Ranging for Backscatter RFID
Chenming Zhou (Disney Research Pittsburgh)
Joshua Griffin (Disney Research Pittsburgh)
A backscatter tag communicates by modulating the scattered electromagnetic wave incident from the reader. The scattered wave is modulated by changing the electrical impedance presented to the tag antenna. A passive backscatter tag receives the power needed to operate from the wave incident from the reader.
A CW radar typically has a small wavelength compared to the distance to be estimated; hence, the CW radar suffers from a cycle ambiguity — i.e., the CW range differs from the ground truth by an unknown number of wavelengths. A DFCW radar’s wavelength, on the other hand, does not contain a cycle ambiguity; however, the distance estimates of a DFCW radar are more susceptible to phase measurement errors of the backscatter tag’s signal than a CW radar.
While identifying an RF tag is relatively straightforward, determining its range or position is challenging, particularly for passive backscatter RF tags. This difficulty is caused, in part, by tag power and bandwidth limitations. This project seeks to measure the distance between a backscatter RF tag and the tag reader with millimeter-level accuracy while respecting the bandwidth limitations of the 5.8 GHz ISM frequency band. The project uses a composite dual-frequency, continuous-wave (DFCW) and continuous wave (CW) radar to determine the range between the tag and reader and a spatial averaging algorithm to reduce the bandwidth required. This technology could be used in entertainment applications when an RF line-of-sight is present. Such applications include game controllers, wireless human computer interaction devices, and motion capture/tracking systems.
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