NLTL RFID Sensor Tag for Indoor Ranging Accuracy
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Solution Overview
Problem
Indoor RFID ranging systems face challenges with accuracy due to multi-path interference and cycle ambiguity, particularly in environments with strong self-interference and limited sensing bandwidth, which affects the precision of distance measurement.
Innovation Solution
The use of nonlinear transmission lines (NLTLs) in RFID tags for broadband harmonic generation, combined with a reader system featuring multiple antennas and demodulators, allows for accurate trilateration by processing phase and magnitude outputs at the second harmonic and subharmonics to resolve distance ambiguity and mitigate interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional CW phase-based ranging methods are used, then high accuracy is achieved, but cycle ambiguity exists when distance is longer than one wavelength
Solution Approach 1:
The patent transitions from single-frequency CW ranging to multi-frequency harmonic ranging, adding the frequency dimension to resolve cycle ambiguity. By measuring phase at multiple harmonics (fundamental, second, third), the system can disambiguate the integer number of wavelengths and achieve accurate ranging beyond one wavelength without losing positional information.
Solution Approach 2:
The patent changes the frequency parameter by utilizing multiple harmonics of the fundamental CW signal. Instead of relying on a single frequency that creates cycle ambiguity, the system uses the fundamental frequency and its harmonics to measure phase differences, enabling resolution of the cycle ambiguity through mathematical processing of multiple frequency components.
2Loss of information
If DFCW (dual-frequency continuous wave) sensing bandwidth is used, then cycle ambiguity can be solved, but reading range is limited within which integer ambiguity can be solved
Solution Approach 1:
The patent employs periodic harmonic generation where the NLTL generates multiple harmonics (fundamental, second, third) of the input CW signal. This periodic structure allows the system to measure phase at multiple frequencies simultaneously, extending the unambiguous ranging range beyond what dual-frequency methods achieve while maintaining cycle ambiguity resolution capability.
Solution Approach 2:
The patent extends the frequency spectrum by utilizing not just two frequencies (as in DFCW) but multiple harmonics of the fundamental frequency. This multi-frequency approach with harmonics provides a broader sensing bandwidth that maintains cycle ambiguity resolution while enabling longer reading ranges through the additional frequency components.
3Ease of manufacture
If passive CW phase ranging by backscattering RFID tags is used, then low-cost and low-energy consumption are achieved, but phase errors caused by multi-path interference are severe
Solution Approach 1:
The patent converts the harmful effect of multi-path interference into a beneficial feature by using the NLTL's harmonic generation property. The nonlinearity of the NLTL creates harmonics that are less susceptible to multipath effects, and by measuring phase at multiple harmonics, the system can distinguish between direct and reflected paths, thereby maintaining accuracy despite the presence of multi-path interference while retaining the low-cost passive backscattering architecture.
4Object-generated harmful factors
If strong self-interference is present in indoor environments, then isolation between transmitting and receiving ends is poor, but phase detection is smeared
Solution Approach 1:
The patent addresses self-interference by utilizing the NLTL's harmonic generation characteristic. The nonlinearity of the NLTL produces harmonics that can be differentiated from the fundamental frequency signal, allowing the receiver to filter out self-interference components at the fundamental frequency while measuring phase at the harmonic frequencies where the interference is minimized, thereby maintaining phase detection accuracy despite strong self-interference.
Solution Approach 2:
The patent introduces harmonic frequencies as an intermediary measurement channel. Instead of measuring phase at the fundamental frequency where self-interference is strongest, the system uses the harmonic frequencies as an intermediary where the phase information is still available but the self-interference is reduced, allowing accurate phase detection even in high-interference environments.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides high-precision, millimeter-range accuracy in indoor localization, effectively addressing multi-path interference and cycle ambiguity, while being cost-effective and suitable for various applications including real-time object tracking and inventory management.
Implementation Method 1
the sensor tag comprising at least one nonlinear transmission line (NLTL) for matched impedance, broadband harmonic generation
Data Source
AI summary
Systems and methods for ranging in indoor environment that are accurate and that are substantially undisturbed by multipath interference. The method includes illuminating a sensor tag with electromagnetic radiation generated from a transceiver; the transceiver are being located a distance away from the sensor tag; the sensor tag comprising at least one nonlinear transmission line (NLTL) for broadband harmonic generation, receiving backscattered electromagnetic radiation from the at least one NLTL at three or more locations; coordinates of the three or more locations being known, obtaining from the phase and magnitude outputs at a second harmonic and at least one sub harmonic of second harmonic, a distance from the sensor tag to each of the three or more locations and trilaterating a location of the sensor tag.