Method and system for distance determination of RF tags

• US 7,511,604 B2
• Filed: 05/04/2003
• Issued: 03/31/2009
• Est. Priority Date: 05/16/2002
• Status: Active Grant

• Current Assignee: Zebra Enterprise Solutions Corp. (Zebra Technologies Corporation)
• Original Assignee: Sandlinks Systems Ltd.
• Inventors: Raphaeli, Ruth, Raphaeli, Dan
• Primary Examiner(s): Zimmerman; Brian A
• Assistant Examiner(s): Brown; Vernal U
• Application Number:
  US10/514,118
• Publication Number:
  US 20070109099A1
• Time in Patent Office: 2,158 Days
• Field of Search: 340/10.1, 340/10.2, 340/10.34, 340/10.4, 340/572.1, 340/10.5, 340/10.3, 340/825.69, 340/825.72, 342/44, 370/342
• US Class Current: 340/10.2
• CPC Class Codes: G01S 13/758 using a signal generator powered by the interrogation signal
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10029 the collision being resolved in the time domain, e.g. using binary tree search or RFID responses allocated to a random time slot
• CPC Class Codes: G06K 7/10079 the collision being resolved in the spatial domain, e.g. temporary shields for blindfolding the interrogator in specific directions
• CPC Class Codes: G06K 7/10306 ultra wide band
• CPC Class Codes: G07C 9/28 the pass enabling tracking or indicating presence
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• 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 74/08 Non-scheduled access, e.g. ALOHA hybrid access H04W74/02

Abstract

The present invention relates to a method and system (10) for distance determination of RF tags (4) and its applications based on measuring the round trip delay, avoiding deterministic collisions between a plurality of wide band response signals transmitted by the tags (4) by adding a pseudo-random time delay to the wide band response signal, and iteratively reducing the collision probability between plurality of wide band response signals.

Claims

1. A method for communicating between a reader and a plurality of RF tags, comprising the steps of:

(a) transmitting a wide band broadcast interrogation signal by a reader;

(b) waiting to receive said wide band broadcast interrogation signal by the plurality of RF tags;

(c) upon receiving said wide band broadcast interrogation signal by a plurality of said RF tags and in response to said wide band broadcast interrogation signal transmitting a plurality of wide band response signals respectively from said RF tags,

(d) receiving said wide band response signals by said reader, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses wherein at least two of said wide band response signals are overlapping wide band response signals, wherein said overlapping wide band response signals are distinguishable when each of said pulses of each of said wide band response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals;

(i) detecting by said reader and thereby distinguishing said sequences of pulses of said overlapping wide band response signals

(ii) estimating by said reader the number of wide band response signals transmitted by said tags in response to said wide band broadcast interrogation signal,

(iii) analyzing by said reader said distinguishable wide band response signals in order to determine their round trip delay,

(e) testing by said reader a predefined statistical criterion indicative of the collision probability of said wide band response signals and according to the result of said testing transmitting by said reader at least one additional wide band broadcast interrogation signal, whereby each said additional wide band broadcast interrogation signal is followed by repeating the steps of said tags said transmitting in response to said additional wide band broadcast interrogation signal and said reader receiving, detecting, estimating, analyzing, testing and transmitting at least one additional wide band broadcast interrogation signal, wherein the collision probability is based at least in part on the number of overlapping wide band response signals, the duration of said pulses and the time interval between said pluses.

2. The method of claim 1, further comprising a step of:

adding a time delay respectively by at least one of said RF tags to at least one of said wide band response signals, whereby said time delay avoids deterministic collisions between the respective pulse sequences of said wide band response signals as transmitted by said tags, wherein the duration of said time delay is less than the duration of a single packet of said wide band response signal.

3. The method of claim 2, wherein said time delay is transmitted to said reader using a method selected from the group consisting of:

said time delay is included in data carried within said at least one wide band response signal, and said time delay is transmitted on a separate signal.

4. The method of claim 2, wherein said time delay is a random or pseudo-random time delay.

5. The method of claim 2, further comprising the steps of:

(f) at least one of said tags is waiting, according to a predefined logic, to receive said at least one additional wide band broadcast interrogation signal, and according to said predefined logic, said at least one tag transmits additional wide band response signals as a response to said at least one additional wide band broadcast interrogation signal;

(g) receiving by said reader the additional at least one wide band response signal, wherein said additional at least one wide band response signal received by said reader include overlapping wide band response signals transmitted by a plurality of said tags, and (h) said reader detecting at least one additional response sequence included in said additional at least one wide band response signal received by said reader.

6. The method of claim 2, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag scanning in the time domain, and whenever said at least one tag is not transmitting or receiving signals, it is deactivated into a sleep mode where it is activated periodically with a predefined duty cycle.

7. The method of claim 2, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag is scanning in the frequency domain by sweeping a center frequency in predefined steps such as to cover a desired frequency range until said wide band broadcast interrogation signal is detected.

8. The method of claim 2, wherein step (c) includes, said at least one tag may transmit said wide band response signal according to a predefined logic.

9. The method of claim 2, wherein said at least one wide band response signal comprising plurality of said wide band response signals, featuring different time delay values, whereby said different time delay are reducing the collisions probability between a plurality of wide band response signals transmitted by said tags.

10. The method of claim 2, wherein said at least one wide band response signal is transmitted on the same channel of said wide band broadcast interrogation signal.

11. The method of claim 2, wherein said at least one wide band response signal is transmitted on a channel selected pseudo-randomly from a predetermined pool of channels.

12. The method of claim 2, wherein said at least one tag having means for determining the identification of the transmitting reader.

13. The method of claim 12, wherein said means for determining said identification of said transmitting reader is selected from the group consisting of:

said wide band broadcast interrogation signal comprises identification of said transmitting reader, and unique predefined physical properties of said wide band broadcast interrogation signal identifying said transmitting reader.

14. The method of claim 12, wherein said tag is programmed to respond only to predefined selected readers.

15. The method of claim 12, wherein said tag stop responding to additional wide band broadcast interrogation signals from a specific reader after said tag responded to wide band broadcast interrogation signals from said specific reader a predetermined number of times in a predetermined duration.

16. The method of claim 15, wherein said tag resumes responding to said wide band broadcast interrogation signals from said specific reader after a fulfillment of a condition selected from the group consisting of:

said predetermined duration past since last respond, and the receive power of said wide band broadcast interrogation signals have been changed by more than a predefined value relative to last received power when said tag responded.

17. The method of claim 2, wherein said testing a predefined statistical criterion indicative of the collision probability between a plurality of said wide band response signals is a predefined statistical test of said estimation of the number of wide band response signals that were received by said reader.

18. The method of claim 17, wherein said statistical test is comparing the value of said estimation of number of responses with a predefined threshold.

19. The method of claim 2, wherein said wide band broadcast interrogation signal is generated by using a series of pulses.

20. The method of claim 2, wherein said wide band broadcast interrogation signal is transmitted in a specific direction, by said reader utilizing a directional antenna.

21. The method of claim 2, wherein said wide band response signal is generated by using a series of pulses.

22. The method of claim 21, wherein the timing of said pulses is determined by timing means.

23. The method of claim 21, wherein said wide band response signal is modulated and said tag is transmitting a predefined number of pulses modulated by a predefined data before transmitting the modulated wide band response signal, whereby the use of said predefined number of pulses modulated by a predefined data by the reader is selected from the group consisting of:

informing said reader of the forthcoming wide band response signal, estimating number of responses and letting PLL of said tag reach better synchronization accuracy before transmitting the data containing response.

24. The method of claim 2, wherein said wide band response signal includes a data portion and the step of analyzing said wide band response signals transmitted by said tags further including extracting said data.

25. The method of claim 2, wherein the value of the power of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, and said at least one additional wide band broadcast interrogation signal, is adjusted by a user.

26. The method of claim 2, wherein the embodiment of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, and said at least one additional wide band broadcast interrogation signal, comprising a method selected from the group consisting of;

pulse signals including pulses separated by no-energy periods, and base-band pulses, and pass-band pulses, and direct sequence spread spectrum signals, and types of wide-band signals achieving a short time resolution.

27. The method of claim 26, wherein said transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, and said at least one additional wide band broadcast interrogation signal, feature transmission data rate of about 100 Kbit/sec, with a bandwidth of about 50 MHz, centered at about 2.4 GHz.

28. The method of claim 2, wherein the transmitted wide band signals selected from the group consisting of said wide band broadcast interrogation signal, and said wide band response signal, and said at least one additional wide band broadcast interrogation signal, comprising a sequence of pulses.

29. The method of claim 2, wherein said wide band broadcast interrogation signal and said at least one additional wide band broadcast interrogation signal, are organized in three intervals comprising:

a preamble portion, a data portion, and a response period portion.

30. The method of claim 29, wherein before said tag is beginning to transmit said wide band response signal in synchronization with said response period of said wide band broadcast interrogation signal, said tag is transmitting during a second synchronization period at the beginning of said response period, a few pulses selected from the group consisting of non-modulated pulses, and modulated pulses with a sequence identical to all tags and known to said reader;

whereby said second synchronization period is increasing the accuracy of said wide band response signal.

31. The method of claim 29, wherein before said tag is beginning to transmit said wide band response signal in synchronization with said response period of said wide band broadcast interrogation signal, said tag is transmitting during a second synchronization period at the beginning of said response period, a few pulses selected from the group consisting of:

non-modulated pulses, and modulated pulses with a sequence identical to all tags and known to said reader;

whereby said reader using said few pulses for increasing the accuracy of said estimating the number of responses to said wide band broadcast interrogation signal.

32. The method of claim 2, wherein said round trip delay is determined by subtracting a predetermined correction factor from the measured time between the transmission of said wide band broadcast interrogation signal and receiving said wide band response signal, whereby said predetermined correction factor compensates for a predefined time delay of said at least one tag between receiving said wide band broadcast interrogation signal and transmitting said wide band response signal and predefined time delay of said reader operation.

33. The method of claim 9, wherein said predetermined correction factor having different values for different tags.

34. The method of claim 2, wherein said RFID system comprising means for dealing with a Multipath effect.

35. The method of claim 34, wherein said RFID system means for dealing with said Multipath effect are selected from the group consisting of:

said reader measuring the delay of the first Multipath component, and said wide band response signal comprising identification of the transmitting tag and said reader reviews the received list of said at least one tag identifications to determine whether a single tag identification appears more than once, wherein said more than once appearance is generally due to said Multipath effect and therefore the surplus received signals having longer delay, are removed from the list of responding tags.

36. The method of claim 2, wherein step (d) further comprising analyzing each collection of pulses belonging to a single said tag for determining that it includes at least a predetermined minimum number of received pulses, whereby said predetermined minimum number of received pulses allows reconstructing the data content of said wide band response signal using error correction methods.

37. The method of claim 2, wherein said at least one additional wide band broadcast interrogation signal is selected from the group consisting of:

wide band broadcast interrogation signal transmitted without a preamble portion, and wide band broadcast interrogation signal including a shorter data content relative to the data portion of the first wide band broadcast interrogation signal, and wide band broadcast interrogation including a short repeat message, and wide band broadcast interrogation signal including only a response portion.

38. The method of claim 2, further comprising said reader comparing a predefined threshold to said round trip delay of at least one of said at least one wide band response signal received by said reader, and performing a task with tags having round trip delay that is passing said predefined threshold test.

39. The method of claim 38, wherein different tags are compared to different predefined thresholds according to their tag identification.

40. The method of claim 38, wherein different tags located in different ranges from said reader receive different handling.

41. The method of claim 38, wherein the determination of said predefined threshold is selected from the group consisting of said reader have a user interface allowing a user to determine said predefined threshold, and said reader is associated with at least one host computer through which the configuration is performed, and said predefined threshold is determined according to a predefined logic initialized by external parameters, and said predefined threshold is determined according to a predefined logic initialized by a sensor measurement.

42. The method of claim 38, wherein different tags are compared to different predefined thresholds.

43. A system for communicating between a reader and a plurality of RF tags, the system comprising:

(a) the reader which transmits a wide band broadcast interrogation signal,

(b) the tags which wait to receive said wide band broadcast interrogation signal,

(c) the tags receive said wide band broadcast interrogation signal and in response transmit respectively a plurality of wide band response signals, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses,

(d) the reader receives said wide band response signals, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses wherein said wide band response signals include overlapping wide band response signals, wherein said overlapping wide band response signals are distinguishable when each of said pulses of each of said wide band response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals and (i) said reader detects and distinguishes the overlapping pulse sequences included in said overlapping wide band response signals (ii) said reader estimates the number of wide band response signals to said wide band broadcast interrogation signal, (iii) said reader analyzes and thereby distinguishes between said overlapping wide band response signals in order to determine their round trip delay,

(e) said reader tests a predefined statistical criterion indicative of the collision probability of said at least one wide hand response signal and according to the result of the test said reader transmits at least one additional wide band broadcast interrogation signal, wherein the collision probability is based at least in part on the number of overlapping wide band response signals, the duration of said pulses and the time interval between said pulses.

44. The system of claim 43, wherein said at least one tag adds a time delay to its wide band response signal, whereby said time delay avoids deterministic collisions between said wide band response signals, wherein said time delay is less than said time interval between said pulses of said wide band response signals.

45. A method for defining a programmable reader cell in RFID system, comprising the steps of:

(a) transmitting a wide band broadcast interrogation signal, by a reader,

(b) a plurality of tags waiting to receive said wide band broadcast interrogation signal,

(c) said tags receiving said wide band broadcast interrogation signal and in response transmitting respectively a plurality of wide band response signals,

(d) receiving by said reader said wide band response signals, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses wherein said wide band response signals received by said reader include overlapping wide band response signals, wherein said overlapping wide band response signals are distinguishable when each of said pulses of each of said wide bhand response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals;

(e) detecting by said reader and thereby distinguishing the pulse sequences included in said overlapping wide band response signal and deciding whether one or more tags answered said wide band broadcast interrogation signal,

(f) analyzing by said reader the at least one detected wide band response signal, in order to determine the round trip delay of said at least one detected wide band response signal,

(g) said reader comparing a predefined threshold to said round trip delay of at least one of said at least one wide band response signal received by said reader, and performing a task with tags having round trip delay that is passing said predefined threshold test.

46. The method of claim 45, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag is scanning in the time domain, and whenever said at least one tag is not transmitting or receiving signals, it is deactivated into a sleep mode where it is activated periodically with a predefined duty cycle.

47. The method of claim 45, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag is scanning in the frequency domain by sweeping a center frequency in predefined steps such as to cover a desired frequency range until said wide band broadcast interrogation signal is detected.

48. The method of claim 45, further comprising the step of:

adding a time delay to said wide band response signal, whereby said time delay avoids deterministic collisions between a plurality of wide band response signals transmitted by said tags, wherein said time delay is less than the duration of a single packet of said wide band response signal.

49. The method of claim 48, wherein said time delay is transmitted to said reader using a method selected from the group consisting of:

said time delay is included in a data carried within said wide band response signal, and said time delay is transmitted on a separate signal.

50. The method of claim 45, wherein said at least one wide band response signal comprising plurality of said wide band response signals, featuring different time delay values, whereby said different time delay are reducing the collisions probability between a plurality of wide band response signals transmitted by said tags.

51. The method of claim 45, wherein said at least one wide band response signal is transmitted on the same channel of said wide band broadcast interrogation signal.

52. The method of claim 45, wherein said at least one wide band response signal is transmitted on a channel selected pseudo-randomly from a predetermined pool of channels.

53. The method of claim 45, wherein said at least one tag having means for determining the identification of the transmitting reader.

54. The method of claim 53, wherein said means for determining said identification of said transmitting reader is selected from the group consisting of:

said wide band broadcast interrogation signal comprises identification of said transmitting reader, and unique predefined physical properties of said wide band broadcast interrogation signal identifying said transmitting reader.

55. The method of claim 53, wherein said tag is programmed to respond only to predefined selected readers.

56. The method of claim 53, wherein said tag stop responding to additional wide band broadcast interrogation signals from a specific reader after said tag responded to wide band broadcast interrogation signals from said specific reader a predetermined number of times in a predetermined duration.

57. The method of claim 56, wherein said tag resumes responding to said wide band broadcast interrogation signals from said specific reader after a fulfillment of a condition selected from the group consisting of:

said predetermined duration past since last respond, and the receive power of said wide band broadcast interrogation signals have been changed by more than a predefined value relative to last received power when said tag responded.

58. The method of claim 45, wherein said wide band broadcast interrogation signal is generated by using a series of pulses.

59. The method of claim 45, wherein said wide band broadcast interrogation signal is transmitted in a specific direction, by said reader utilizing a directional antenna.

60. The method of claim 45, wherein said wide band response signal is generated by using a series of pulses.

61. The method of claim 60, wherein the timing of said pulses is determined by timing means.

62. The method of claim 60, wherein said wide band response signal is modulated and said tag is transmitting a predefined number of non-modulated pulses before transmitting the modulated wide band response signal, whereby said predefined number of non-modulated pulses are informing said reader of the forthcoming wide band response signal and help in synchronizing a PLL of said reader.

63. The method of claim 45, wherein said wide band response signal includes a tag identification field and the step of analyzing said wide band response signals transmitted by said tags further including extracting said tag identification field.

64. The method of claim 45, wherein the value of the power of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, is adjusted by a user.

65. The method of claim 45, wherein the embodiment of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, comprising a method selected from the group consisting of;

pulse signals including pulses separated by no-energy periods, and base-band pulses, and pass-band pulses, and direct sequence spread spectrum signals, and types of wide-band signals achieving a short time resolution.

66. The method of claim 65, wherein said transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, feature transmission data rate of about 100 Kbit/sec, with a bandwidth of about 50 MHz, centered at about 2.4 GHz.

67. The method of claim 45, wherein the transmitted wide band signals selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, comprising a sequence of pulses.

68. The method of claim 45, wherein said wide band broadcast interrogation signal, is organized in three intervals comprising:

a preamble portion, a data portion, and a response period portion.

69. The method of claim 45, wherein said round trip delay is determined by subtracting a predetermined correction factor from the measured time between the transmission of said wide band broadcast interrogation signal and receiving said wide band response signal, whereby said predetermined correction factor compensates for a predefined time delay of said at least one tag between receiving said wide band broadcast interrogation signal and transmitting said wide band response signal and predefined time delay of said reader operation.

70. The method of claim 50, wherein said predetermined correction factor having different values for different tags.

71. The method of claim 45, wherein said RFID system comprising means for dealing with a Multipath effect.

72. The method of claim 71, wherein said RFID system means for dealing with said Multipath effect are selected from the group consisting of:

said reader measuring the delay of the first Multipath component, and said wide band response signal comprising identification of the transmitting tag and said reader reviews the received list of said at least one tag identifications to determine whether a single tag identification appears more than once, wherein said more than once appearance is generally due to said Multipath effect and therefore the surplus received signals having longer delay, are removed from the list of responding tags.

73. The method of claim 45, further comprising analyzing each collection of pulses belonging to a single said tag for determining that it includes at least a predetermined minimum number of received pulses, whereby said predetermined minimum number of received pulses allows reconstructing the data content of said wide band response signal using error correction methods.

74. The method of claim 45, wherein said at least one additional wide band broadcast interrogation signal is selected from the group consisting of:

wide band broadcast interrogation signal transmitted without a preamble portion, and wide band broadcast interrogation signal including a shorter data content relative to the data portion of the first wide band broadcast interrogation signal, and wide band broadcast interrogation including a short repeat message, and wide band broadcast interrogation signal including only a response portion.

75. The method of claim 45, further comprising said reader comparing a predefined threshold to said round trip delay of at least one of said at least one wide band response signal received by said reader, and performing a task with tags having round trip delay that is passing said predefined threshold test.

76. The method of claim 75, wherein different tags are compared to different predefined thresholds according to their tag identification.

77. The method of claim 75, wherein different tags located in different ranges from said reader receive different handling.

78. The method of claim 75, wherein the determination of said predefined threshold is selected from the group consisting of:

said reader have a user interface allowing a user to determine said predefined threshold, and said reader is associated with at least one host computer through which the configuration is performed, and said predefined threshold is determined according to a predefined logic initialized by external parameters, and said predefined threshold is determined according to a predefined logic initialized by a sensor measurement.

79. The method of claim 75, wherein different tags are compared to different predefined thresholds.

80. The method of claim 45, further comprising the step of:

said reader estimating the number of wide band response signals to said wide band broadcast interrogation signal, testing by said reader a predefined statistical criterion indicative of the collision probability of said at least one wide band response signal and according to the result of said test, transmitting by said reader at least one additional wide band broadcast interrogation signal, wherein the collision probability is based at least in part on the number of overlapping wide band response signals, the duration of said pulses and the time interval between said pulses.

81. The method of claim 45, further comprising whenever said reader deciding that more than a predefined number of tags answered to said wide band broadcast interrogation signal, there is repeating all steps until the number of tags answering additional wide band broadcast interrogation signal decrease to less than said predefined number of tags.

82. The method of claim 81, wherein the value of said predefined number of tags is set by a statistical criterion in accordance with the collision probability between a plurality of wide band response signals transmitted by a plurality of said tags.

83. A system for defining a programmable reader cell in RFID system, comprising:

(a) a reader which transmits a wide band broadcast interrogation signal,

(b) a plurality of tags which wait to receive said wide band broadcast interrogation signal,

(c) the tags wait to receive said wide band broadcast interrogation signal and in response to said wide band broadcast interrogation signal transmit respectively a plurality of wide band response signals, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pluses;

(d) said reader receives said wide band response signals, wherein said wide band response signals include overlapping wide band response signals, wherein said overlapping wide band response signals are distinguishable when each of said pulses of each of said wide band response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals;

(e) said reader detects and distinguishes the overlapping pulse sequences included in said overlapping wide band response signals, and decides whether one or more tags answered said wide band broadcast interrogation signal,

(f) said reader analyzes the at least one detected wide band response signal, in order to determine its round trip delay,

(g) said reader compares a predefined threshold to said round trip delay of said at least one detected wide band response signal, and performs a task with at least a portion of said tags having round trip delay that passes said predefined threshold test.

84. In an RFID system including at least two readers and a plurality of RF tags, a method for locating the position of at least one of the RF tags in relation to the respective positions of the at least two readers, the method comprising the steps of:

(a) transmitting respectively wide band broadcast interrogation signals, by said at least two readers,

(b) the tags waiting to receive said wide band broadcast interrogation signals,

(c) in response to said wide band broadcast interrogation signals transmitting respectively wide band response signals, by said at least one tag, in response to each of said received wide band broadcast interrogation signals, wherein each of said wide band response signals includes respective identification information about the interrogating reader and about the at least one RF tag, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses;

(d) receiving respectively, at least one of said wide band response signals at each of said at least two readers, wherein said at least one wide band response signals respectively received by said readers includes overlapping wide band response signals from the RF tags, wherein said overlapping wide band response signals are distinguishable when each of said pulses of each of said wide band response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals;

(e) detecting by distinguishing said at least one wide band response signal from said overlapping wide band response signals at each of said at least two readers, deciding whether one or more of the RF tags answered its wide band broadcast interrogation signal, and analyzing said at least one wide band response signal, in order to determine the round trip delay to and from the at least one RF tag and said identification information thereof,

(f) transferring, from each of said at least two readers, distance and said identification information of the at least one RF tags and of said reader to a central processing unit,

(g) by using the information transferred from said at least two readers, locating the position of the at least one RF tag in relation to the respective positions of said at least two readers, wherein said locating is performed by said central processing unit.

85. The method of claim 84, wherein the step of transmitting said wide band broadcast interrogation signal is by each of said at least two readers independently.

86. The method of claim 84, wherein each of said at least two readers deciding whether a collision between a plurality of said wide band response signals occurred, and if said collision occurred, said reader is transmitting at least one additional wide band broadcast interrogation signal, whereby said at least one additional wide band broadcast interrogation signal is initiating a new communicating session between said reader and said plurality of RF tags in order to reduce the collision probability between said plurality of wide band response signals transmitted by said tags.

87. The method of claim 84, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag is scanning in the time domain, and whenever said at least one tag is not transmitting or receiving signals, it is deactivated into a sleep mode where it is activated periodically with a predefined duty cycle.

88. The method of claim 84, wherein the step of said at least one tag is waiting to receive said wide band broadcast interrogation signal includes said at least one tag is scanning in the frequency domain by sweeping a center frequency in predefined steps such as to cover a desired frequency range until said wide band broadcast interrogation signal is detected.

89. The method of claim 84, wherein said at least one tag may transmit said wide band response signal according to a predefined logic.

90. The method of claim 84, wherein a time delay is added to said wide band response signal, whereby said time delay is avoiding deterministic collisions between a plurality of wide band response signals transmitted by said tags.

91. The method of claim 90, wherein said time delay is transmitted to said reader using a method selected from the group consisting of:

said time delay is included in a data carried within said wide band response signal, and said time delay is transmitted on a separate signal and said time delay is in large steps relative to the possible round trip delay values whereby said time delay can be removed by said at least two readers from the measured round trip delay value, without knowing the value of said time delay.

92. The method of claim 84, wherein said at least one wide band response signal comprising plurality of said wide band response signals, featuring different time delay values, whereby said different time delay are reducing the collisions probability between a plurality of wide band response signals transmitted by said tags.

93. The method of claim 84, wherein said at least one wide band response signal is transmitted on the same channel of said wide band broadcast interrogation signal.

94. The method of claim 84, wherein said at least one wide band response signal is transmitted on a channel selected pseudo-randomly from a predetermined pool of channels.

95. The method of claim 84, wherein said at least one tag having means for determining the identification of the transmitting reader.

96. The method of claim 95, wherein said means for determining said identification of said transmitting reader is selected from the group consisting of:

said wide band broadcast interrogation signal comprises identification of said transmitting reader, and unique predefined physical properties of said wide band broadcast interrogation signal identifying said transmitting reader.

97. The method of claim 95, wherein said tag is programmed to respond only to predefined selected readers.

98. The method of claim 95, wherein said tag stop responding to additional wide band broadcast interrogation signals from a specific reader after said tag responded to wide band broadcast interrogation signals from said specific reader a predetermined number of times in a predetermined duration.

99. The method of claim 98, wherein said tag resumes responding to said wide band broadcast interrogation signals from said specific reader after a fulfillment of a condition selected from the group consisting of:

said predetermined duration past since last respond, and the receive power of said wide band broadcast interrogation signals have been changed by more than a predefined value relative to last received power when said tag responded.

100. The method of claim 86, wherein said decision whether a collision between a plurality of said wide band response signals occurred is positive whenever more than a predetermined number of responses were received by said at least two readers.

101. The method of claim 100, wherein the value of said predetermined number of responses is set by a statistical criterion in accordance with the collision probability between a plurality of wide band response signals transmitted by a plurality of said tags.

102. The method of claim 84, wherein said wide band broadcast interrogation signal is generated by using a series of pulses.

103. The method of claim 84, wherein said wide band broadcast interrogation signal is transmitted in a specific direction, by said at least two readers utilizing a directional antenna.

104. The method of claim 84, wherein said wide band response signal is generated by using a series of pulses.

105. The method of claim 104, wherein the timing of said pulses is determined by timing means.

106. The method of claim 104, wherein said wide band response signal is modulated and said tag is transmitting a predefined number of non-modulated pulses before transmitting the modulated wide band response signal, whereby said predefined number of non-modulated pulses are informing said at least two readers of the forthcoming wide band response signal and help in synchronizing a PLL of said at least two readers.

107. The method of claim 84, wherein said wide band response signal includes a tag identification field and the step of analyzing said wide band response signals transmitted by said tags further including extracting said tag identification field.

108. The method of claim 84, wherein the value of the power of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, is adjusted by a user.

109. The method of claim 84, wherein the embodiment of the transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, comprising a method selected from the group consisting of;

pulse signals including pulses separated by no-energy periods, and base-band pulses, and pass-band pulses, and direct sequence spread spectrum signals, and types of wideband signals achieving a short time resolution.

110. The method of claim 109, wherein said transmissions selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, feature transmission data rate of about 100 Kbit/sec, with a bandwidth of about 50 MHz, centered at about 2.4 GHz.

111. The method of claim 84, wherein the transmitted wide band signals selected from the group consisting of:

said wide band broadcast interrogation signal, and said wide band response signal, comprising a sequence of pulses.

112. The method of claim 84, wherein said round trip delay is determined by subtracting a predetermined correction factor from the measured time between the transmission of said wide band broadcast interrogation signal and receiving said wide band response signal, whereby said predetermined correction factor compensates for a predefined time delay of said at least one tag between receiving said wide band broadcast interrogation signal and transmitting said wide band response signal and predefined time delay of said at least two readers operation.

113. The method of claim 92, wherein said predetermined correction factor having different values for different tags.

114. The method of claim 84, wherein said method comprising means for dealing with a Multipath effect.

115. The method of claim 114, wherein said means for dealing with said Multipath effect are selected from the group consisting of:

said at least two readers measuring the delay of the first Multipath component, and said wide band response signal comprising identification of the transmitting tag and said at least two readers reviews the received list of said at least one tag identifications to determine whether a single tag identification appears more than once, wherein said more than once appearance is generally due to said Multipath effect and therefore the surplus received signals having longer delay, are removed from the list of responding tags.

116. The method of claim 84, further comprising each of said at least two readers analyzing each collection of pulses belonging to a single said tag for determining that it includes at least a predetermined minimum number of received pulses, whereby said predetermined minimum number of received pulses allows reconstructing the data content of said wide band response signal using error correction methods.

117. The method of claim 84, wherein said at least one additional wide band broadcast interrogation signal is selected from the group consisting of:

wide band broadcast interrogation signal transmitted without a preamble portion, and wide band broadcast interrogation signal including a shorter data content relative to the data portion of the first wide band broadcast interrogation signal, and wide band broadcast interrogation including a short repeat message, and wide band broadcast interrogation signal including only a response portion.

118. The method of claim 84, further comprising said at least two readers comparing a predefined threshold to said round trip delay of at least one of said at least one wide band response signal received by said at least two readers, and performing a task with tags having round trip delay that is passing said predefined threshold test.

119. The method of claim 118, wherein different tags are compared to different predefined thresholds according to their tag identification.

120. The method of claim 118, wherein different tags located in different ranges from said at least two readers receive different handling.

121. The method of claim 118, wherein the determination of said predefined threshold is selected from the group consisting of:

said at least two readers have a user interface allowing a user to determine said predefined threshold, and said at least two readers are associated with at least one host computer through which the configuration is performed, and said predefined threshold is determined according to a predefined logic initialized by external parameters, and said predefined threshold is determined according to a predefined logic initialized by a sensor measurement.

122. The method of claim 118, wherein different tags are compared to different predefined thresholds.

123. The method of claim 84, wherein said central processing unit each of said at least two readers transferring distance and identity information of said tags and said reader itself to a central processing unit.

124. An RFID system including at least two readers and a plurality of RF tags, the system for locating the position of at least one of the RF tags in relation to the relative positions of the at least two readers, the system comprising:

(a) each of said at least two readers transmits a wide band broadcast interrogation signal,

(b) the RF tags waits to receive said wide band broadcast interrogation signals,

(c) the RF tags transmit a wide band response signal to each of said wide band broadcast interrogation signals received, wherein each of said wide band response signals includes identification information about the interrogating reader and about said transmitting RF tag, wherein said wide band response signals each include a sequence of pulses, wherein the duration of said pulses is much less than the time interval between said pulses;

(d) each of said at least two readers receives at least one of said wide band response signals, wherein said at least one wide band response signal received by said readers include overlapping wide band response signals, wherein said overlapping wide band response signal are distinguishable when each of said pulses of each of said wide band response signals is detectable during the time intervals between the pulses of all other said overlapping wide band response signals;

(e) each of said at least two readers detects said at least one wide band response signal, decides whether the at least one of the RF tags answered its wide band broadcast interrogation signal, and analyzes said at least one wide band response signal, transmitted by said at least one RF tag, in order to determine the round trip delay to and from the at least one RF tag and said identification information thereof,

(f) each one of said at least two readers transfers distance and identity information of said at least one RF tag and of said reader itself to a central processing unit,

(g) by using the information transferred from said at least two readers, said central processing unit locates the position of said at least one tag in relation to the relative positions of said at least two readers.