Long Range Radio Frequency Identification System

• US 20110032081A1
• Filed: 07/06/2010
• Published: 02/10/2011
• Est. Priority Date: 08/07/2009
• Status: Active Grant

• Current Assignee: Ring LLC (Amazon.com, Inc.)
• Original Assignee: Ben Wild
• Inventors: Wild, Ben
• Granted Patent: US 8,699,636 B2
• Time in Patent Office: Days
• US Class Current: 340/10.42
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• 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: H04Q 2213/13095 PIN / Access code, authentication

Abstract

A method and apparatus for building a long range RFID system is disclosed. A new signaling structure called Block Pseudo Noise is described that allows for more computationally efficient decoding. A novel approach to synchronize the RFID reader local oscillator with the RFID tag oscillator using an on board GPS receiver on the RFID tags and RFID reader is also disclosed. A novel positioning technique called Asynchronous Time Difference of Arrival used to located RFID tags is also disclosed.

Claims

1. A system for long-range RFID communication, the system comprising one or more RFID readers and one or more RFID tags, wherein at least some of the readers and at least some of the tags each have an oscillator frequency synchronizing circuit.

2. The system of claim 1 wherein the system is capable of operating at a range of over about 2000 m.

3. The system of claim 1 where the RFID readers and the RFID tags communicate, after oscillator frequency synchronization, using block PN sequences.

4. The system of claim 1 wherein the oscillator frequency synchronizing circuit further comprise:

(a) a counter for measuring the period of an external synchronization signal;

(b) a processor to compute an oscillator frequency adjustment factor; and

(c) a digital to analog converter for adjusting the frequency of an oscillator.

5. A method for frequency synchronizing an RFID tag comprising:

(a) receiving a stable periodic reference signal;

(b) measuring the period of the stable periodic reference signal; and

(c) adjusting the oscillator output frequency of the RFID tag, based on the period of the stable periodic reference signal.

6. The method of claim 5 wherein the stable periodic reference signal is the 1 part per second (1PPS) output of a GPS receiver.

7. The method of claim 5 wherein the stable periodic reference signal is a 60 Hz AC power line signal.

8. The method of claim 5 wherein the stable periodic reference signal is generated by one of the RFID readers and wirelessly transmitted to one or more of the RFID tags.

9. The method of claim 5 wherein the RFID tag is synchronized with one or more RFID readers.

10. The method of claim 5 wherein the RFID tag is synchronized with one or more other RFID tags.

11. A method for frequency synchronizing an RFID reader comprising:

(a) receiving a stable periodic reference signal;

(b) measuring the period of the stable periodic reference signal; and

(c) adjusting the oscillator output frequency of the RFID reader, based on the period of the stable periodic reference signal.

12. The method of claim 11 wherein the RFID reader is frequency synchronized with one or more other RFID readers.

13. The method of claim 11 wherein the RFID reader is frequency synchronized with one or more RFID tags.

14. A method for generating a block pseudo noise sequence comprising:

(a) generating an inner pseudo noise sequence;

(b) generating an outer pseudo noise sequence; and

(c) adding together time shifted and scaled versions of the inner pseudo noise sequence, wherein the scaled versions are generated by multiplying the inner pseudo noise sequence with an element of the outer pseudo noise sequence.

15. The method of claim 14, wherein the time shifts of the first pseudo noise sequence are set to the length of the first pseudo noise sequence.

16. A method for decoding a block pseudo noise sequence, wherein the block pseudo noise sequence comprises an inner sequence and an outer sequence, comprising:

(a) inputting data to an inner matched filter matched to the inner sequence;

(b) outputting inner filter matched data to an outer matched filter, matched to the outer sequence; and

(c) outputting outer filter matched data.

17. The method of claim 16 wherein the outer filter is modified by summing one or more of the output taps and multiplying the output of these adders with one or more frequency correction factors to correct for any frequency offset between the RFID reader and RFID tag.

18. A method for asynchronously determining an RFID tag'"'"'s position comprising:

(a) sending, from the RFID tag, a tag message to each of a first RFID reader, second RFID reader and third RFID reader;

(b) the first RFID reader decoding the tag message and sending a reader message to the second RFID reader and the third RFID reader;

(c) the second RFID reader decoding the tag message and sending a second reader message to the first RFID reader and the third RFID reader;

(d) the third RFID reader decoding the tag message and sending a third reader message to the second RFID reader and the first RFID reader;

(e) at the first RFID reader, computing the time differences between the arrival of the tag message and the arrival of the second and the third reader messages;

(f) at the second RFID reader, computing the time differences between the arrival of the tag message and the arrival of the first and third reader messages;

(g) at the third RFID reader, computing the time differences between the arrival of the tag message and the arrival of the first and the second reader messages; and

(h) using the time differences in (e) (f) and (g) to determine the RFID tag'"'"'s location.

19. The method of claim 18, wherein the RFID tag sends a tag message to each of three or more RFID readers;

each of the RFID readers decoding the tag message and sending reader messages to each of the other RFID readers;

at each of the RFID readers, computing the time difference between the arrival of the tag messages and the arrival of the reader messages; and

using the time differences to determine the RFID tag'"'"'s location.

20. The method of claim 18 wherein each RFID reader sends its reader message after a deterministic period of time.

21. The method of claim 18 wherein the RFID tags and the RFID readers are transmitting and receiving on the Ultra Wideband frequency band between 3-10 GHz.

22. A method for reducing RFID tag to RFID reader interference, comprising:

(a) receiving, at the RFID tag, reader location;

(b) obtaining, at the RFID tag, the RFID tag'"'"'s location using an on-board GPS chip;

(c) computing, at the RFID tag, the distance to the nearest RFID reader and adjusting transmit power as a function of distance; and

(d) using, at the RFID tag, an external timing signal to transmit only on pre-assigned time and frequency slots.

23. The method of claim 22 wherein the external timing signal is obtained from the output of a GPS chip.

24. The method of claim 22 wherein the external timing signal is transmitted from the RFID reader to the RFID tag.