Optimization of variable coherence integration for the location of weak signals

• US 8,922,430 B2
• Filed: 12/22/2011
• Issued: 12/30/2014
• Est. Priority Date: 12/22/2011
• Status: Active Grant

• Current Assignee: Skyhook Holding, Inc. (Liberty Broadband Corp.)
• Original Assignee: TruePosition Incorporated
• Inventors: Gander, Edward J., Mia, Rashidus S.
• Primary Examiner(s): Phan, Dao
• Application Number:
  US13/334,727
• Publication Number:
  US 20130162480A1
• Time in Patent Office: 1,104 Days
• Field of Search: 342/173, 342/357.78, 342/387, 342/442, 342/451, 342/457, 455/456.1, 455/456.6, 340/515
• US Class Current: 342/387
• CPC Class Codes: G01S 5/0218 Multipath in signal reception
• CPC Class Codes: G01S 5/0273 using multipath or indirect path propagation signals in position determination

Abstract

In a network-based Wireless Location System (WLS), geographically distributed Location Measurement Units (LMUs) must be able to detect and use reverse channel (mobile to network) signals across multiple BTS coverage areas. By using Matched Replica correlation processing with the local and reference signals subdivided into discrete segments prior to correlation, the effects of mobile clock drift and Doppler shifts can be mitigated allowing for increased processing gain. By using historical network and real-time data about the radio signal and/or radio channel, the segmentation and computation scheme may be optimized to reduce latency and enhance capacity while maximizing location accuracy.

Claims

1. A method for generating a variable coherence scheme for use in a variable coherence integration process employing partial coherence processing paths to determine a precise time of arrival (TOA) of a transmission of a wireless device, comprising:

prior to entering the variable coherence integration process and in response to receiving tasking information, using historical information and real-time information to generate a variable coherence processing scheme, including determining a number of partial coherence processing paths and discrete subdivisions most likely to generate a highest processing gain; and

entering the variable coherence integration process.

2. A method as recited in claim 1, wherein the historical information includes local cell density information, geographic data, propagation data and historical drive test data.

3. A method as recited in claim 1, wherein the real-time information includes information determined from multiple demodulation sites, said real-time information including received signal power information, multi-path fading estimates, sounding channel information, and frequency offset information.

4. A method as recited in claim 3, wherein the frequency offset information is employed to calculate a velocity for the wireless device.

5. A method as recited in claim 1, wherein the real-time information includes frequency band information provided with the tasking information.

6. A method as recited in claim 3, wherein the multipath fading estimates are determined from a RAKE receiver.

7. A method as recited in claim 3, wherein the sounding channel information is determined from a Sounding Reference Signal.

8. A method as recited in claim 1, wherein the generation of a variable coherence processing scheme includes use of a default scheme and modifying the default scheme.

9. A method as recited in claim 8, wherein the default scheme is initially tailored to a service area and wherein modifying the default scheme includes using carrier network data to determine a largest expected Doppler shift.

10. A method as recited in claim 9, wherein a default scheme is prepared for each of a plurality of operational frequency bands.

11. A method as recited in claim 9, wherein the default scheme is dependent on cell density.

12. A method as recited in claim 11, wherein larger cells are assumed to be handling calls for faster moving mobile devices.

13. A method as recited in claim 8, wherein modifying the default scheme comprises use of collected radio signal and location data, and modeling, to tailor the default scheme.

14. A method as recited in claim 13, further comprising using drive test data to test and improve a modeled schema, wherein the default scheme is dependent on multipath and Doppler data, and moving drive test calls are made at normal road speeds to enable collection of data to create or adjust the default scheme.

15. A method as recited in claim 14, wherein, for each cell and sector available from drive testing, a variable coherence scheme that gives a successful time difference of arrival (TDOA) estimate is developed.

16. A method as recited in claim 6, wherein RAKE finger tracking is used to estimate whether fading is fast or slow and to select smaller maps for fast fading and larger maps for slow fading.

17. A method as recited in claim 16, wherein a selected time difference of arrival (TDOA) map from a reference location measuring unit (LMU) is used as a guide to select the maps/paths at co-op LMUs.

18. A method as recited in claim 17, wherein the co-op LMUs compute an entire set but only select a subset of the computed maps for search based on a reference map.

19. A method as recited in claim 3, wherein processor loading is considered as real-time information in the decision on construction of the variable coherence scheme, wherein a small set of paths is selected in overload conditions to allow greater location capacity at a lower average location accuracy and yield.

20. A method as recited in claim 1, wherein the variable coherence integration process comprises:

receiving a transmission of a wireless device;

generating a first digital sample set representing discrete samples of the transmission over a collection duration;

providing to first and second partial coherence processing paths copies of a reference and copies of the first digital sample set, wherein the copies are divided into m1 and m2 discrete subdivisions, wherein m1 and m2 are integers greater than 1 and m2 is greater than m1;

executing, in the first partial coherence processing path, a first correlation process in which a first set of sample segments is correlated with said reference, said first set of sample segments comprising a first plurality (m1) of segments, each of the m1 segments comprising a subset of said first digital sample set, wherein the execution of said first correlation process yields m1 outputs, wherein the m1 outputs comprise m1 complex numbers;

non-coherently summing the m1 outputs of the first correlation process to produce a number representing the sum of the magnitudes of the m1 complex numbers;

executing, in the second partial coherence processing path, a second correlation process in which a second set of sample segments is correlated with said reference, said second set of sample segments comprising a second plurality (m2) of segments, each of the m2 segments comprising a subset of said first digital sample set, wherein the execution of said second correlation process yields m2 outputs, wherein the m2 outputs comprise m2 complex numbers;

non-coherently summing the m2 outputs of the second correlation process to produce a number representing the sum of the magnitudes of the m2 complex numbers;

searching outputs of the correlation processes to identify a peak corresponding to an earliest arriving signal in each output set;

selecting one of said earliest arriving signals to determine a TOA value for use in location processing, wherein the selecting is based on the peaks in the outputs of the correlation processes; and

using the TOA value in location processing to determine a precise geographic location of said wireless device.

21. A method as recited in claim 20, wherein the variable coherence integration process further comprises executing, in a full coherence path, a third correlation process in which said first digital sample set is correlated with said reference over the collection duration.

22. A wireless location system (WLS), comprising:

(a) a subsystem for generating a variable coherence scheme for use in a variable coherence integration process employing partial coherence processing paths to determine a precise time of arrival (TOA) of a transmission of a wireless device, wherein the subsystem is configured to perform the following computer-implemented steps;

prior to entering the variable coherence integration process and in response to receiving tasking information, using historical information and real-time information to generate a variable coherence processing scheme, including determining a number of partial coherence processing paths and discrete subdivisions most likely to generate a highest processing gain; and

entering the variable coherence integration process; and

(b) a plurality of location measuring units (LMUs), wherein at least one LMU comprises means for carrying out the variable coherence integration process.

23. A WLS as recited in claim 22, wherein the at least one LMU further comprises:

a receiver for receiving a transmission of a wireless device;

means for generating a first digital sample set representing discrete samples of the transmission over a collection duration;

means for providing copies of a reference and copies of the first digital sample set, wherein the copies are divided into m1 and m2 discrete subdivisions, wherein m1 and m2 are integers greater than 1 and m2 is greater than m1;

a first partial coherence path including means for executing a first correlation process in which a first set of sample segments is correlated with said reference, said first set of sample segments comprising a first plurality (m1) of segments, each of the m1 segments comprising a subset of said first digital sample set, wherein the execution of said first correlation process yields m1 outputs, wherein the m1 outputs comprise m1 complex numbers;

means for non-coherently summing the m1 outputs of the first correlation process to produce a number representing the sum of the magnitudes of the m1 complex numbers;

a second partial coherence path including means for executing a second correlation process in which a second set of sample segments is correlated with said reference, said second set of sample segments comprising a second plurality (m2) of segments, each of the m2 segments comprising a subset of said first digital sample set, wherein the execution of said a second correlation process yields m2 outputs, wherein the m2 outputs comprise m2 complex numbers;

means for non-coherently summing the m2 outputs of the second correlation process to produce a number representing the sum of the magnitudes of the m2 complex numbers;

means for searching outputs of the correlation processes to identify an earliest arriving signal in each output set; and

means for comparing the identified earliest arriving signal in the outputs of the correlation processes and selecting one of said earliest arriving signals to determine a TOA value for use in location processing.

24. A WLS as recited in claim 22, wherein the historical information includes local cell density information, geographic data, propagation data and historical drive test data.

25. A WLS as recited in claim 22, wherein the real-time information includes information determined from multiple demodulation sites, said real-time information including received signal power information, multi-path fading estimates, sounding channel information, and frequency offset information.

26. A WLS as recited in claim 25, wherein the frequency offset information is employed to calculate a velocity for the wireless device.

27. A WLS as recited in claim 22, wherein the real-time information includes frequency band information provided with the tasking information.

28. A WLS as recited in claim 25, wherein the multipath fading estimates are determined from a RAKE receiver.

29. A WLS as recited in claim 25, wherein the sounding channel information is determined from a Sounding Reference Signal.

30. A WLS as recited in claim 22, wherein the generation of a variable coherence processing scheme includes use of a default scheme and modifying the default scheme.

31. A WLS as recited in claim 30, wherein the default scheme is initially tailored to a service area and wherein modifying the default scheme includes using carrier network data to determine a largest expected Doppler shift.

32. A WLS as recited in claim 31, wherein a default scheme is prepared for each of a plurality of operational frequency bands.

33. A WLS as recited in claim 31, wherein the default scheme is dependent on cell density.

34. A method as recited in claim 33, wherein larger cells are assumed to be handling calls for faster moving mobile devices.

35. A WLS as recited in claim 30, wherein modifying the default scheme comprises use of collected radio signal and location data, and modeling, to tailor the default scheme.

36. A WLS as recited in claim 35, further comprising using drive test data to test and improve a modeled schema, wherein the default scheme is dependent on multipath and Doppler data, and moving drive test calls are made at normal road speeds to enable collection of data to create or adjust the default scheme.

37. A WLS as recited in claim 36, wherein, for each cell and sector available from drive testing, a variable coherence scheme that gives a successful time difference of arrival (TDOA) estimate is developed.

38. A WLS as recited in claim 28, wherein RAKE finger tracking is used to estimate whether fading is fast or slow and to select smaller maps for fast fading and larger maps for slow fading.

39. A WLS as recited in claim 38, wherein a selected time difference of arrival (TDOA) map from a reference location measuring unit (LMU) is used as a guide to select the maps/paths at co-op LMUs.

40. A WLS as recited in claim 39, wherein the co-op LMUs compute an entire set but only select a subset of the computed maps for search based on a reference map.

41. A WLS as recited in claim 25, wherein processor loading is considered as real-time information in the decision on construction of the variable coherence scheme, wherein a small set of paths is selected in overload conditions to allow greater location capacity at a lower average location accuracy and yield.

42. A WLS as recited in claim 23, wherein the at least one LMU further comprises means for executing, in a full coherence path, a third correlation process in which said first digital sample set is correlated with said reference over the collection duration.