METHOD FOR SIMULTANEOUS DETECTION OF A PLURALITY OF RFID TAGS USING MULTIUSER DETECTION

• US 20100039233A1
• Filed: 08/12/2009
• Published: 02/18/2010
• Est. Priority Date: 08/12/2008
• Status: Active Grant

• Current Assignee: Signal Decode. Inc.
• Original Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Inventors: Niedzwiecki, Joshua D., Basu, Prabahan
• Granted Patent: US 9,769,547 B2
• Time in Patent Office: Days
• US Class Current: 340/10.100
• CPC Class Codes: H04Q 2209/47 using RFID associated with sensors
• CPC Class Codes: H04Q 2209/753 where the polling of the sub-stations is synchronous
• CPC Class Codes: H04Q 9/00 Arrangements in telecontrol or telemetry systems for selectively calling a substation from a main station, in which substation desired apparatus is selected for applying a control signal thereto or for obtaining measured values therefrom

Abstract

A method and apparatus are disclosed that apply multiuser detection (MUD) analysis to an aggregated RF response from a plurality of simultaneously queried RFID tags, so as to distinguish the individual tag responses. The claimed method thereby significantly reduces RFID detection latency when multiple tags are simultaneously queried. Some embodiments transmit carrier waves at more than one frequency, such as a plurality of equally-spaced frequencies, so as to enhance the MUD analysis by incorporating a multi-frequency dimension. Other embodiments incorporate additional spatial dimensions by deploying multiple RF detection antennae at separated locations. The number of colliding tag responses must be estimated before MUD analysis can be applied. In some embodiments other signal parameters must be estimated, such as signal bias and an impulse function for each responding tag that characterizes alterations of the RF signal while in transit due to propagation distance, passage through intervening objects, and reflections.

Claims

1. A method for distinguishing responses received by an RFID detector from a plurality of simultaneously queried RFID tags, the method comprising:

querying the plurality of RFID tags;

receiving an aggregated RF response from the plurality of RFID tags;

characterizing the aggregated RF response according to a plurality of characteristics, the plurality of characteristics including at least RF amplitude and RF phase;

according to the characterization of the aggregated RF response, estimating at least one parameter, the at least one parameter including a number of RFID tag responses included in the aggregated RF response; and

applying multiuser detection to the aggregated RF response according to the at least one estimated parameter so as to distinguish each of the RFID tag responses included in the aggregated RF response.

2. The method of claim 1, further comprising repeating the steps of querying, receiving, characterizing, estimating, and applying until the responses of all of the plurality of simultaneously queried RFID tags have been distinguished.

3. The method of claim 1, wherein the at least one parameter includes a signal bias.

4. The method of claim 3, wherein the signal bias is estimated by averaging a bias of the aggregated RF response over a plurality of symbol intervals.

5. The method of claim 3, wherein the estimated signal bias is subtracted from the aggregated RF response preceding further analysis of the aggregated RF response.

6. The method of claim 1, wherein estimating the number of RFID tag responses included in the aggregated RF response includes applying a statistical analysis method so as to detect clustering of the aggregated RF response, as characterized according to the plurality of characteristics.

7. The method of claim 6, wherein the statistical analysis method comprises one of:

a K-Means algorithm;

a Maximum likelihood solution; and

a T test.

8. The method of claim 1, wherein:

the aggregated RF response includes a plurality of unique identifying sequences transmitted simultaneously by each of the responding RFID tags, the identifying sequences being statically independent of each other, and

estimating the number of RFID tag responses included in the aggregated RF response includes counting a number of unique eigenvalues in a covariance matrix derived from a portion of the aggregate RF signal that includes the simultaneously transmitted identifying sequences.

9. The method of claim 1, wherein for each of the RFID tags that contributed to the aggregated RF response, the at least one parameter includes a corresponding channel impulse response function that characterizes an alteration of an RF response from the corresponding RFID tag during transit of the RF response from the RFID tag to the RFID detector.

10. The method of claim 1, wherein:

receiving the aggregated RF response from the plurality of RFID tags includes simultaneously transmitting to the plurality of RFID tags a plurality of RF carrier waves at a plurality of RF frequencies; and

characterizing the aggregated RF response according to a plurality of characteristics includes characterizing the aggregated RF response according to frequency.

11. The method of claim 10, wherein the plurality of RF carrier waves is a plurality of equal amplitude RF carrier waves, and the plurality of RF frequencies is a plurality of equally spaced RF frequencies.

12. The method of claim 1, wherein:

receiving the aggregated RF response from the plurality of RFID tags includes obtaining a plurality of spatially separated responses received using a plurality of spatially separated RF receiving antennae; and

characterizing the aggregated RF response according to a plurality of characteristics includes characterizing the aggregated RF response according to the plurality of spatially separated responses.

13. An apparatus for distinguishing responses received from a plurality of simultaneously queried RFID tags, the apparatus comprising:

an RF transmitter configured to transmit a querying RF signal to the plurality of RFID tags;

an RF receiver configured to receive an aggregated RF response from the plurality of RFID tags;

an RF characterizer configured to characterize the aggregated RF response according to a plurality of characteristics, the plurality of characteristics including at least RF amplitude and RF phase; and

a computer controlled by software configured to direct the computer to estimate at least one parameter pertaining to the characterized aggregated RF response, the at least one parameter including a number of RFID tag responses included in the aggregated RF response, the software being further configured to direct the computer to apply multiuser detection to the aggregated RF response according to the at least one estimated parameter, so as to distinguish each of the RFID tag responses included in the aggregated RF response.

14. The apparatus of claim 13, wherein the at least one parameter includes a signal bias.

15. The apparatus of claim 14, wherein the signal bias is estimated by averaging a bias of the aggregated RF response over a plurality of symbol intervals.

16. The apparatus of claim 14, wherein the estimated signal bias is subtracted from the aggregated RF response preceding further analysis of the aggregated RF response.

17. The apparatus of claim 13, wherein estimating the number of RFID tag responses included in the aggregated RF response includes applying a statistical analysis method so as to detect clustering of the aggregated RF, as characterized according to the plurality of characteristics.

18. The apparatus of claim 17, wherein the statistical analysis method comprises one of:

a K-Means algorithm;

a Maximum likelihood solution; and

a T test.

19. The apparatus of claim 13, wherein:

the aggregated RF response includes a plurality of unique identifying sequences transmitted simultaneously by each of the responding RFID tags, the identifying sequences being statically independent of each other; and

estimating the number of RFID tag responses included in the aggregated RF response includes counting a number of unique eigenvalues in a covariance matrix derived from a portion of the aggregate RF signal that includes the simultaneously transmitted identifying sequences.

20. The apparatus of claim 13, wherein for each of the RFID tags that contributed to the aggregated RF response, the at least one parameter includes a corresponding channel impulse response function that characterizes an alteration of an RF response from the corresponding RFID tag during transit of the RF response from the RFID tag to the RFID detector.

21. The apparatus of claim 13, wherein:

the RF transmitter is able to simultaneously transmit to the plurality of RFID tags a plurality of RF carrier waves at a plurality of RF frequencies; and

the software is able to direct the computer to characterize the aggregated RF response according to the plurality of RF frequencies.

22. The apparatus of claim 13, further comprising a plurality of spatially separated RF receiving antennae that are able to receive the aggregated RF response as a plurality of spatially separated responses, the software being able to direct the computer to characterize the aggregated RF response according to the plurality of spatially separated responses.