Abstract
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 ( $6.7~\lambda _{0}$ ), showing a realized gain of 24.6 dBi and a maximum RCS of −9.22 dBm2 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 dBm2 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.
Highlights
Retroreflectors are devices providing radar cross section (RCS) augmentation and are widely used for automotive radars and measurement calibration [1], tracing objects [2], indoor localization [3] and recently, they have been explored as devices which can help to overcome high path losses at 5G millimeter waves and optical communication systems [4]
The measurement of the monostatic RCS of the retroreflective Luneburg lens is performed with a WR-10 horn antenna, which serves as a transmitting and receiving antenna and the reflector is placed on a turntable at a distance of 1.3 m
In this article, we presented a perforated Luneburg lens retroreflector for millimeter-wave frequency-coded indoor localization
Summary
Retroreflectors are devices providing radar cross section (RCS) augmentation and are widely used for automotive radars and measurement calibration [1], tracing objects [2], indoor localization [3] and recently, they have been explored as devices which can help to overcome high path losses at 5G millimeter waves and optical communication systems [4]. Kadera et al.: Gradient-Index-Based Frequency-Coded Retroreflective Lenses for mm-Wave Indoor Localization nearby environment, i.e., clutter, such as naturally occurring retrodirective reflections in the corners of a room or among furniture To overcome this issue, high quality factor (high-Q) resonators can be exploited for their ability to store electromagnetic (EM) energy for a longer time inside the resonator, which helps to filter out the undesired clutter reflections by a time-gating technique as presented in [13]–[15]. Its constant relative permittivity ( r ≈ 4) limits its use to small diameters of 4-5 wavelengths due to a high-density of parasitic resonances, which are caused by an impedance mismatch at its surface [17] To overcome this limiting factor, a Luneburg lens with a gradient permittivity profile can be used as theoretically introduced in [18]. The proposed designs are manufactured, and the measurement results are shown in Section IV, following a discussion of the results and their intended improvement with a threedimensional (3-D) printing technology
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