Identifying RFID tag moving coherently with reader

• US 7,619,524 B2
• Filed: 07/20/2007
• Issued: 11/17/2009
• Est. Priority Date: 07/20/2007
• Status: Active Grant

• Current Assignee: Symbol Technologies, LLC (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Calvarese, Russell
• Primary Examiner(s): TWEEL JR, JOHN ALEXANDER
• Application Number:
  US11/781,026
• Publication Number:
  US 20090021376A1
• Time in Patent Office: 851 Days
• Field of Search: 340/572.1, 340/572.2, 340/572.7, 340/10.1, 340/539.23
• US Class Current: 340/572.1
• CPC Class Codes: G06Q 10/00 Administration; Management

Abstract

A system and method to distinguish an RFID tag which is moving synchronously with an RFID reader from other RFID tags which are not moving synchronously with the reader. In one embodiment, the reader emits read signals at a fixed frequency with a corresponding period. A determination is made by the RFID reader as to the signal strengths received from an RFID tag at time intervals which are integer multiples of the half-period of the signal. Signal strengths from the tag which are the same or substantially the same at these prescribed measurement intervals are taken as an indication that the tag is moving synchronously with the RFID reader. In an alternative embodiment, the frequency of the read signals may vary over time. Correlations between received tag signal strengths with particular frequencies are an indication that the RFID tag is moving synchronously with the RFID reader.

Claims

1. A method for identifying an RFID tag moving with an RFID reader, comprising:

(a) transmitting by the reader over a period of time a plurality of reading signals, wherein said signals have a signal period;

(b) receiving by the reader reply signals from any RFID tags within range of the reader;

(c) determining for each reply;

a particular RFID tag from which the reply was received; and

a received signal strength from the RFID tag;

(d) analyzing the received signal strengths and noting any consistencies of signal strengths correlated with the period of the signal period; and

(e) determining from the consistencies whether the particular RFID tag is moving coherently with the RFID reader.

2. A method according to claim 1, further comprising determining whether the particular RFID tag consistently does not reply on a period basis which has the same period as the signal period.

3. The method of claim 1, wherein step (d) comprises:

identifying a first read of the particular tag at a first time and a second read of the particular tag at a second time, wherein a time interval between the first read and the second read is an integer multiple of the signal period; and

determining a degree to which the signal strength is substantially the same for the first read and the second read.

4. The method of claim 3, further comprising repeating the identifying step and the determining step for a plurality of pairs of first reads at first times and respective second reads at respective second times.

5. The method of claim 4, further comprising determining an aggregate level of consistency of signal strengths over a plurality of reads.

6. The method of claim 5, further comprising that if over the plurality of reads the degree of consistency exceeds a designated consistency threshold, then the RFID tag is moving coherently with the RFID reader.

7. The method of claim 1, further comprising:

(f) determining over the period of time a distribution of reading signals for which the particular RFID tag is RF visible to the reader;

(f) determining based on the distribution a probability that the particular RFID tag was in range of the reader from the beginning of the period of time to the end of the period of time; and

(g) determining whether the particular RFID tag is moving coherently with the RFID reader based on both the consistencies and the probability.

8. The method of claim 1, further comprising:

(f) determining a degree of correlation between received signal strengths and a plurality of respective frequencies of the read signals; and

(g) determining whether the particular RFID tag is moving coherently with the RFID reader based on both the consistencies and the correlation.

9. A method for identifying an RFID tag moving with an RFID reader, comprising:

(a) transmitting by the reader over a period of time a plurality of reading signals at a plurality of frequencies within a transmission band;

(b) receiving by the reader reply signals from any RFID tags within range of the reader;

(c) determining for each reply;

a particular RFID tag from which the reply was received;

a received signal strength from the RFID tag; and

a frequency associated therewith;

(d) analyzing the received signal strengths and noting any correlations of signal strengths with particular RFID tags and particular frequencies; and

(e) determining from the correlations whether the particular RFID tag is moving coherently with the RFID reader.

10. A method according to claim 9, further comprising determining whether the particular RFID tag consistently does not reply at a particular frequency.

11. The method of claim 9, wherein step (d) comprises determining for a first read of the particular tag at a particular frequency and a second read of the particular tag at the particular frequency the degree to which the signal strength is the same or substantially the same for the first read and the second read.

12. The method of claim 9, wherein noting any correlations of signal strengths with particular RFID tags and particular frequencies comprises at least one of:

determining a degree to which the particular RFID tag consistently does not reply at a particular frequency; and

determining a degree to which the particular RFID tag consistently returns a substantially consistent signal strength at a particular frequency;

wherein is determined a degree of correlation between the particular RFID tag and at least a frequency.

13. The method of claim 12, further comprising determining that if over the plurality of reading signals the correlation between the particular RFID tag and the at least a frequency exceeds a designated correlation threshold, then the RFID tag is moving coherently with the RFID reader.

14. The method of claim 13, further comprising determining that if over a plurality of frequencies the degree of correlation exceeds the designated correlation threshold, then the RFID tag is moving coherently with the RFID reader.

15. A method for identifying an RFID tag moving with an RFID reader, comprising:

(a) transmitting by the reader over a period of time a plurality of reading signals at a plurality of frequencies within a transmission band;

(b) receiving by the reader reply signals from any RFID tags within range of the reader;

(c) determining over the plurality of reading signals a distribution of reading signals for which a specific RFID tag is within range of the reader; and

(d) determining based on the distribution whether the particular RFID tag is moving coherently with the RFID reader.

16. The method of claim 15, further comprising:

determining a percentage of the period of time when the particular RFID tag is in range of the reader; and

determining based on the percentage and the distribution whether the particular RFID tag is moving coherently with the RFID reader.

17. The method of claim 15, further comprising:

determining a degree to which the distribution is a substantially uniform distribution; and

determining based on the degree to which the distribution is a substantially uniform distribution whether the particular RFID tag is moving coherently with the RFID reader.

18. The method of claim 15, further comprising:

determining a particular frequency of the plurality of frequencies at which the particular RFID tag consistently does not reply at the particular frequency; and

decreasing a weighting of reading signals at said particular frequency when determining the distribution.

19. A system for identifying an RFID tag moving with an RFID reader, comprising:

the RFID reader, wherein said RFID reader is configured to transmit over a period of time a plurality of reading signals;

an RF receiver associated with said RFID reader, wherein said RFID receiver is configured to receive reply signals from any RFID tags within range of the reader;

a processor associated with said RFID reader, wherein said processor is configured to determine at least one of;

any consistencies of a received signal strength from a particular RFID tag correlated with a period of the plurality of reading signals;

a correlation between a signal strength received from the particular RFID tag and a particular frequency of the plurality of reading signals; and

a distribution of reading signals for which the particular RFID tag is within range of the reader.

20. The system of claim 19, further comprising a signal strength measuring component associated with said RFID reader, said signal strength measuring component configured to determine the signal strength received from the particular RFID tag.

21. The system of claim 20, wherein said processor is further configured to determine whether the particular RFID tag is moving coherently with the RFID reader based on at least one of:

the consistencies of the received signal strength from the particular RFID tag correlated with a period of the plurality of reading signals;

the correlation between the signal strength received from the particular RFID tag and the particular frequency of the plurality of reading signals; and

the distribution of reading signals for which the particular RFID tag is within range of the reader.

22. The system of claim 21, wherein said processor is further configured to determine whether the particular RFID tag is moving coherently with the RFID reader based on at least one of a plurality of consistencies and a plurality of correlations.