Signal constellations of a retrodirective array phase modulator
Koo, Gregory Andre
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A quadrature phase shift keying (QPSK) retrodirective array phase modulator (RAPM) was designed and fabricated to characterize its backscatter signal constellation when placed near objects with varying conductivities and relative permittivities. The signal constellations produced when the RAPM was placed near objects were compared to a constellation in free space to determine relative magnitude and phase changes. When conductors and high permittivity dielectrics were placed close behind the RAPM, constellation points were found to shrink in magnitude by up to twenty percent and shift in phase by up to eight degrees. When conductors were placed between the RAPM and an interrogator, the signal constellation was found to collapse, shrinking by up to 95.6 percent. For materials similar to free space, minimal constellation shrinkage resulted, but signal constellation rotation by up to 68 degrees occurred. The power consumption of a RAPM was also characterized and found to decrease as the number of bits per symbol increased. This result demonstrates that in comparison to conventional backscatter tags, which implement one bit per symbol, the RAPM can implement a greater number of bits per symbol, reduce its power consumption, and increase its range in a passive backscatter communication system. To characterize the beamwidth of the RAPM's retrodirective array, a radar cross section (RCS) measurement of the RAPM was performed over a scan angle range of -90 to +90 degrees. The structural component generated by the RAPM's patch antenna ground plane was found to dominate the antenna mode of the retrodirective array. As a result, a novel homodyne receiver based RCS measurement was performed to filter out the structural RCS component and measure the pure antenna mode of the RAPM.