Distance estimation between an RFID tag and an RFID reader

1. A system for estimating a distance, comprising:

a Radio Frequency Identifier (RFID) reader;

an RFID tag;

measurement equipment for measuring a plurality of phase differences at different frequencies between transmitted Radio Frequency (RF) signals from the RFID reader and corresponding received RF signals at the RFID tag; and

a processor configured to

calculate normalized phases from the plurality of phase differences;

calculate corrected phases by resolving one or more ambiguities from the normalized phases;

obtain a characteristic curve using the corrected phases; and

provide an estimate of the distance based on the characteristic curve and the corrected phases.

2. The system of claim 1 , wherein at least one of the plurality of phase differences is equivalent to a difference between a phase of an unmodulated backscattered signal and a phase of an unmodulated transmitted signal.

3. The system of claim 1 , wherein the RFID tag is a tag selected from the group consisting of a battery operated tag, an active tag that uses backscattering modulation, and a passive tag.

4. The system of claim 1 , wherein a given one of the corrected phases is calculated by adding an integer multiple of an ambiguity factor to a given one of the normalized phases to resolve the ambiguity of the given one of the normalized phases.

5. The system of claim 1 , wherein at least one of the plurality of phase differences for a given one of the different frequencies is normalized as a function of the given one of the different frequencies.

6. The system of claim 5 , wherein a difference between two of the corrected phases for a given one of the different frequencies is less than the ambiguity factor.

7. The system of claim 5 , wherein the corrected phases are calculated to follow the characteristic curve.

8. The system of claim 7 , wherein a mean squared difference between the corrected phases and the characteristic curve is minimized.

9. The system of claim 1 , wherein the transmitted RF signals are transmitted using a plurality of wavelengths, and wherein the characteristic curve, relating to the normalized phases versus the plurality of wavelengths, is an affine function.

10. The system of claim 1 , wherein the transmitted RF signals are transmitted using a plurality of wavelengths, and wherein the characteristic curve, relating to the normalized phases versus the plurality of wavelengths, is a hyperbolic function.

11. The system of claim 1 , wherein the one or more ambiguities are resolved by performing an intra-frequency ambiguity resolution process and an inter-frequency ambiguity resolution process.

12. The system of claim 1 , wherein the estimate of the distance is used to identify the approximate position of an object to which the RFID tag is attached.

13. The system of claim 12 , wherein the approximate position is within a checkout bin at a checkout station.

14. The system of claim 12 , wherein a plurality of estimates of the distance for the RFID tag from multiple RFID readers, including the RFID reader, is used to identify the placement of an object.

15. The system of claim 12 , wherein the approximate position is used to identify the proximity of two RFID tags that include the RFID tag.

16. The system of claim 12 , wherein the position of the RFID tag is tracked.

17. The system of claim 1 , wherein a status of the RFID tag as moving or stationary is identified.

18. The system of claim 17 , wherein when the RFID tag is stationary, a presence of an object within an area between the RFID reader and the RFID tag is identified.

19. A computer-implemented method for estimating a distance between a Radio Frequency Identifier (RFID) tag and an RFID reader, comprising:

measuring, by measurement equipment, a plurality of phase differences at different frequencies between transmitted RF signals from the RFID reader and corresponding received RF signals at the RFID tag;

calculating, by a processor, normalized phases from the plurality of phase differences;

calculating, by the processor, corrected phases by resolving one or more ambiguities from the normalized phases;

obtaining, by the processor, a characteristic curve using the corrected phases; and

providing, by the processor, an estimate of the distance based on the characteristic curve and the corrected phases.

20. A computer program product for estimating a distance between an Radio Frequency Identifier (RFID) tag and an RFID reader, the computer program product comprising a non-transitory computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to perform a method comprising:

measuring, by measurement equipment, a plurality of phase differences at different frequencies between transmitted RF signals from the RFID reader and corresponding received RF signals at the RFID tag;

calculating, by a processor, normalized phases from the plurality of phase differences;

calculating, by the processor, corrected phases by resolving one or more ambiguities from the normalized phases;

obtaining, by the processor, a characteristic curve using the corrected phases; and

providing, by the processor, an estimate of the distance based on the characteristic curve and the corrected phases.