Interference control method for RFID systems

• US 6,566,997 B1
• Filed: 12/03/1999
• Issued: 05/20/2003
• Est. Priority Date: 12/03/1999
• Status: Expired due to Term

• Current Assignee: Assa Abloy AB
• Original Assignee: HID Corporation (Assa Abloy AB)
• Inventors: Bradin, John P.
• Primary Examiner(s): Lieu, Julie
• Application Number:
  US09/454,127
• Time in Patent Office: 1,264 Days
• Field of Search: 340/10.2, 340/10.1, 340/572.1, 340/3.41, 370/445
• US Class Current: 340/10.2
• 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

Abstract

A reader unit for use in a radio frequency identification (RFID) system delays the initiation of an interrogation operation when potentially interfering radio frequency (RF) energy is detected within an operational frequency range of the system. The reader unit waits a quasi-random period of time after the detection and then senses the spectral environment again to determine whether the energy is still present. If the energy is still present, the reader unit waits another quasi-random period and the process repeats. If there is no energy present or the energy is below a threshold value, the reader unit immediately initiates the interrogation operation. By delaying the initiation of the interrogation operation until the operative frequency range is free of potential interferers, the likelihood of harmful interference effects is significantly reduced.

Claims

1. A reader unit for use in a radio frequency identification (RFID) system having a plurality of radio frequency (RF) tags, said reader unit comprising:

a transmitter for periodically generating an interrogation signal for transmission to the plurality of RF tags during an interrogation operation, said interrogation signal for causing at least one of said RF tags to transmit an identification signal back to said reader unit;

a radio frequency (RF) sensor for sensing, during an inactive state of said transmitter, the presence of RF energy within an operational frequency range of the RFID system in an environment surrounding said reader unit; and

a delay unit for delaying initiation of an interrogation operation when said RF sensor indicates the presence of RF energy satisfying a predetermined condition within said operational frequency range of the RFID system.

2. The reader unit, as claimed in claim 1, wherein:

said delay unit delays initiation of said interrogation operation for a quasi-random delay period.

3. The reader unit, as claimed in claim 2, wherein:

said quasi-random delay period is selected to allow another RFID system to complete a corresponding interrogation operation before expiration of said quasi-random delay period.

4. The reader unit, as claimed in claim 2, wherein:

said delay unit checks said RF sensor after said quasi-random delay period to determine whether RF energy satisfying said predetermined condition is still present and, if so, delays initiation of said interrogation operation for another quasi-random delay period.

5. The reader unit, as claimed in claim 1, wherein:

said interrogation signal has a first carrier frequency; and

said operational frequency range of said RFID system includes said first carrier frequency.

6. The reader unit, as claimed in claim 1, wherein:

said identification signal transmitted back to said reader unit has a second carrier frequency; and

said operational frequency range of said RFID system includes said second carrier frequency.

7. The reader unit, as claimed in claim 1, further comprising:

a receiver for receiving said identification signal transmitted back to said reader unit from said at least one RF tag, wherein said operational frequency range of said RFID system includes an operational bandwidth of said receiver.

8. The reader unit, as claimed in claim 1, wherein:

said predetermined condition includes said RF energy sensed by said RF sensor exceeding a predetermined threshold value.

9. The reader unit, as claimed in claim 8, further comprising:

a threshold generator for dynamically varying said predetermined threshold value based on changing system conditions.

10. A method for managing interrogation activities in a radio frequency identification (RFID) system having an RF reader unit and a plurality of RF tag devices, said method comprising the steps of:

determining that an interrogation operation needs to be performed in the RFID system, said interrogation operation requiring the transmission of an interrogation signal from the RF reader unit to the plurality of RF tag devices;

checking a spectral environment about the reader unit to determine whether an RF signal meeting a predetermined criterion exists within an operative frequency range of the RFID system;

when an RF signal meeting the predetermined criterion does not exist within the operative frequency range of the RFID system, immediately transmitting said interrogation signal;

when an RF signal meeting the predetermined criterion does exist within the operative frequency range of the RFID system, delaying transmission of the interrogation signal for a quasi-random delay period; and

when transmission of the interrogation signal has been delayed a predetermined number of times, transmitting said interrogation signal regardless of whether an RF signal meeting the predetermined criterion does exist within the operating frequency range of the RFID system.

11. The method, as claimed in claim 10, further comprising:

second checking the spectral environment about the reader unit to determine whether an RF signal meeting the predetermined criterion exists within the operative frequency range of the RFID system after the quasi-random delay period has expired; and

immediately transmitting said interrogation signal when said step of second checking indicates that an RF signal meeting the predetermined criterion does not exist within the operative frequency range of the RFID system.

12. The method, as claimed in claim 10, wherein:

said predetermined criterion includes a peak value of said RF signal exceeding a predetermined threshold value.

13. A method for use within a radio frequency identification (RFID) system having an RF reader unit and a plurality of RF tag devices, said method comprising the steps of:

determining that an interrogation operation needs to be performed in the RFID system, said interrogation operation requiring the transmission of an interrogation signal from the RF reader unit to the plurality of RF tag devices;

measuring a magnitude of radio frequency (RF) energy in a region surrounding the RF reader unit;

comparing said magnitude of said RF energy to a predetermined threshold value; and

transmitting said interrogation signal from said RF reader unit to said plurality of RF tag devices when said magnitude of said RF energy is less than said predetermined threshold value.

14. The method claimed in claim 13, further comprising:

delaying transmission of said interrogation signal from said RF reader unit when said magnitude of said RF energy exceeds said predetermined threshold level.

15. The method claimed in claim 13, wherein:

said step of measuring a magnitude of RF energy includes detecting RF energy in a receiver and determining a peak value of said detected RF energy in a peak detector.

16. The method claimed in claim 13, further comprising:

repeating said steps of measuring and comparing until said magnitude of said RF energy is less than said predetermined threshold value or a predetermined number of repetitions have been performed.

17. The method claimed in claim 16, further comprising:

transmitting said interrogation signal from said RF reader unit to said plurality of RF tag devices regardless of whether said magnitude of said RF energy is less than said predetermined threshold value when it is determined that a continuously hostile spectral environment exists about the RF reader unit.

18. The method claimed in claim 16, wherein:

said RF reader unit determines that a continuously hostile spectral environment exists about the RF reader unit when said repeating step ends before said magnitude of said RF energy drops below said predetermined threshold value.

19. The method claimed in claim 18, further comprising:

reporting said continuously hostile spectral environment to a central location when said RF reader unit determines that a continuously hostile spectral environment exists about the RF reader unit.

20. The method claimed in claim 13, further comprising:

collecting information regarding a spectral environment about said reader unit using said magnitude measured in said measuring step; and

reporting said information to a central location.