing the quality of the range-free position estimate, a second quality parameter expressing the quality of said range-based position estimate; and combine the range-free position estimate and the range-based position estimate using said first and second quality parameters.
7. A method of determining the position of a mobile device relative to a plurality of gateways whose positions are known, comprising:
determining for each gateway a time of arrival of the signal,
assigning to each gateway a weight factor dependent from the rank it occupies when the gateways are sorted by their time of arrival, whereby those gateways whose time of arrival is earlier are assigned a heavier weight factor,
computing the position of the mobile device by a weighted sum of the position of the gateways using said weight factors.
8. The method of claim 7 , wherein the weight factors are computed based on the rank and the time-difference of arrival of the signal at the gateways.
9. The method of claim 8 , wherein the time-difference of arrival is divided by a scale factor dependent from the average distance between the gateways that received the signal.
10. The method of claim 7 , wherein the weight factors are always positive or zero, and/or they are zero when the time-difference of arrival exceeds a determined value.
11. The method of claim 7 , wherein the weight factors (W i ) of each gateway are calculated by:
W i =R i 2 ·max(0, k−t i /Δ)
where r i represents the rank of the gateway, t i its time-difference of arrival, Δ the average distance between the gateways that received the signal, and k is a predetermined value.
12. The method of claim 7 , comprising: computing also a range-based position estimation from said times of arrival, determining a first quality parameter expressing the quality of the range-free position estimate, and a second quality parameter expressing the quality of said range-based position estimate, and combining the range-free position estimate and the range-based position estimate using said first and second quality parameters.
13. The method of claim 7 , comprising a step of correcting the times of arrival towards earlier arrival times, based on an indication of multipath.
14. The method of claim 13 , wherein the steps of correcting the times of arrival and of determining a refined position are repeated iteratively until a determined convergence criterion is satisfied.
15. The method of claim 13 , wherein the indication of the multipath that the signal has suffered in the propagation to the corresponding gateway is a multipath prediction based on a propagation model.
16. The method of claim 13 , wherein the indication of the multipath that the signal has suffered in the propagation to the corresponding gateway is derived from a path loss exponent.