| Summary: | This article introduces retroreflective lenses for millimeter-wave radio-frequency indoor localization. A three-dimensional (3D) gradient-index Luneburg lens is employed to increase radar cross section (RCS) of photonic-crystal high-Q resonators and its performance is compared to conventional radar retroreflectors. A classic Luneburg lens with and without a reflective layer is realized with 25 mm diameter (6.7 λ<sub>0</sub>), showing a realized gain of 24.6 dBi and a maximum RCS of -9.22 dBm<sup>2</sup> at 80 GHz. The proposed Luneburg lens with embedded high-Q resonators as frequency-coded particles in a photonic crystal structure, operating as a reflective layer, achieved a maximum RCS of -15.84 dBm<sup>2</sup> at the resonant frequency of 76.5 GHz and showed a repeatable response each 18° over ±36° in two perpendicular planes. With this high RCS of the Luneburg lens, a maximum readout range of 1.3 m could be achieved compared to 0.15 m without the lens at 76.5 GHz for the same transmit power, receiver sensitivity, and gain of the reader antenna.
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