Cellular telephone anti-fraud system

• US 5,448,760 A
• Filed: 06/08/1993
• Issued: 09/05/1995
• Est. Priority Date: 06/08/1993
• Status: Expired due to Term

• Current Assignee: Authorize Net LLC (Visa, Inc.)
• Original Assignee: Corsair Communications, Inc. (Visa, Inc.)
• Inventors: Frederick, Max B.
• Primary Examiner(s): Urban, Edward F.
• Assistant Examiner(s): TO, DORIS HA
• Application Number:
  US08/074,083
• Time in Patent Office: 819 Days
• Field of Search: 455/54.1, 455/54.2, 455/58.1, 455/56.1, 455/33.1, 455/38.1, 455/67.1, 455/227, 455/228, 455/115, 379/62, 380/23
• US Class Current: 455/410
• CPC Class Codes: H04W 12/06 Authentication
• CPC Class Codes: H04W 12/126 Anti-theft arrangements, e.g. protection against subscriber identity module [SIM] cloning
• CPC Class Codes: H04W 12/72 Subscriber identity

Abstract

Authorized use of a wireless subscriber system, e.g. cellular telephone system, is distinguished from unauthorized use by electronically observing a plurality of external signal traits which represent alterations in the radio frequency signal due to physical characteristics identified with a particular transmitter. Examples of such external signal traits include those which are produced as a consequence of manipulating the carrier to encode information upon it and those produced by transmitter timing circuit idiosyncrasies. The external signal traits are converted into a signature code or security pattern which is then stored along with the existing subscriber system identification code to generate a historic security pattern database for use in subsequent authorized use validation.

Claims

1. A transmitter identification system for identifying a transmitter which produces a transmitter signal in which information is encoded by manipulating a carrier, the transmitter signal including external signal traits resulting from said manipulation that are characteristic of said transmitter, comprising:

receiving means located remote from said transmitter for receiving said transmitter signal;

characterizing means connected to said receiving means for generating a plurality of characterizing signals for said transmitter by measuring features of said external signal traits resulting from said manipulation that are characteristic of said transmitter; and

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern wherein said current security pattern is used to identify said transmitter,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said features codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values.

2. The transmitter identification system of claim 1 further comprising:

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of transmitters;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern of said transmitter to corresponding feature codes of at least one of said historic security patterns stored in said historic security pattern storing means,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall transmitter confidence level from said feature confidence levels.

3. The transmitter identification system of claim 2 wherein said first comparing means generates one of a mismatch signal, a possible mismatch signal, and a match signal for said transmitter by establishing a scale dividing a range of possible values of said overall transmitter confidence level as follows;

(a) if said overall transmitter confidence level is above a third predetermined value, generating said mismatch signal;

(b) if said overall transmitter confidence level is below a fourth predetermined value, generating said match signal; and

(c) if said overall transmitter confidence level is not above said third predetermined value and not below said fourth predetermined value, generating said possible mismatch signal, wherein said third predetermined value is greater than said fourth predetermined value.

4. Apparatus for identifying a wireless subscriber unit and for granting or denying said wireless subscriber unit access to a subscriber service, said wireless subscriber unit generating a wireless subscriber signal in which data is encoded by manipulating a carrier, said wireless subscriber signal containing external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit, and said data including an identification code, comprising:

receiving and transmitting means located remote from said wireless subscriber unit for receiving said wireless subscriber signal;

characterizing means connected to said receiving and transmitting means for generating a plurality of characterizing signals for said wireless subscriber unit by measuring features of said external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit; and

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern wherein said current security pattern and said identification code are used to identify said wireless subscriber unit,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values.

5. The apparatus of claim 4 further comprising:

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of wireless subscriber units;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern of said wireless subscriber unit to corresponding feature codes of at least one of said wireless subscriber units stored in said historic security pattern storing means,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall wireless subscriber unit confidence level from said feature confidence levels.

6. The apparatus of claim 5 further including:

access means connected to said first comparing means for granting or denying access to said wireless subscriber unit to said subscriber service based upon said overall wireless subscriber unit confidence level.

7. The apparatus of claim 6 wherein said first comparing means generates one of a mismatch signal, a possible mismatch signal, and a match signal for said wireless subscriber unit by establishing a scale dividing a range of possible values of said overall wireless subscriber unit confidence level as follows:

(a) if said overall wireless subscriber unit confidence level is above a third predetermined value, generating said mismatch signal;

(b) if said overall wireless subscriber unit confidence level is below a fourth predetermined value, generating said match signal; and

(c) if said overall wireless subscriber unit confidence level is not above said third predetermined value and not below said fourth predetermined value, generating said possible mismatch signal wherein said third predetermined value is greater than said fourth predetermined value.

8. The apparatus of claim 7 further comprising:

fraudulent user history storing means for storing fraudulent user security patterns, each including a plurality of feature codes, for a plurality of fraudulent wireless subscriber units; and

second comparing means connected to said first comparing means for comparing said feature codes of said current security pattern of said wireless subscriber unit to corresponding feature codes of at least one of said fraudulent user security patterns when said first comparing means generates the possible mismatch signal.

9. The apparatus of claim 8 wherein said second comparing means generates fraud feature confidence levels related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one fraudulent user security pattern, and produces an overall fraudulent confidence level from said fraudulent feature confidence levels, wherein said second comparing means generates one of a fraud match signal or a fraud mismatch signal as follows:

(a) if said overall fraud confidence level is above a fifth predetermined value, generating said fraud match signal; and

(b) if said overall fraud confidence level is below said fifth predetermined value, generating said fraud mismatch signal.

10. The apparatus of claim 9 wherein said access means grants access to said wireless subscriber unit if said first comparing means generates the match signal, or if said first comparing means generates the possible mismatch signal and said second comparing means generates the fraud mismatch signal.

11. The apparatus of claim 10 wherein said access means denies access to said wireless subscriber unit if said first comparing means generates the mismatch signal, or if the first comparing means generates the possible mismatch signal and said second comparing means generates the fraud match signal.

12. The apparatus of claim 11 wherein said fraudulent user history storing means is updated with said current security pattern of said wireless subscriber unit if said second comparing means generates the fraud match signal.

13. The apparatus of claim 10 wherein said said historic security pattern stored in said historic security pattern storing means corresponding to said identification code is updated with said current security pattern of said wireless subscriber unit if said second comparing means generates the fraud mismatch signal.

14. The apparatus of claim 4 wherein said converting means compares said plurality of characterizing signals to a plurality of standard signals and generates a plurality of multibit signals representing deviation of each of said characterizing signals from each of said standard signals.

15. The apparatus of claim 14 wherein said converting means combines said plurality of multibit signals into said current security pattern.

16. The apparatus of claim 14 wherein said converting means combines said plurality of multibit signals and said identification code into said current security pattern.

17. The apparatus of claim 4 wherein said characterizing means generates at least one of said characterizing signals by timing a feature in said wireless subscriber signal.

18. The apparatus of claim 17 wherein said converting means compares said at least one of said characterizing signals representing said timing of said feature to a standard timing signal and generates a first multibit signal representing deviation of said characterizing signal from said standard timing signal.

19. The apparatus of claim 4 wherein said characterizing means generates at least one of said characterizing signals by performing a time comb on a portion of said wireless subscriber signal.

20. The apparatus of claim 4 wherein said characterizing means generates at least one of said characterizing signals by generating a frequency histogram on a portion of said wireless subscriber signal to determine RF shift of said wireless subscriber signal.

21. The apparatus of claim 4 wherein said characterizing means generates at least one of said characterizing signals by fitting a line by a least-means-squared method to a mark portion of said wireless subscriber signal.

22. The apparatus of claim 4 wherein said characterizing means generates at least one of said characterizing signals by fitting a line by a least-means-squared method to a space portion of said wireless subscriber signal.

25. The method of claim 21 further comprising the steps of:

generating a current security pattern, including a plurality of feature codes, for said requesting wireless subscriber unit;

comparing said feature codes of said requesting wireless subscriber unit with corresponding feature codes of said security pattern stored in said ordered relationship with said identification code;

generating feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said requesting wireless subscriber unit and said corresponding feature codes of said security pattern stored in said ordered relationship with said identification code;

generating an overall wireless subscriber unit confidence level from said feature confidence levels.

26. The method of claim 25 further comprising the steps of:

granting or denying access to said wireless subscriber unit to said subscriber service based upon said overall wireless subscriber unit confidence level.

23. Apparatus for identifying a wireless subscriber unit and for granting or denying said wireless subscriber unit access to a subscriber service, said wireless subscriber unit generating a wireless subscriber signal in which data is encoded by manipulating a carrier, the wireless subscriber signal containing external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit, and said data including an identification code, comprising:

receiving and transmitting means located remote from said wireless subscriber unit for receiving said wireless subscriber signal;

characterizing means connected to said receiving and transmitting means for generating a plurality of characterizing signals for said wireless subscriber unit by measuring features of said external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit;

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern, wherein said current security pattern and said identification code are used to identify said wireless subscriber unit,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values;

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of wireless subscriber units;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern of said wireless subscriber unit to corresponding feature codes of at least one of said wireless subscriber units stored in said historic security pattern storing means corresponding to said identification code of said wireless subscriber unit,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall wireless subscriber unit confidence level from said feature confidence levels;

access means connected to said first comparing means for granting or denying access to said wireless subscriber unit to said subscriber service based upon said overall wireless subscriber unit confidence level.

24. A method for identifying a wireless subscriber unit and for granting or denying said wireless subscriber unit access to a subscriber service, said wireless subscriber unit generating a wireless subscriber signal in which data is encoded by manipulating a carrier, the wireless subscriber signal containing external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit, and said data including an identification code, comprising the steps of:

providing receiving and transmitting means located remote from said wireless subscriber unit for receiving said wireless subscriber signal;

characterizing said wireless subscriber unit by generating a plurality of characterizing signals for said wireless subscriber unit by measuring a plurality of features of said external signal traits resulting from said manipulation that are characteristic of said wireless subscriber unit;

converting said plurality of characterizing signals to a security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values;

storing said security pattern and said identification code in an ordered relationship;

subsequently utilizing said security pattern to grant or deny access to said subscriber service to a requesting wireless subscriber unit with same identification code.

27. The method of claim 24 further comprising the step of:

monitoring at least one usage parameter indicative of fraudulent use.

28. The method of claim 27 wherein said usage parameter includes at least one of a number of times a wireless subscriber unit requests access to said subscriber service in a first period, and a length of time said wireless unit uses said subscriber service in second period.

29. A transmitter identification system for identifying a transmitter which produces a transmitter signal in which information is encoded by manipulating a carrier, the transmitter signal including external signal traits resulting from transmitter timing idiosyncrasies that are characteristic of said transmitter, comprising:

receiving means located remote from said transmitter for receiving said transmitter signal;

characterizing means connected to said receiving means for generating a plurality of characterizing signals for said transmitter by measuring features of said external signal traits resulting from said transmitter timing idiosyncrasies that are characteristic of said transmitter; and

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern wherein said current security pattern is used to identify said transmitter,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values.

30. The transmitter identification system of claim 29 further comprising:

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of transmitters;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern of said transmitter to corresponding feature codes of at least one of said historic security patterns stored in said historic security pattern storing means,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall transmitter confidence level from said feature confidence levels.

31. The transmitter identification system of claim 30 wherein said first comparing means generates one of a mismatch signal, a possible mismatch signal, and a match signal for said transmitter by establishing a scale dividing a range of possible values of said overall transmitter confidence level as follows:

(a) if said overall transmitter confidence level is above a third predetermined value, generating said mismatch signal;

(b) if said overall transmitter confidence level is below a fourth predetermined value, generating said match signal; and

(c) if said overall transmitter confidence level is not above said third predetermined value and not below said fourth predetermined value, generating said possible mismatch signal wherein said third predetermined value is greater than said fourth predetermined value.

32. Apparatus for identifying a wireless subscriber unit and for granting or denying said wireless subscriber unit access to a subscriber service, said wireless subscriber unit generating a wireless subscriber signal in which data is encoded by manipulating a carrier, said wireless subscriber signal containing external signal traits resulting from wireless subscribing unit timing idiosyncrasies that are characteristic of said wireless subscriber unit, and said data including an identification code, comprising:

receiving and transmitting means located remote from said wireless subscriber unit for receiving said wireless subscriber signal;

characterizing means connected to said receiving and transmitting means for generating a plurality of characterizing signals for said wireless subscriber unit by measuring features of said external signal traits resulting from said wireless subscriber unit timing idiosyncrasies that are characteristic of said wireless subscriber unit; and

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern wherein said current security pattern and said identification code are used to identify said wireless subscriber unit,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein said feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if a characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values.

33. The apparatus of claim 32 further comprising:

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of wireless subscriber units;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern of said wireless subscriber unit to corresponding feature codes of at least one of said wireless subscriber units stored in said historic security pattern storing means,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall wireless subscriber unit confidence level from said feature confidence levels.

34. The apparatus of claim 33 further including:

access means connected to said first comparing means for granting or denying access to said wireless subscriber unit to said subscriber service based upon said overall wireless subscriber unit confidence level.

35. The apparatus of claim 34 wherein said first comparing means generates one of a mismatch signal, a possible mismatch signal, and a match signal for said wireless subscriber unit by establishing a scale dividing a range of possible values of said overall wireless subscriber unit confidence level as follows:

(a) if said overall wireless subscriber unit confidence level is above a third predetermined value, generating said mismatch signal;

(b) if said overall wireless subscriber unit confidence level is below a fourth predetermined value, generating said match signal; and

(c) if said overall wireless subscriber unit confidence level is not above said third predetermined value and not below said fourth predetermined value, generating said possible mismatch signal, wherein said third predetermined value is greater than said fourth predetermined value.

36. The apparatus of claim 35 further comprising:

fraudulent user history storing means for storing fraudulent user security patterns, each including a plurality of feature codes, for a plurality of fraudulent wireless subscriber units; and

second comparing means connected to said first comparing means for comparing said feature codes of said current security pattern of said wireless subscriber unit to corresponding feature codes of at least one of said fraudulent user security patterns when said first comparing means generates the possible mismatch signal.

37. The apparatus of claim 36 wherein said second comparing means generates fraud feature confidence levels related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one fraudulent user security pattern, and produces a fraud confidence level from said fraudulent feature confidence levels, wherein said second comparing means generates one of a fraud match signal or a fraud mismatch signal as follows:

(a) if said fraud confidence level is above a fifth predetermined value, generating said fraud match signal; and

(b) if said fraud confidence level is below said fifth predetermined value, generating said fraud mismatch signal.

38. The apparatus of claim 37 wherein said access means grants access to said wireless subscriber unit if said first comparing means generates the match signal, or if said first comparing means generates the possible mismatch signal and said second comparing means generates the fraud mismatch signal.

39. The apparatus of claim 38 wherein said security pattern stored in said historic security pattern storing means for said identification code is updated with said current security pattern of said wireless subscriber unit if said second comparing means generates the fraud match signal.

40. The apparatus of claim 37 wherein said access means denies access to said wireless subscriber unit if said first comparing means generates the mismatch signal, or if the first comparing means generates the possible mismatch signal and said second comparing means generates the fraud match signal.

41. The apparatus of claim 40 wherein said fraudulent user history storing means is updated with said current security pattern of said wireless subscriber unit if said second comparing means generates the fraud match signal.

42. The apparatus of claim 32 wherein said characterizing means generates at least one of said characterizing signals by timing a feature in said wireless subscriber signal.

43. The apparatus of claim 42 wherein said converting means compares said at least one of said characterizing signals representing said timing of said feature to a standard timing signal and generates a first multibit signal representing deviation of said characterizing signal from said standard timing signal.

44. Apparatus for identifying a wireless subscriber unit and for granting or denying said wireless subscriber unit access to a subscriber service, said wireless subscriber unit generating a wireless subscriber signal in which data is encoded by manipulating a carrier, the wireless subscriber signal containing external signal traits resulting from wireless subscriber unit timing idiosyncrasies that are characteristic of said wireless subscriber unit and said data including an identification code, comprising:

receiving and transmitting means located remote from said wireless subscriber unit for receiving said wireless subscriber signal;

characterizing means connected to said receiving and transmitting means for generating a plurality of characterizing signals for said wireless subscriber unit by measuring features of said external signal traits resulting from said wireless subscriber unit timing idiosyncrasies that are characteristic of said wireless subscriber unit;

converting means connected to said characterizing means for converting said plurality of characterizing signals into a current security pattern, wherein said current security pattern and said identification code are used to identify said wireless subscriber unit,

wherein said converting means produces said current security pattern by generating feature codes for said plurality of characterizing signals by establishing, separately for each feature, a scale dividing a range of possible values of a characterizing signal for said feature into a predetermined number of divisions, each division being associated with a unique code, said scale divisions including an upper division and a lower division, and

wherein a feature code for said feature is produced by assigning said characterizing signal to one of said predetermined number of divisions, as follows;

(a) if said characterizing signal is above a first predetermined value, assigning said characterizing signal a feature code corresponding to said upper division;

(b) if said characterizing signal is below a second predetermined value, assigning said characterizing signal a feature code corresponding to said lower division; and

(c) if said characterizing signal is not above said first predetermined value and not below said second predetermined value, assigning said characterizing signal a feature code, intermediate said feature codes which fall between said upper and lower divisions, reflecting the relationship said characterizing signal bears to said first and second predetermined values;

historic security pattern storing means for storing historic security patterns, each including a plurality of feature codes, for a plurality of wireless subscriber units;

first comparing means connected to said converting means and said historic security pattern storing means for comparing each of said feature codes of said current security pattern to corresponding feature codes of at least one of said wireless subscriber units stored in said historic security pattern storing means corresponding to said identification code of said wireless subscriber unit,

wherein said first comparing means generates feature confidence levels, separately for each feature, related to a difference between each of said feature codes of said current security pattern and said corresponding feature codes of said at least one of said historic security patterns, and

wherein said first comparing means generates an overall wireless subscriber unit confidence level from said feature confidence level;

access means connected to said first comparing means for granting or denying access to said wireless subscriber unit to said subscriber service based upon said overall wireless subscriber unit confidence level.