System and method for interference cancellation in a location calculation, for use in a wireless location system

• US 6,765,531 B2
• Filed: 08/13/2002
• Issued: 07/20/2004
• Est. Priority Date: 01/08/1999
• Status: Expired due to Term

• Current Assignee: Skyhook Holding, Inc. (Liberty Broadband Corp.)
• Original Assignee: TruePosition Incorporated
• Inventors: Anderson, Robert J.
• Primary Examiner(s): PHAN, DAO LINDA
• Application Number:
  US10/217,782
• Publication Number:
  US 20030016174A1
• Time in Patent Office: 707 Days
• Field of Search: 342/378, 342/450, 342/457, 342/463, 342/465, 342/14, 342/16, 342/17, 455/63.1, 455/67.13
• US Class Current: 342/378
• CPC Class Codes: G01S 1/026 of associated receivers
• CPC Class Codes: G01S 5/011 Identifying the radio environment
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/0215 Interference
• CPC Class Codes: G01S 5/0218 Multipath in signal reception
• CPC Class Codes: G01S 5/02213 Receivers arranged in a network for determining the position of a transmitter
• CPC Class Codes: G01S 5/0284 Relative positioning
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence
• CPC Class Codes: H04B 1/7107 Subtractive interference cancellation
• CPC Class Codes: H04W 4/90 Services for handling of emergency or hazardous situations, e.g. earthquake and tsunami warning systems [ETWS]
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management
• CPC Class Codes: H04W 76/50 for emergency connections

Abstract

In a Wireless Location System, reception and demodulation of signals is often performed at many sites. A collision may occur, e.g., in a TDMA, AMPS, GSM, GPRS, EDGE, CDMA, CDMA 2000, or WCDMA system, when a signal of interest (a transmission from a mobile transmitter to be located) and an interfering signal are received at the same time and at the same site. In the event such a collision occurs, the interfering signal may have been demodulated by a receiver system, or SCS, at another site, and the demodulated data be known by the TLP. This interferer can be reconstructed and subtracted from the raw data representing the combined signal of interest and interferer at the site where the collision occurred. This technique can provide significant (e.g., approximately 10-15 dB) interference rejection, with little degradation to the signal of interest.

Claims

1. An interference cancellation method for use in a wireless location system, comprising:

receiving a transmission at a first receiving site from a mobile transmitter to be located;

determining a probability of collision whereby an interfering signal has corrupted the signal received at the first receiving site; and

if the probability of collision exceeds a threshold, removing at least a substantial part of the interfering signal from the signal received at the first receiving site.

2. A method as recited in claim 1, wherein the signal received at the first receiving site is digitized and then processed in accordance with a predetermined algorithm to determine the probability of collision.

3. A method as recited in claim 2, wherein the predetermined algorithm comprises determining a signal-to-noise ratio (SNR) associated with the received signal.

4. A method as recited in claim 3, wherein the predetermined algorithm further comprises determining whether a variance of the SNR exceeds a predetermined threshold.

5. A method as recited in claim 3, wherein the predetermined algorithm further comprises determining whether the SNR has increased or decreased in a single step and by an amount greater than a predetermined threshold.

6. A method as recited in claim 3, wherein the predetermined algorithm further comprises basing the probability of collision on whether an average SNR of a signal received at a remote site is greater than an average SNR predicted by a propagation model.

7. A method as recited in claim 1, wherein the step of removing at least a substantial part of the interfering signal from the signal received at the first receiving site comprises reconstructing a representation of an interfering signal.

8. A method as recited in claim 7, wherein the step of removing at least a substantial part of the interfering signal from the signal received at the first receiving site further comprises processing the reconstructed signal prior to subtracting it from the signal received at the first receiving site.

9. A method as recited in claim 8, wherein the processing of the reconstructed signal includes time shifting.

10. A method as recited in claim 8, wherein the processing of the reconstructed signal includes frequency shifting.

11. A method as recited in claim 8, wherein the processing of the reconstructed signal includes weighting.

12. A method as recited in claim 8, wherein the processing of the reconstructed signal includes processing in accordance with a predetermined propagation model such that the signal subtracted from the signal received at the first receiving site is a substantially accurate representation of an interfering signal.

13. A method as recited in claim 8, further comprising repeating the process of removing at least a substantial part of an interfering signal from the signal received at the first receiving site so as to remove substantial parts of multiple interfering signals.

14. A method as recited in claim 13, wherein the process of repeating entails selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a threshold value of SNR is reached.

15. A method as recited in claim 13, wherein the process of repeating entails selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a time limit is exceeded.

16. A method as recited in claim 13, wherein the process of repeating entails selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a potential location determination has reached a specified accuracy and confidence.

17. A method as recited in claim 7, and further comprising distributing a representation of the reconstructed signal to one or more other receiving sites, whereby said other receiving site(s) may employ the reconstructed signal in removing interference from a signal received at that site.

18. A method as recited in claim 1, and further comprising, after determining that the probability of collision is above a predetermined threshold, verifying that received signal data from each of a plurality of antenna ports at the first receiving site is from a mobile transmitter to be located.

19. A method as recited in claim 18, wherein the step of verifying comprises demodulating segments of the received signal to verify that identifying information in the signal is correct.

20. A method as recited in claim 19, wherein the identifying information includes at least one of the MIN and MSID.

21. A method as recited in claim 18, wherein the step of verifying comprises demodulating segments of the received signal to verify that message characteristics in the signal match those received by another receiver site.

22. A method as recited in claim 21, wherein the message characteristics include dialed digits.

23. A method as recited in claim 1, wherein the method is carried out in a wireless location system including a plurality of receiving sites, and wherein a pair of said receiving sites define a baseline for time difference of arrival (TDOA) calculations, and wherein an interfering signal is cancelled at one receiving site by using a reconstructed representation of the interfering signal derived from a signal received at the other receiving site, whereby the accuracy of a TDOA determination is improved.

24. A method as recited in claim 1, wherein the method is carried out to improve the accuracy of an angle of arrival (AOA) determination in a wireless location system including a plurality of receiving sites.

25. A method as recited in claim 1, wherein the step of determining a probability of collision comprises correlation of the signal received at the first receiving site with a reference signal.

26. A method as recited in claim 25, further comprising correlation of a reconstructed version of an interferer with a reference from a nearby site, across time to find an optimal time-shifted version of the interferer.

27. A method as recited in claim 25, further comprising correlation of a reconstructed version of an interferer with a reference from a nearby site, across frequency to find an optimal frequency-shifted version of the interferer.

28. A method as recited in claim 25, further comprising correlation of a reconstructed version of an interferer with a reference from a nearby site, across time and frequency to find an optimal time- and frequency-shifted version of the interferer.

29. A method as recited in claim 1, wherein the steps of determining a probability of collision and removing at least a substantial part of the interfering signal from the signal received at the first receiving site are performed at a receiving site.

30. A method as recited in claim 1, wherein the steps of determining a probability of collision and removing at least a substantial part of the interfering signal from the signal received at the first receiving site are performed at a central site.

31. A method as recited in claim 1, wherein the predetermined algorithm comprises using knowledge of a mobile transmitter, other than the mobile transmitter to be located, communicating on the same channel as the mobile transmitter to be located to determine that interference is present.

32. A method as recited in claim 31, wherein said knowledge is derived from a wireless communications system.

33. A method as recited in claim 21, wherein the message characteristics include channel synchronization fields.

34. A method as recited in claim 1, wherein the step of determining a probability of collision comprises determining whether received signal power is above a threshold and a correlation of the signal received at the first receiving site with a reference signal is below a threshold.

35. A wireless location system, comprising:

a plurality of signal receiving sites, each of said sites including a receiver capable of receiving and digitizing signals from mobile transmitters, and a demodulator for demodulating the received signals;

means for determining a probability of collision whereby an interfering signal has corrupted the signal received at a first receiving site; and

means for removing at least a substantial part of the interfering signal from the signal received at the first receiving site when the probability of collision exceeds a threshold.

36. A wireless location system as recited in claim 35, further comprising means for processing the signal received at the first receiving site in accordance with a predetermined algorithm to determine the probability of collision.

37. A wireless location system as recited in claim 36, wherein the predetermined algorithm comprises determining a signal-to-noise ratio (SNR) associated with the received signal.

38. A wireless location system as recited in claim 37, wherein the predetermined algorithm further comprises determining whether a variance of the SNR exceeds a predetermined threshold.

39. A wireless location system as recited in claim 37, wherein the predetermined algorithm further comprises determining whether the SNR has increased or decreased in a single step and by an amount greater than a predetermined threshold.

40. A wireless location system as recited in claim 37, wherein the predetermined algorithm further comprises basing the probability of collision on whether an average SNR of a signal received at a remote site is greater than an average SNR predicted by a propagation model.

41. A wireless location system as recited in claim 35, wherein the means for removing at least a substantial part of the interfering signal from the signal received at the first receiving site comprises means for reconstructing a representation of an interfering signal.

42. A wireless location system as recited in claim 41, wherein the means for removing at least a substantial part of the interfering signal from the signal received at the first receiving site further comprises means for processing the reconstructed signal prior to subtracting it from the signal received at the first receiving site.

43. A wireless location system as recited in claim 42, wherein the processing of the reconstructed signal includes time shifting.

44. A wireless location system as recited in claim 42, wherein the processing of the reconstructed signal includes frequency shifting.

45. A wireless location system as recited in claim 42, wherein the processing of the reconstructed signal includes weighting.

46. A wireless location system as recited in claim 42, wherein the processing of the reconstructed signal includes processing in accordance with a predetermined propagation model such that the signal subtracted from the signal received at the first receiving site is a substantially accurate representation of an interfering signal.

47. A wireless location system as recited in claim 42, further comprising means for repeating the process of removing at least a substantial part of an interfering signal from the signal received at the first receiving site so as to remove substantial parts of multiple interfering signals.

48. A wireless location system as recited in claim 47, wherein the means for repeating includes means for selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a threshold value of SNR is reached.

49. A wireless location system as recited in claim 47, wherein the means for repeating includes means for selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a time limit is exceeded.

50. A wireless location system as recited in claim 47, wherein the means for repeating includes means for selecting a strongest interferer to remove first, followed by one or more other interferers in decreasing order of interference until a potential location determination has reached a specified accuracy and confidence.

51. A wireless location system as recited in claim 41, and further comprising means for distributing a representation of the reconstructed signal to one or more other receiving sites, whereby said other receiving site(s) may employ the reconstructed signal in removing interference from a signal received at that site.

52. A wireless location system as recited in claim 35, and further comprising means for verifying that received signal data from each of a plurality of antenna ports at the first receiving site is from a mobile transmitter to be located.

53. A wireless location system as recited in claim 52, wherein the means for verifying comprises means for demodulating segments of the received signal to verify that identifying information in the signal is correct.

54. A wireless location system as recited in claim 53, wherein the identifying information includes at least one of the MIN and MSID.

55. A wireless location system as recited in claim 52, wherein the means for verifying comprises means for demodulating segments of the received signal to verify that message characteristics in the signal match those received by another receiver site.

56. A wireless location system as recited in claim 55, wherein the message characteristics include dialed digits.

57. A wireless location system as recited in claim 35, wherein a pair of said receiving sites define a baseline for time difference of arrival (TDOA) calculations, and wherein an interfering signal may be cancelled at one receiving site by using a reconstructed representation of the interfering signal derived from a signal received at the other receiving site, whereby the accuracy of a TDOA determination is improved.

58. A wireless location system as recited in claim 35, wherein the means for determining a probability of collision comprises means for correlating the signal received at the first receiving site with a reference signal.

59. A wireless location system as recited in claim 58, further comprising means for correlating a reconstructed version of an interferer at the first site with a reference from a nearby site, across time to find an optimal time-shifted version of the interferer.

60. A wireless location system as recited in claim 58, further comprising means for correlating a reconstructed version of an interferer at the first site with a reference from a nearby site, across frequency to find an optimal frequency-shifted version of the interferer.

61. A wireless location system as recited in claim 58, further comprising means for correlating a reconstructed version of an interferer at the first site with a reference from a nearby site, across time and frequency to find an optimal time- and frequency-shifted version of the interferer.

62. A wireless location system as recited in claim 35, wherein the means for determining a probability of collision and removing at least a substantial part of the interfering signal from the signal received at a first receiving site are located at a receiving site.

63. A wireless location system as recited in claim 35, wherein the means for determining a probability of collision and removing at least a substantial part of the interfering signal from the signal received at the first receiving site are located at a central site.

64. A wireless location system as recited in claim 35, wherein the predetermined algorithm comprises using knowledge of a mobile transmitter, other than the mobile transmitter to be located, communicating on the same channel as the mobile transmitter to be located to determine that interference is present.

65. A wireless location system as recited in claim 64, wherein said knowledge is derived from a wireless communications system.

66. A wireless location system as recited in claim 55, wherein the message characteristics include channel synchronization fields.

67. A wireless location system as recited in claim 35, wherein the means for determining a probability of collision comprises means for determining whether received signal power is above a threshold and a correlation of the signal received at the first receiving site with a reference signal is below a threshold.