Adaptive waveform matching for use in transmitter identification

• US 5,715,518 A
• Filed: 03/06/1996
• Issued: 02/03/1998
• Est. Priority Date: 03/06/1996
• Status: Expired due to Fees

• Current Assignee: Cellular Technical Services Co., Inc. (Asensus Surgical, Inc.)
• Original Assignee: Cellular Technical Services Co., Inc. (Asensus Surgical, Inc.)
• Inventors: Barrere, Wm. Gerrit, Green, Evan R., Kaplan, Dmitry
• Primary Examiner(s): Pham, Chi H.
• Assistant Examiner(s): ARTHUR JEANGLAUDE, GERTRUDE
• Application Number:
  US08/611,429
• Time in Patent Office: 699 Days
• Field of Search: 455/33.1, 455/49.1, 455/54.1, 455/54.2, 455/56.1, 455/67.1, 455/103, 455/115, 379/58-60, 379/62, 340/825.31, 340/825.34, 380/21, 380/23
• US Class Current: 340/5.8
• CPC Class Codes: H04W 12/06 Authentication
• CPC Class Codes: H04W 12/12 Detection or prevention of fraud
• CPC Class Codes: H04W 12/122 Counter-measures against attacks; Protection against rogue devices
• CPC Class Codes: H04W 12/79 Radio fingerprint

Abstract

A system and method extracts a portion of a transmitted waveform and compares the transmission characteristics of the extracted waveform to determine if they match a stored fingerprint associated with the transmitter, such as a cellular telephone. The system adaptively creates fingerprint data by storing a plurality of waveforms believed to be from a particular transmitter and calculates the variability of the individual transmissions with respect to a composite mean waveform of the transmission characteristics. The system emphasizes the variability between the individual waveforms and the composite mean waveform and determines variability values for each of the individual waveforms in the fingerprint. When the system receives an additional unidentified transmission purported to be from the transmitter with the stored fingerprint, the system determines whether the variability of the unidentified waveform fits within the predetermined margins of variability for the fingerprint. If the variability of the unidentified waveform is less than a predetermined variability threshold, the system considers the waveforms to be a suitable match. The system can adaptively include new waveforms in the fingerprint by determining whether the newly-received transmission characteristics meet a series of criteria for inclusion in the fingerprint. In addition, the system analyzes the newly-received waveform and all waveforms forming the fingerprint to determine if any of the waveforms exceed a predetermined maximum variability value. A waveform that exceeds the maximum variability value is eliminated from the fingerprint. The system is easily automated, thus providing a useful analysis technique for communications network such as cellular telephone systems where a large population of transmitters with similar transmission characteristics are operating at any given time.

Claims

1. A system for the identification of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters, the system comprising:

a radio frequency receiver circuit receiving a radio transmission from the unauthenticated transmitter that can be the authentic transmitter or the unauthorized transmitter, said received radio transmission including identification data identifying the unauthenticated transmitter as the authentic transmitter;

a demodulator demodulating said received radio transmission to generate a demodulated signal, said demodulated signal including a first unauthenticated response signal;

a data storage area which stores a plurality of response waveforms designated as reference waveforms for the authentic transmitter;

a norrealizer processor which receives each of said reference waveforms from said data storage area and normalizes each of said reference waveforms, said normalizer processor also receiving and normalizing said first unauthenticated response signal;

a mean processor which computes a composite mean waveform from said normalized waveforms;

a waveform processor which computes difference values between each of said individual normalized waveforms and said composite mean waveform, said waveform processor applying a weighting function to each of said difference values to generate weighted values and summing each of said weighted values to generate a variability value for each of said individual normalized waveforms, said variability values indicating a variability of each of said individual normalized waveforms from said composite mean waveform, said waveform processor also computing first unauthenticated difference values between said first unauthenticated normalized waveform and said composite mean waveform and applying said weighting function to each of said unauthenticated difference values to generate first unauthenticated weighted values and summing each of said first unauthenticated weighted values to generate a first unauthenticated variability value for said first unauthenticated normalized waveform, said first unauthenticated variability value indicating a variability of said first unauthenticated normalized waveform from said composite mean waveform; and

a comparison processor which compares said first unauthenticated variability value to said variability values for said individual reference waveforms to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

2. The system of claim 1, further including a statistical analyzer to compute statistical measures of said variability values for said individual reference waveforms and said unauthenticated variability value, said comparison processor using said statistical measures to determine whether the unauthenticated transmitter is the authentic transmitter.

3. The system of claim 2 wherein said statistical analyzer calculates a mean variability value of said variability values for said individual mean waveforms and a standard deviation of said variability values for said individual mean waveforms, said comparison processor using said mean variability value and said standard deviation to determine whether the unauthenticated transmitter is the authentic transmitter.

4. The system of claim 2 wherein said comparison processor calculates a confidence value indicative of the level of confidence with which the unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU4## where CV is said confidence value, V_U is said first unauthenticated variability value, V is a mean variability value of said variability values for said individual reference waveforms, and σ

_V is a standard deviation of said variability values for said individual reference waveforms.

5. The system of claim 4 wherein said comparison processor generates an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

6. The system of claim 5 wherein said predetermined threshold is between two and three.

7. The system of claim 1 wherein each of said reference waveforms has a response start as a function of time, the system further including an alignment processor to shift said response start and temporally align said reference waveforms, said data storage area storing said temporally aligned reference waveforms.

8. The system of claim 1 wherein said first unauthenticated response signal is a transmitter transient comprising one of a group including a turn-on transient and a turn-off transient.

9. The system of claim 1 wherein said demodulated signal includes a second unauthenticated response signal, and said normalizer processor receives and normalizes said second unauthenticated response signal, said waveform processor computes second unauthenticated difference values between said second unauthenticated response mean waveform and said composite mean waveform to generate second difference values and applies said weighting function to said second difference values to generate second unauthenticated weighted values and sums each of said second unauthenticated weighted values to generate a second unauthenticated variability value indicating a variability of said second unauthenticated normalized waveform from said composite mean waveform, and said comparison processor compares said first and second unauthenticated variability values to said variability values of said individual reference waveforms to determine whether the unauthenticated transmitter is the authentic transmitter.

10. The system of claim 9 wherein said second unauthenticated response signal is a transmitter transient comprising one of a group of a turn-on transient and a turn-off transient.

11. The system of claim 1 wherein said waveform processor applies said weighting function by raising said difference values to a power greater than one.

12. The system of claim 11, further including a look-up table having a input index with a range corresponding to a range of values for said difference values and corresponding output values with a range corresponding to said difference values raised to said power greater than one, said waveform processor applying said weighting function by transmitting particular values for said difference to said look-up table input index and reading said corresponding output value corresponding to said difference values raised to said power greater than one.

13. The system of claim 1 wherein said waveform processor applies said weighting function raising said difference values to a power of 1.2.

14. The system of claim 1, further including a modification processor to analyze said first unauthenticated response signal and said individual reference waveforms and to include said first unauthenticated response signal as an additional one of said reference waveforms only if said first unauthenticated response signal has a predefined acceptable characteristic, said mean processor computing a new composite mean waveform if said first unauthenticated response signal is included as said additional reference waveform.

15. The system of claim 14 wherein said radio transmission from the unauthenticated transmitter has a received signal strength and a signal strength variability, said modification processor analyzing said received signal strength and said signal strength variability as said predefined acceptable characteristic, said modification processor including said first unauthenticated response signal as said additional reference waveform only if said received signal strength is above a predetermined signal threshold and said signal strength variability is below a predetermined variability threshold.

16. The system of claim 14 wherein said first unauthenticated response signal has a measured length and an initial response start as a function of time, the system further including an alignment processor to shift said initial response start and temporally align said first unauthenticated response signal with respect to said composite mean waveform, said modification processor including said first unauthenticated response signal as said additional reference waveform only if said measured length is substantially equal to a measured length of said composite mean waveform and said temporal alignment is below a predetermined alignment threshold.

17. The system of claim 1, further including a modification processor which analyzes said reference waveforms and determines a confidence value for each of said reference waveforms, said modification processor comparing said calculated confidence values to a maximum confidence value and removing a particular reference waveform from said data storage area if said calculated confidence value for said particular reference waveform exceeds said maximum confidence value.

18. The system of claim 17 wherein said maximum confidence value is dependent on a number of said reference waveforms in said data storage area, said maximum confidence value decreasing as said number of reference waveforms in said data storage area increases.

19. The system of claim 18 wherein said waveform processor applies said weighting function by raising said first unauthenticated difference values to a power greater than one.

20. The system of claim 18 wherein the statistical values related to the reference waveforms and the composite mean waveform are variability values for each of individual reference waveforms derived from the respective individual reference waveforms and said comparison processor uses said variability values to determine whether the unauthenticated transmitter is the authentic transmitter.

21. The system of claim 19 wherein the statistical values related to the reference waveforms and the composite mean waveform are a mean variability value of said variability values for each of said individual mean waveforms and a standard deviation of said variability values for each of said individual mean waveforms, said comparison processor using said mean variability value and said standard deviation to determine whether the unauthenticated transmitter is the authentic transmitter.

22. The system of claim 19 wherein said comparison processor calculates a confidence value indicative of the level of confidence with which the unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU5## where CV is said confidence value, V_U is said first unauthenticated response variability value, V is a mean variability value of said variability values of said individual reference waveforms, and σ

_V is a standard deviation of said variability value for said individual reference waveforms.

23. The system of claim 22 wherein said comparison processor generates an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

24. The system of claim 23 wherein said predetermined threshold is between two and three.

25. A system for the authentication of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters, the system including a plurality of stored response signals from the authentic transmitter designated as reference waveforms for the authentic transmitter, a composite mean waveform derived from the reference waveforms, and statistical values related to the reference waveforms and the composite mean waveform, the system comprising:

a receiver circuit which receives a transmission from the unauthenticated transmitter that can be the authentic transmitter or the unauthorized transmitter, said received transmission including a first unauthenticated response signal and identification data identifying the unauthenticated transmitter as the authentic transmitter;

a waveform processor which computes first unauthenticated difference values between a first unauthenticated normalized waveform and the composite mean waveform, said waveform processor applying a weighting function to each of said first unauthenticated difference values to generate a first unauthenticated variability value for said first unauthenticated mean waveform, said first unauthenticated variability value indicating a variability of said first unauthenticated mean waveform from the composite mean waveform; and

a comparison processor which compares said first unauthenticated variability value to the statistical values related to the reference waveforms and the composite mean waveform to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

26. The system of claim 25 wherein the statistical values related to the reference waveforms and the composite mean waveform are derived from normalized reference waveforms, the system further including a normalizer processor which receives and normalizes said first unauthenticated response signal.

27. The system of claim 25, further including a demodulator to demodulate said transmission, said first unauthenticated response signal being derived from said demodulated transmission.

28. The system of claim 27 wherein said first unauthenticated response signal is a transmitter turn-on transient.

29. The system of claim 27 wherein said first unauthenticated response signal is a transmitter turn-off transient.

30. The system of claim 25 wherein said received transmission includes a second unauthenticated response signal, and said normalization processor computes a second unauthenticated response normalized waveform from said second unauthenticated waveform, said waveform processor computes second unauthenticated difference values between said second unauthenticated normalized waveform and the composite mean waveform and applies said weighting function to each of said second unauthenticated difference values to generate a second unauthenticated variability value for said second unauthenticated mean waveform, and said comparison processor compares said first and second unauthenticated variability values to the statistical values related to the reference waveforms and the composite mean waveform to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

31. A system for the development of a reference fingerprint used in the authentication of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters where the unauthenticated transmitter transmits identification data identifying the unauthenticated transmitter as the authentic transmitter, the transmission including an unauthenticated response signal, the system comprising:

a data storage area which stores a plurality of response waveforms designated as reference waveforms for the authentic transmitter;

a mean processor which computes a composite mean waveform from said reference waveforms; and

a waveform processor which computes difference values between each of said reference waveforms and said composite mean waveform, said waveform processor applying a weighting function to each of said difference values to generate weighted values, said waveform processor using said weighted values to generate a variability value for each of said reference waveforms, said variability values indicating a variability of each of said reference waveforms from said composite mean waveform, whereby the system processes the unauthenticated response signal to generate an unauthenticated variability value for the unauthenticated response signal and compares the unauthenticated variability value with the variability values for each of said reference waveforms to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

32. The system of claim 31 wherein said waveform processor sums each of said weighted values to generate said variability values for each of said reference waveforms.

33. The system of claim 31, further including a statistical analyzer to compute a statistical measure of said variability values for said reference waveforms, whereby said statistical measure is used to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

34. The system of claim 33 wherein said statistical analyzer calculates a mean variability value of said variability values for said reference waveforms and a standard deviation of said variability values for said reference waveforms, whereby said mean variability value and said standard deviation are used to determine whether the unauthenticated transmitter the authentic transmitter or the unauthorized transmitter.

35. The system of claim 33, further including a comparison processor which calculates a confidence value indicative of the level of confidence with which the unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU6## where CV is said confidence value, V_U is the unauthenticated response variability value, V is a mean variability value of said variability values for said reference waveforms, and σ

_V is a standard deviation of said variability values for said reference waveforms.

36. The system of claim 35 wherein said comparison processor generates an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

37. The system of claim 36 wherein said predetermined threshold is between two and three.

38. The system of claim 31 wherein said waveform processor applies said weighting function by raising said difference values to a power greater than one.

39. The system of claim 31, further including a normalizer processor which receives and normalizes each of said reference waveforms, said mean processor computing said composite mean waveform from said normalized reference waveforms.

40. The system of claim 31, further including a modification processor which analyzes the unauthenticated response signal and said reference waveforms for possible inclusion of the unauthenticated response signal as an additional one of said reference waveforms only if the unauthenticated response signal has a predefined acceptable characteristic, said mean processor computing a new composite mean waveform if the unauthenticated response signal is included as said additional reference waveform.

41. The system of claim 40 wherein the transmission from the unauthenticated transmitter has a received signal strength and a signal strength variability, said modification processor analyzing said received signal strength and said signal strength variability as said predefined acceptable characteristic, said modification processor including said unauthenticated response signal as said additional reference waveform only if said received signal strength is above a predetermined signal threshold and said signal strength variability is below a predetermined variability threshold.

42. The system of claim 38 wherein each of said reference waveforms has a response start as a function of time, the system further including an alignment processor which shifts said response start to temporally align said reference waveforms, said data storage area storing said temporally aligned reference waveforms.

43. The system of claim 40 wherein the unauthenticated response signal has a measured length and an initial response start as a function of time, said alignment processor temporally shifting said initial response start with respect to said composite mean waveform to temporally align the unauthenticated response signal with said composite mean waveform, said modification processor including the unauthenticated response signal as said additional reference waveform only if said measured length is substantially equal to a measured length of said composite mean waveform and said temporal shifting is below a predetermined shifting threshold.

44. The system of claim 31, further including a modification processor which analyzes said reference waveforms and determines a confidence value for each of said reference waveforms, said modification processor comparing said calculated confidence values to a maximum confidence value and removing a particular reference waveform from said data storage area if said calculated confidence value for said particular reference waveform exceeds said maximum confidence value.

45. The system of claim 44 wherein said maximum confidence value is dependent on a number of said reference waveforms in said data storage area, said maximum confidence value decreasing as said number of reference waveforms in said data storage area increases.

46. A method for the identification of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters, the method comprising the steps of:

storing a plurality of response waveforms designated as reference waveforms for the authentic transmitter in a data storage area;

computing a composite mean waveform from said reference waveforms;

calculating difference values between said reference waveforms and said composite mean waveform;

applying a weighting function to said difference values to generate weighted difference values;

determining a reference variability value based on said weighted difference values, said reference variability value being indicative of the degree of variation of said reference waveforms from said composite mean waveform;

receiving a transmission from the unauthenticated transmitter that can be the authentic transmitter or the unauthorized transmitter, said transmission including a first unauthenticated response signal and identification data identifying the unauthenticated transmitter as the authentic transmitter; and

calculating first unauthenticated difference values between said first unauthenticated response signal and said composite mean waveform;

applying said weighting function to said first unauthenticated difference values to generate first unauthenticated weighted difference values;

determining a first unauthenticated variability value based on said first unauthenticated weighted difference values, said first unauthenticated variability value being indicative of the degree of variation of said first unauthenticated response signal from said composite mean waveform; and

comparing said first unauthenticated variability value to said reference variability value to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

47. The method of claim 46, further including the step of calculating normalized waveforms for each respective reference waveform wherein said step of calculating difference values uses said normalized waveforms and said composite mean waveform.

48. The method of claim 47 wherein said step of computing said reference variability value includes the steps of computing individual reference variability value for each of said normalized waveforms and calculating a statistical measure of said individual reference variability values as said reference variability value.

49. The method of claim 48 wherein said step of calculating said statistical measure includes calculating a mean variability value of said individual reference variability values and a standard deviation of said individual reference variability values as said reference variability value, said step of comparing using said mean variability value and said standard deviation to determine whether said unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

50. The method of claim 48 wherein said step of comparing calculates a confidence value indicative of the level of confidence with which said unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU7## where CV is said confidence value, V_U is said first unauthenticated variability value, V is a mean variability value of said variability values for said normalized waveforms, and σ

_V is a standard deviation of said variability values for said individual mean waveforms.

51. The method of claim 50, further including the step of generating an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

52. The method of claim 51 wherein said predetermined threshold is between two and three.

53. The method of claim 46 wherein each of said reference waveforms have a response start as a function of time, the method further including the step of shifting said response starts to temporally align said reference waveforms, said data storage area storing said temporally aligned reference waveforms.

54. The method of claim 46 wherein said first unauthenticated response is a transmitter turn-on transient.

55. The method of claim 46 wherein said first unauthenticated response is a transmitter turn-off transient.

56. The method of claim 46 wherein said received transmission includes a second unauthenticated response signal, the method further including the steps of:

calculating second unauthenticated difference values between said second unauthenticated response signal and said composite mean waveform;

applying said weighting function to said second unauthenticated difference values to generate second unauthenticated weighted difference values; and

determining a second unauthenticated variability value based on said second unauthenticated weighted difference values, said second unauthenticated variability value being indicative of the degree of variation of said second unauthenticated response signal from said composite mean waveform, said step of comparing using both said first and second unauthenticated variability values to determine whether said unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

57. The method of claim 46 wherein said step of applying said weighting function raises said first unauthenticated difference values to a power greater than one to generate said first unauthenticated weighted difference values.

58. The method of claim 46 wherein said step of applying said weighting function raises said first unauthenticated difference values to a power to a power of 1.2 to generate said first unauthenticated weighted difference values.

59. The method of claim 46, further including the steps of analyzing said first unauthenticated response signal and said reference waveforms and including said first unauthenticated response signal as an additional one of said reference waveforms only if said first unauthenticated response signal has a predefined acceptable characteristic, said step of computing a composite mean waveform computing a new composite mean waveform if said first unauthenticated response signal is included as said additional reference waveform.

60. The method of claim 59 wherein said received transmission from the unauthenticated transmitter has a received signal strength and a signal strength variability, wherein said step of analyzing said first unauthenticated response signal includes the steps of analyzing said received signal strength and said signal strength variability as said predefined characteristic, said step of including said first unauthenticated response as said additional reference waveform including said first unauthenticated response only if said received signal strength is above a predetermined signal threshold and said signal strength variability is below a predetermined variability threshold.

61. The method of claim 59 wherein said first unauthenticated response signal has a measured length and an initial response start as a function of time, the method further including the step of temporally shifting said initial response start with respect to said composite mean waveform to temporally align said first unauthenticated response signal with said composite mean waveform, said step of including said first unauthenticated response signal as said additional reference waveform including said first unauthenticated response signal only if said measured length is substantially equal to a measured length of said composite mean waveform and said temporal shifting is below a predetermined shifting threshold.

62. The method of claim 46, further including the steps of analyzing said reference waveforms to determine a confidence value for each of said reference waveforms, comparing said calculated confidence values to a maximum confidence value, and removing a particular reference waveform from said data storage area if said calculated confidence value for said particular reference waveform exceeds said maximum confidence value, said step of computing a composite waveform computing a new composite waveform if said particular waveform is removed from said data storage area.

63. The method of claim 62 wherein said maximum confidence value is dependent on a number of said reference waveforms in said data storage area, said maximum confidence value decreasing as said number of reference waveforms in said data storage area increases.

64. A method using a receiver system for the authentication of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters, the receiver system including a plurality of stored response signals from the authentic transmitter designated as reference waveforms for the authentic transmitter, a composite mean waveform derived from the reference waveforms, and statistical values related to the performance waveforms and the composite mean waveform, the method comprising the steps of:

receiving a transmission from the unauthenticated transmitter that can be the authentic transmitter or the unauthorized transmitter, said received transmission including a first unauthenticated response signal and identification data identifying the unauthenticated transmitter as the authentic transmitter;

calculating first unauthenticated difference values between said first unauthenticated response signal and the composite mean waveform;

applying a weighting function to said first unauthenticated difference values to generate first unauthenticated weighted difference values;

determining a first unauthenticated variability value based on said first unauthenticated weighted difference values, said first unauthenticated variability value being indicative of the degree of variation of said first unauthenticated response signal from said composite mean waveform; and

comparing said first unauthenticated response variability value to the statistical values related to the reference waveforms and the composite mean waveform to determine whether said unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

65. The method of claim 64 wherein said step of applying said weighting function raises said first unauthenticated difference values to a power greater than one to generate said first unauthenticated weighted difference values.

66. The method of claim 64 wherein said step of applying said weighting function raises said first unauthenticated difference values to a power to a power of 1.2 to generate said first unauthenticated weighted difference values.

67. The method of claim 64 wherein said statistical values include an individual reference variability value for each of the reference waveforms, a mean variability value of said individual reference variability values, and a standard deviation of said individual reference variability values as said reference variability value, said step of comparing using said mean variability value and said standard deviation to determine whether said unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

68. The method of claim 64 wherein the receiver system includes normalized waveforms for each respective reference waveform and the statistical values are related to the normalized waveforms and the composite mean waveform, the method further including the step of computing a first unauthenticated normalized waveform from said first unauthenticated response signal, said step of calculating difference values calculating said difference values between said first unauthenticated normalized waveform and said composite mean waveform, said step of comparing said first unauthenticated response variability value to the statistical values using the statistical values related to the normalized waveforms and the composite mean waveform to determine whether said unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

69. The method of claim 68 wherein said step of comparing calculates a confidence value indicative of the level of confidence with which the unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU8## where CV is said confidence value, V_U is said first unauthenticated variability value, V is a mean variability value of said variability values for said normalized waveforms, and σ

_V is a standard deviation of said variability values for said normalized waveforms.

70. The method of claim 69, further including the step of generating an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

71. The method of claim 70 wherein said predetermined threshold is between two and three.

72. A method for the development of a reference fingerprint used in the authentication of an individual unauthenticated transmitter as an authentic transmitter or an unauthorized transmitter when operating in an environment containing a plurality of similar transmitters when the unauthenticated transmitter that can be the authentic transmitter or the unauthorized transmitter transmits identification data identifying the unauthenticated transmitter as the authentic transmitter, the transmission including an unauthenticated response signal, the method comprising the steps of:

storing a plurality of response waveforms designated as reference waveforms for the authentic transmitter in a data storage area;

computing a composite mean waveform from said reference waveforms;

calculating difference values between said reference waveforms and said composite mean waveform;

applying a weighting function to said difference values to generate weighted difference values; and

determining a reference variability value based on said weighted difference values, said reference variability value being indicative of the degree of variation of said reference waveforms from said composite mean waveform, whereby the unauthenticated response signal is processed to generate an unauthenticated variability value for the unauthenticated response signal and is compared with said reference variability value to determine whether the unauthenticated transmitter is the authentic transmitter or the unauthorized transmitter.

73. The method of claim 72 wherein said step of determining said reference variability value sums each of said weighted values to generate an individual variability value for each of said reference waveforms and generates a statistical measure of said individual variability values as said reference variability value.

74. The method of claim 73 wherein said statistical measure includes a mean variability value of said individual variability values and a standard deviation of said individual variability values, whereby said mean variability value and said standard deviation are to determine whether said unauthenticated transmitter the authentic transmitter or the unauthorized transmitter.

75. The method of claim 73, further including the step of calculating a confidence value indicative of the level of confidence with which the unauthenticated transmitter is identified as the authentic transmitter, said confidence value having the following form:

##EQU9## where CV is said confidence value, V_U is the unauthenticated response variability value, V is a mean variability value of said individual variability values, and σ

_V is a standard deviation of said individual variability values.

76. The method of claim 75 wherein said comparison processor generates an indication that the unauthenticated transmitter is the authentic transmitter if said confidence value is less than or equal to a predetermined threshold.

77. The method of claim 72 wherein said step of applying said weighting function raises said difference values to a power greater than one to generate said weighted difference values.

78. The method of claim 72, further including the steps of analyzing the unauthenticated response signal and said reference waveforms and including the unauthenticated response signal as an additional one of said reference waveforms only if the unauthenticated response signal has a predefined acceptable characteristic, said step of computing said composite mean waveform computing a new composite mean waveform if the unauthenticated response signal is included as said additional reference waveform.

79. The method of claim 78 wherein the received transmission from the unauthenticated transmitter has a received signal strength and a signal strength variability, said step of analyzing the unauthenticated response signal includes the steps of analyzing said received signal strength and said signal strength variability, said step of including the unauthenticated response as said additional reference waveform including the unauthenticated response only if said received signal strength is above a predetermined signal threshold and said signal strength variability is below a predetermined variability threshold.

80. The method of claim 78 wherein each of said reference waveforms have a response start as a function of time, the method further including the step of shifting said response starts to temporally align said reference waveforms, said data storage area storing said temporally aligned reference waveforms.

81. The method of claim 80 wherein the unauthenticated response signal has a measured length and an initial response start as a function of time, said step of shifting also shifting the unauthenticated response signal with respect to said composite mean waveform to temporally align the unauthenticated response signal with said composite mean waveform, said step of analyzing and including the unauthenticated response signal including the unauthenticated response signal as said additional reference waveform only if said measured length is substantially equal to a measured length of said composite mean waveform and said temporal shifting is below a predetermined shifting threshold.

82. The method of claim 72, further including the steps of analyzing said reference waveforms to determine a confidence value for each of said reference waveforms, comparing said calculated confidence values to a maximum confidence value, and removing a particular reference waveform from said data storage area if said calculated confidence value for said particular reference waveform exceeds said maximum confidence value, said step of computing a composite waveform computing a new composite waveform if said particular waveform is removed from said data storage area.

83. The method of claim 82 wherein said maximum confidence value is dependent on a number of said reference waveforms in said data storage area, said maximum confidence value decreasing as said number of reference waveforms in said data storage area increases.