RFID tag location systems
First Claim
1. A method of locating an RFID tag in a multipath environment including reflected signal paths, the method comprising:
- transmitting tag location signals from a plurality of different transmit antennas, wherein said antennas are spaced apart by a distance of more than a near field limit at a frequency of a said signal, wherein said near field limit is D2/(4λ
) where λ
is a wavelength at said frequency of said signal and D is a maximum dimension of one of said antennas, wherein D is expressed in centimeters and a quotient of the near field limit is greater than 1 centimeter;
receiving a corresponding plurality of return signals from said tag; and
processing said tag return signals to determine a range to said tag;
wherein said transmitting comprises transmitting at a plurality of different frequencies, relative phase, or both;
wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range by evaluating a phase-frequency gradient Δ
φ
/Δ
f where Δ
φ
is said phase difference between frequencies with a frequency difference Δ
f, and wherein said determining of said phase difference determines a phase difference between either;
i) two of said transmit signals orii) a first transmit signal and its corresponding return signal; and
wherein said determining of said range to said tag employs multiple transmissions from said plurality of different transmit antennas and receiving from said plurality of different transmit antennas with a phase-frequency gradient weighting for uses said return signals determined responsive to respective received signal strengths of the return signals to mitigate erroneous range measurements resulting from said reflected signal paths weighted responsive to a respective received signal strength of the return signal.
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Abstract
We describe a methods of locating an RFID tag. One method transmits tag location signals at a plurality of different frequencies from a plurality of different antennas spaced apart by more than a near field limit distance. The processing determines a phase difference at the plurality of different frequencies by determining a phase difference between either i) two or more of the transmit signals resulting in a maxima in the returned signal RSSI or ii) a first transmit signal and its corresponding return signal. The range determining uses return signals weighted by signal strength. Further data which may be used for averaging may be generated by using the above techniques along with changes in the polarisation state of the transmit and receive antennas and/or physical reconfiguration of the antennas (e.g. switch the transmit and receive elements).
30 Citations
13 Claims
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1. A method of locating an RFID tag in a multipath environment including reflected signal paths, the method comprising:
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transmitting tag location signals from a plurality of different transmit antennas, wherein said antennas are spaced apart by a distance of more than a near field limit at a frequency of a said signal, wherein said near field limit is D2/(4λ
) where λ
is a wavelength at said frequency of said signal and D is a maximum dimension of one of said antennas, wherein D is expressed in centimeters and a quotient of the near field limit is greater than 1 centimeter;receiving a corresponding plurality of return signals from said tag; and processing said tag return signals to determine a range to said tag; wherein said transmitting comprises transmitting at a plurality of different frequencies, relative phase, or both; wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range by evaluating a phase-frequency gradient Δ
φ
/Δ
f where Δ
φ
is said phase difference between frequencies with a frequency difference Δ
f, and wherein said determining of said phase difference determines a phase difference between either;i) two of said transmit signals or ii) a first transmit signal and its corresponding return signal; and wherein said determining of said range to said tag employs multiple transmissions from said plurality of different transmit antennas and receiving from said plurality of different transmit antennas with a phase-frequency gradient weighting for uses said return signals determined responsive to respective received signal strengths of the return signals to mitigate erroneous range measurements resulting from said reflected signal paths weighted responsive to a respective received signal strength of the return signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A system for locating an RFID tag in a multipath environment including reflected signal paths, the system comprising:
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a transmitter to transmit tag location signals from a plurality of different transmit antennas, wherein said antennas are spaced apart by a distance of more than a near new field limit distance at a frequency of a said signal, wherein said near field limit is D2/(4λ
) where λ
is a wavelength at said frequency of said signal and D is a maximum dimension of said antennas, wherein D is expressed in centimeters and a quotient of the near field limit is greater than 1 centimeter; anda processor to process said tag return signals to determine a range to said tag;
wherein said transmitting comprises transmitting at a plurality of different frequencies, relative phase, or both;wherein said processing comprises determining a phase difference at said plurality of different frequencies to determine said range by evaluating a phase-frequency gradient Δ
φ
/Δ
f where Δ
φ
is said phase difference between frequencies with a frequency difference Δ
f, and wherein said determining of said phase difference determines a phase difference between either;i) two of said transmit signals or ii) a first transmit signal and its corresponding return signal; and
-4-Serial No. wherein said determining of said range to said tag employs multiple transmissions from said plurality of different transmit antennas and receiving from said plurality of different transmit antennas with a phase-frequency gradient weighting for uses said return signals determined responsive to respective received signal strengths of the return signals to mitigate erroneous range measurements resulting from said reflected signal paths weighted responsive to a respective received signal strength of the return signal.
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Specification