RFID READER SYSTEMS DETECTING PILOT TONE

• US 20070109129A1
• Filed: 01/11/2007
• Published: 05/17/2007
• Est. Priority Date: 05/24/2005
• Status: Abandoned Application

• Current Assignee: Impinj, Inc.
• Original Assignee: Impinj, Inc.
• Inventors: Cooper, Scott, Sarajedini, Amir, Esterberg, Aanand, Diorio, Christopher, Humes, Todd, Sundstrom, Kurt
• Application Number:
  US11/622,092
• Publication Number:
  US 20070109129A1
• Time in Patent Office: Days
• US Class Current: 340/572.200
• 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

RFID tags are commanded to generate a pilot tone in their backscatter. When the backscattered pilot tone is received in the reader, the pilot tone is used to estimate the tag period/frequency. Then, the estimate is used to seed and lock a symbol timing recovery loop, which provides a detected signal to one or more correlators for detecting the tag preamble. A delayed version of the received tag signal is compared against a baseline signal threshold established from the received signal to detect the pilot tone.

Claims

1. A method for a Radio Frequency Identification (RFID) reader system, comprising:

Receiving from an RFID tag a backscattered signal;

estimating a frequency of a pilot tone in the backscattered signal;

recovering a timing of symbols in the backscattered signal using the estimated frequency; and

demodulating the backscattered signal based on the symbol timing to recover data.

2. The method of claim 1, further comprising:

detecting the pilot tone.

3. The method of claim 2, in which

the signal is backscattered from the RFID tag responsive to a command transmitted by the reader, and

in which the pilot tone is detected only if the command belongs in a subset of commands.

4. The method of claim 2, in which

the pilot tone is detected by:

generating a baseline version of the backscattered signal;

generating a version of the backscattered signal that is delayed with respect to the backscattered signal; and

establishing detection when the delayed version reaches a detection relationship with the baseline version.

5. The method of claim 4, in which

the detection relationship is that the delayed version exceed the baseline version.

6. The method of claim 4, in which

the baseline version of the tag signal is generated as a preset portion of a magnitude of the tag signal.

7. The method of claim 6, in which

the preset portion is substantially equal to 50%.

8. The method of claim 6, in which

the baseline version has a waveform that rises up to a cusp, and then continues at substantially the same value, and

the delayed version is delayed enough so that detection is established after the cusp.

9. The method of claim 4, in which

detection is established only if both the delayed version and the baseline version exceed a detection threshold.

10. The method of claim 9, in which

the detection threshold is defined in terms of detected ambient noise.

11. The method of claim 9, in which

the detection threshold is updated.

12. The method of claim 9, in which

the backscattered signal is received responsive to a command signal transmitted by the RFID reader system,

the command signal causes the tag to be silent during a silent period, and

the detection threshold is defined from a power measurement made during the silent period.

13. The method of claim 1, in which

the frequency is estimated by employing a Discrete Fourier Transform (DFT) of the received pilot tone.

14. The method of claim 1, in which

the frequency is estimated by employing a Fast Fourier Transform (FFT) of the received pilot tone.

15. The method of claim 1, in which

the frequency is estimated by correlating the backscattered tag signal with an expected version of the backscattered signal.

16. The method of claim 1, further comprising:

identifying in the backscattered signal a preamble using the symbol timing, and

in which the backscattered signal is demodulated based on a timing of the preamble.

17. The method of claim 16, in which

the preamble is identified using a single correlator.

18. The method of claim 16, further comprising:

determining from the preamble timing a Time of Arrival (TOA) for a remainder of the backscattered signal; and

in which the data is recovered from the remainder.

19. The method of claim 18, further comprising:

delaying the remainder of the backscattered signal the preamble by a delay time; and

adjusting the delay time to compensate for an amount of time taken to estimate the frequency.

20. A Radio Frequency Identification (RFID) reader system component operable to receive a backscattered RFID tag signal that includes a pilot tone, the component comprising:

a frequency estimator operable to estimate a frequency of the pilot tone and to generate from it a frequency estimation signal;

a buffer/delay operable to generate a delayed version of the backscattered signal;

a symbol timing recovery loop controlled according to the frequency estimation signal, the symbol timing recovery loop operable to output, responsive to the delayed version of the backscattered signal, a TS signal that has recovered a timing of symbols in the backscattered signal; and

a decoder for decoding the TS signal to recover data.

21. The component of claim 20, further comprising:

a detector for detecting the pilot tone.

22. The component of claim 20, in which

the frequency estimator further detects the pilot tone.

23. The component of claim 20, in which

the frequency is estimated by the frequency estimator by employing a Discrete Fourier Transform (DFT) of the received pilot tone.

24. The component of claim 20, in which

the frequency is estimated by the frequency estimator by employing a Fast Fourier Transform (FFT) of the received pilot tone.

25. The component of claim 20, in which

the frequency is estimated by the frequency estimator by correlating the backscattered tag signal with an expected version of the backscattered signal.

26. The component of claim 20, in which a frequency estimate signal is generated according to a frequency of the backscattered signal, and

a frequency of the symbol timing recovery loop is locked responsive to the frequency estimate signal.

27. The component of claim 20, in which

a symbol phase estimate signal is generated according to a symbol phase of the backscattered signal, and

a phase of the symbol timing recovery loop is controlled according to the symbol phase estimate signal.

28. The component of claim 20, comprising:

at least one correlator operable to correlate the TS signal with an expected preamble signal for generating a correlation signal, and

in which the TS signal is demodulated only if the correlation signal exceeds a threshold.

29. The component of claim 28, in which

only one correlator is used by the reader for correlating the TS signal.

30. The component of claim 20, in which

the buffer/delay is implemented by a variable delay.

31. The component of claim 20, in which

the buffer/delay is implemented by a variable rate buffer.

32. The component of claim 20, in which

the delayed version of the tag signal is controlled to impose a nonzero delay time suitable for compensating for a delay from the frequency estimator.

33. The component of claim 20, in which

the symbol timing recovery loop comprises:

a digital rate converter circuit arranged to;

obtain sample values of a tag response signal waveform at sample time points;

reconstruct signal values at target time points from the sample values using the frequency estimate signal; and

output the reconstructed signal values as the TS signal;

a timing error detector coupled to the digital rate converter;

a loop filter coupled to the phase detector; and

a numerically controlled oscillator (NCO) coupled to the loop filter and to the digital rate converter such that the timing error detector, the loop filter, and the NCO form a feedback loop.

34. The component of claim 33, in which

the symbol timing recovery loop further comprises:

a matched filter coupled to an input of the digital rate converter.

35. The component of claim 33, in which

the symbol timing recovery loop further comprises:

a matched filter coupled to between the digital rate converter and the timing error detector.

36. The component of claim 33, in which

the digital rate converter includes:

an interpolating filter arranged to receive the sample time points, interpolate, and generate offset sample points;

a decimator coupled to the interpolating filter arranged to decimate a portion of the sample points such that offset sample points corresponding to the target time points are provided by the digital rate converter; and

a timing processor arranged to provide the interpolating filter and the decimator with a first and second control signals based on an output of the NCO.

37. A Radio Frequency Identification (RFID) reader system component operable to receive a backscattered RFID tag signal that includes a pilot tone, the component comprising:

means for receiving from an RFID tag a backscattered signal;

means for detecting a pilot tone from the backscattered signal;

means for estimating a frequency of the pilot tone in the backscattered signal;

means for recovering a timing of symbols in the backscattered signal using the estimated frequency; and

means for demodulating the backscattered signal based on the symbol timing to recover data.

38. The component of claim 37, in which

the means for detecting the pilot tone comprises:

means for generating a baseline version of the backscattered signal;

means for generating a version of the backscattered signal that is delayed with respect to the backscattered signal; and

means for establishing detection when the delayed version reaches a detection relationship with the baseline version.

39. The component of claim 37, further comprising:

means for identifying in the backscattered signal a preamble using the symbol timing, in which the backscattered signal is demodulated based on a timing of the preamble.

40. The component of claim 37, further comprising:

means for determining from the preamble timing a Time Of Arrival (TOA) for a remainder of the backscattered signal, in which the data is recovered from the remainder.

41. The component of claim 40, further comprising:

means for determining from the preamble timing a Time Of Arrival (TOA) for a remainder of the backscattered signal, in which the data is recovered from the remainder.

42. The component of claim 41, further comprising:

means for delaying the remainder of the backscattered signal the preamble by a delay time; and

means for adjusting the delay time to compensate for an amount of time taken to estimate the frequency.