This paper 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 0), showing a realized gain of 24.6 dBi and a maximum RCS of-9.22 dBm 2 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 2 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.

<p>This paper presents and compares two promising approaches for realizing liquid crystal (LC)-based phase shifting topologies for Reconfigurable Intelligent Surfaces (RIS). Main requirements for the phase shifters include high Figure of Merit (FoM), compact size, low response times and constant group delay. The first approach is based on defective ground structure inverted microstrip lines (DGS-IMSL) and shows high FoM above 74 °/dB between 25 and 30 GHz. In addition, it promises low response times due to its thin 4.6 µm LC layer and features moderate physical length below 0.45 λ0. However, it is limited by a non-constant group delay. The second approach, the bandpass inverted microstrip line (BP-IMSL), has a lower FoM of approximately 26 °/dB between 28 to 30 GHz with a thicker LC layer of 20 µm. Nevertheless, a shorter physical size below 0.25 λ0 is accomplished and potential for a flat group delay is given. </p>

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<p>This paper presents and compares two promising approaches for realizing liquid crystal (LC)-based phase shifting topologies for Reconfigurable Intelligent Surfaces (RIS). Main requirements for the phase shifters include high Figure of Merit (FoM), compact size, low response times and constant group delay. The first approach is based on defective ground structure inverted microstrip lines (DGS-IMSL) and shows high FoM above 74 °/dB between 25 and 30 GHz. In addition, it promises low response times due to its thin 4.6 µm LC layer and features moderate physical length below 0.45 λ0. However, it is limited by a non-constant group delay. The second approach, the bandpass inverted microstrip line (BP-IMSL), has a lower FoM of approximately 26 °/dB between 28 to 30 GHz with a thicker LC layer of 20 µm. Nevertheless, a shorter physical size below 0.25 λ0 is accomplished and potential for a flat group delay is given. </p>