System and method for ultra-high-resolution ranging using RFID
A radio-frequency method for range finding includes modulating a reference signal having an intermediate frequency to a downlink signal having a carrier frequency using a clock signal. The downlink signal is transmitted to a tag using a transceiver. An uplink signal backscattered from the tag is received and demodulated using the clock signal. The uplink signal has a frequency that is a harmonic of the carrier frequency. A distance between the tag and the transceiver is calculated based on a phase of the demodulated uplink signal. A system for range finding includes a transceiver and a processor. The transceiver modulates a reference signal to a downlink signal and transmits the downlink signal. The transceiver receives and demodulates an uplink signal. The processor is configured to receive the demodulated uplink signal and calculate a distance between the tag and the transceiver using a phase of the demodulated uplink signal.
1. A radio-frequency method for range finding, the method comprising:
modulating a reference signal having an intermediate frequency, f IF , to a downlink signal having a carrier frequency, f c , using a clock signal;
transmitting the downlink signal to a tag using a transceiver, the tag being located at a distance from the transceiver;
receiving an uplink signal backscattered from the tag, the uplink signal having a frequency that is a harmonic of the carrier frequency;
demodulating the uplink signal using the clock signal;
calculating a distance between the tag and the transceiver based on a phase of the demodulated uplink signal;
modulating the reference signal to one or more additional downlink signals each having an additional carrier frequency and each of the additional carrier frequencies generated using a corresponding clock signal, and wherein each of the additional carrier frequencies is not equal to f c ;
transmitting, using a corresponding one or more additional transceivers, the one or more additional downlink signals to the tag;
receiving one or more additional uplink signals backscattered from the tag, each of the one or more additional uplink signals being at a second harmonic of a corresponding one of the one or more additional carrier frequencies;
demodulating each of the one or more additional uplink signals using the clock signal of the corresponding one or more carrier frequencies; and
calculating a distance between the tag and each additional transceiver based on a difference between a phase of the reference signal and a phase of a corresponding one of the one or more demodulated uplink signals.
2. The method of claim 1 , wherein the uplink signal is at a second harmonic of the carrier frequency to lower phase noise interference from leakage of the downlink signal.
3. The method of claim 1 , wherein each step of calculating the distance is repeated at a sampling rate to update the distance.
4. The method of claim 1 , wherein f IF is greater than a frequency where Flicker noise power density is equal to the thermal noise density.
5. The method of claim 3 , further comprising calculating a moving average comprising a predetermined number of most recent calculated distances.
6. The method of claim 1 , wherein calculating the distance between the tag and the transceiver further comprises dividing a result by the square root of a relative permittivity of a medium between the transceiver and the tag, wherein the relative permittivity of the medium is known and relative to a vacuum.
7. The method of claim 1 , further comprising determining a relative permittivity of a medium in which the tag disposed, using the calculated distance and a known range of the tag, wherein the relative permittivity of the medium is relative to a vacuum.
8. The method of claim 1 , wherein the uplink signal has a unique digital identification code to provide isolation from ambient noise.
9. The method of claim 8 , wherein the unique digital identification code is encoded using a code-division multiple access (CDMA) protocol to provide isolation from other tags.
10. The method of claim 1 , wherein the distance between the tag and the transceiver is calculated by comparing the reference signal to the demodulated uplink signal.
11. A radio-frequency system for range finding, comprising:
a transceiver configured to:
modulate a reference signal having an intermediate frequency, f IF , to a downlink signal having a carrier frequency, f c , using a clock signal;
transmit the downlink signal;
receive a backscattered uplink signal from a tag, wherein the uplink signal is at a harmonic frequency of the carrier frequency;
demodulate the uplink signal using the clock signal such that transceiver is a coherent transceiver; and
a processor configured to receive the demodulated uplink signal and calculate a distance between the tag and the transceiver using a phase of the demodulated uplink signal;
a tag configured to receive the downlink signal at f c and to backscatter the uplink signal at the harmonic frequency of the carrier frequency;
one or more additional transceivers, each configured to modulate and transmit the reference signal at a corresponding one or more additional carrier frequencies, and to receive and demodulate corresponding uplink signals, each uplink signal at a harmonic of a corresponding one of the one or more additional carrier frequencies; and
wherein the processor is further configured to calculate a distance between the tag and each additional transceiver of the one or more additional transceivers based on a difference between a phase of the reference signal and a phase of a corresponding one of the one or more demodulated corresponding uplink signals.
12. The system of claim 11 , wherein the harmonic frequency of the carrier frequency is the second harmonic.
13. The system of claim 11 , wherein the tag is configured to encode a digital identification code onto the uplink signal.
14. The system of claim 11 , where the tag is configured to encode the uplink signal using a code-division multiple access (CDMA) protocol.
15. The system of claim 13 , further comprising one or more additional tags, each configured with a unique digital identification or CDMA code.
16. The system of claim 11 , wherein the tag is configured to transform the downlink signal to the uplink signal without offsetting a phase of the downlink signal.
17. The system of claim 11 , wherein the tag comprises a non-linear transmission line.
18. The system of claim 11 , the processor is further configured to repeatedly calculate each distance at a sampling rate.
19. The system of claim 18 , wherein the processor is further configured to calculate a moving average comprising a predetermined number of most recent calculated distances.
20. The system of claim 11 , further comprising an analog-to-digital converter to convert the demodulated uplink signal to a digital signal, wherein the analog-to-digital converter is configured to preserve a carrier phase of the uplink signal.
21. The system of claim 11 , further comprising a clock for generating the clock signal, wherein the clock is in communication with the transceiver.
22. The system of claim 11 , wherein the processor is further configured to determine a relative permittivity of a medium along a path between the tag and each transceiver of the one or more additional transceivers, using an obtained distance from the corresponding transceiver and a known range of the tag from the corresponding transceiver, wherein the relative permittivity of the medium is relative to a vacuum.