RF fingerprinting for location estimation
The location of a terminal is estimated within an area of coverage of a wireless network comprising at least one fixed node by measuring a complex frequency response of a radio channel between the terminal and one of said fixed nodes and estimating the location of the terminal on the basis of at least a comparison between data representing a first metric of the measured complex frequency response and data representing a plurality of stored metrics, each of said plurality of stored metrics being related to one of a plurality of different locations within the network and each stored metric being of a complex frequency response measured between the said one of said fixed nodes and the location within the network to which the metric relates.
1. A method of estimation of a location of a terminal within an area of coverage of a wireless network comprising at least one fixed node, the method comprising:
measuring a complex frequency response of a radio channel between the terminal and one of said fixed nodes; and
estimating the location of the terminal on the basis of at least a comparison between data representing a first metric of the measured complex frequency response and data representing a plurality of stored metrics,
each of said plurality of stored metrics being related to one of a plurality of different locations within the network and each stored metric being of a complex frequency response measured between the said one of said fixed nodes and the location within the network to which the metric relates,
wherein the first metric and the plurality of stored metrics are autocorrelation functions.
2. A method according to claim 1 , wherein the first metric and the plurality of stored metrics are frequency coherence functions.
3. A method according to claim 2 , wherein the first metric and the plurality of stored metrics are normalised frequency coherence functions.
4. A method according to claim 1 , wherein said measuring the complex frequency response of the radio channel between the terminal and said one of said fixed nodes is performed on the basis of a signal transmitted from a selected one of the terminal and the said one of the said fixed nodes, the signal being received by the other of the terminal and the said one of the said fixed nodes.
5. A method according to claim 4 , wherein the signal comprises an Orthogonal Frequency Division Multiplex symbol, wherein said measuring comprises:
receiving pilot tone values of the Orthogonal Frequency Division Multiplex symbol;
estimating the complex frequency response of the radio channel on the basis of the received pilot tones; and
evaluating the first metric on the basis of an autocorrelation function of the estimated complex frequency response.
6. A method according to claim 1 , in which said estimating comprises:
evaluating a measure of the difference between the first metric and each of the stored metrics;
estimating the position of the terminal on the basis of the location related to the stored metric for which the evaluated measure of the difference is a minimum.
7. A method according to claim 1 , wherein the wireless network further comprises a plurality of fixed nodes, the method further comprising;
measuring a set of a complex frequency responses, the set comprising a complex frequency response for a radio channel between the terminal and each of two or more of said plurality of fixed nodes;
determining a set of first metrics, each first metric corresponding to one of the measured set of complex frequency responses;
estimating the location of the terminal on the basis of a comparison between each first metric in the set of first metrics and corresponding metrics in a plurality of stored sets of metrics,
each stored set of metrics being related to one of a plurality of different locations within the network and each set of stored metrics comprising a stored metric for a complex frequency response measured between each of the said two or more of the plurality of fixed nodes and the location within the network to which the stored set of metrics relates.
8. A method according to claim 7 , in which said estimating comprises:
evaluating a measure of the difference between each first metric in the set of first metrics and the corresponding metric in each of said plurality of stored sets of metrics;
calculating a mean squared difference between the set of first metrics and each of the stored sets of metrics, on the basis of the evaluated measures;
estimating the position of the terminal on the basis of one of said plurality of locations for which the evaluated mean squared difference is a minimum.
9. A method according to claim 7 , in which said estimating comprises:
evaluating a measure of the difference between each first metric in the set of first metrics and the corresponding metric in each of said plurality of stored sets of metrics;
calculating a mean squared difference between the set of first metrics and each of the stored sets of metrics, on the basis of the evaluated measures;
selecting two or more locations from said plurality of locations having lower evaluated mean squared difference than the other locations; and
estimating the position of the terminal on the basis of an interpolation between the selected locations.
10. A method of compiling survey data for use in the estimation of a location of a terminal within an area of coverage of a wireless network comprising at least one fixed node, the method comprising:
measuring a complex frequency response of a radio channel between a measurement node and one of said fixed nodes at a plurality of different locations within the network; and
deriving data representing a metric of each measured complex frequency associated with each of the plurality of different locations;
storing the derived data for each of said plurality different locations together with an indication of the location within the network to which said metric relates,
wherein said metric is an autocorrelation function.
11. A method according to claim 10 , wherein said metric is a frequency coherence function.
12. A method according to claim 11 , wherein said metric is a normalised frequency coherence function.
13. A method according to claim 10 , wherein said measuring the complex frequency response of the radio channel between the measurement node and said one of said fixed nodes is performed on the basis of a signal transmitted from a selected one of the measurement node and to the said one of the said fixed nodes, the signal being received by the other of the measurement node and the said one of the said fixed nodes.
14. A processor arranged to estimate a location of a terminal within an area of coverage of a wireless network comprising at least one fixed node,
wherein the processor is arranged to receive a measurement of a complex frequency response of a radio channel between the terminal and one of said fixed nodes, and
the processor is arranged to estimate the location of the terminal on the basis of at least a comparison between data representing a first metric of the measured complex frequency response and data representing a plurality of stored metrics,
each of said plurality of stored metrics being related to one of a plurality of different locations within the network and each stored metric being of a complex frequency response measured between the said one of said fixed nodes and the location within the network to which the metric relates,
wherein the first metric and the plurality of stored metrics are autocorrelation functions.
15. A processor according to claim 14 , wherein the first metric and the plurality of stored metrics are frequency coherence functions.
16. A processor according to claim 15 , wherein the first metric and the plurality of stored metrics are normalised frequency coherence functions.
17. A processor according to claim 14 , wherein said measuring the complex frequency response of the radio channel between the terminal and said one of said fixed nodes is performed on the basis of a signal transmitted from a selected one of the terminal and the said one of the said fixed nodes, the signal being received by the other of the terminal and the said one of the said fixed nodes.
18. A non-transitory computer readable medium encoded with computer executable instructions for causing a processor to estimate a location of a terminal within an area of coverage of a wireless network comprising at least one fixed node by
receiving a measurement of a complex frequency response of a radio channel between the terminal and one of said fixed nodes, and
estimating the location of the terminal on the basis of at least a comparison between data representing a first metric of the measured complex frequency response and data representing a plurality of stored metrics,
each of said plurality of stored metrics being related to one of a plurality of different locations within the network and each stored metric being of a complex frequency response measured between the said one of said fixed nodes and the location within the network to which the metric relates,
wherein the first metric and the plurality of stored metrics are autocorrelation functions.