Beam Scanning Capability of Passive Smart Eelctromagnetic Skins

The next-generation communication systems will require new network paradigms due to their unique characteristics. The use of mm-waves or sub- THz frequencies appears to be promising due to the high bandwidth they can guarantee. However, these frequencies experience higher losses along the propagation path. As a result, coverage can be significantly degraded or even absent in areas that are not in the line of sight (LOS) of the base station antenna. To address this issue, the propagation environment must be considered as an additional design parameter while keeping networks simple, cost-effective, and low-power-consuming. This can be achieved by using Smart Electromagnetic Skins (SESs), which are thin surfaces able to provide a non-specular reflection to reach blind spots or cover desired areas. When they can also provide reconfigurability, they are called Reconfigurable Intelligent Surfaces (RISs). RISs generally require a control network to vary the phase provided by each unit cell in which the surface is discretized. Since their number is typically in the thousands, the latter requires to manage RISs is quite sophisticated. To avoid this drawback, we exploit the idea of design a passive SES that can change the provided pointing direction when its position is mechanically varied with respect to the direction of arrival of the incident field. While this solution seems particularly suitable when a curved SES mounted on a street/light pole is used [A. Freni, M. Beccaria, A. Mazzinghi, A. Massaccesi, P. Pirinoli, “Low-Profile and Low-Visual Impact Smart Electromagnetic Curved Passive Skins for Enhancing Connectivity in Urban Scenarios,” Electronics, 2023], since it could be simply rotated around the pole, at this initial step a planar surface is considered. In particular, the latter is discretized with $50\times 74$ unit cells, with size 3.6 mm $=0.327\lambda$ at the design frequency of 27.25 GHz. The unit cell, shown in the blow-up of Fig.la, consists of a layer of Diclad527 $(\epsilon_{\mathrm{r}}=2.55,\ \tan\delta=0.0022)$ with thickness $h=0.508$ mm, metalized on both sides. The metallization on the top presents a slot with the shape of a cross, with varying size $W$ Above this, there is a further layer, with a thickness of 1 mm, made of Lexan $(\epsilon_{\mathrm{r}}=2.8,\ \tan\delta=0.01)$ that acts as radome. The SES is designed to provide a phase distribution obtained as a linear combination of those required to have two different pointing directions, each associated with a different direction of arrival of the incident field. To check the feasibility of this approach, the reflecting surface in Fig. 1 is designed: the pointing directions are assumed to be characterized by $\varphi_{\mathrm{n}\mathrm{a}\mathrm{x}}=90^{\mathrm{o}}, \theta_{\mathrm{n}\mathrm{a}\mathrm{x}}=15^{\mathrm{o}}$ and $\theta_{\max}=40^{\mathrm{o}}$ (see the reference system in Fig. 1a) while the incident plane waves impinge with angles $\wp_{\mathrm{n}\mathrm{c}}=_{-}90^{\mathrm{o}},\theta_{1\mathrm{n}\mathrm{c}}= -10^{\mathrm{o}}$ and $\theta_{\text{inc}}=-30^{\mathrm{o}}$, respectively. Then, the position of the SES is changed, to vary the direction of arrival of the field, and the RCS plotted in Fig. 1 b for different positions is obtained. A scanning larger than $20^{\mathrm{o}}$ achieved in the $yz$ plane, with a reduction of the maximum RCS no larger than 1.3 dB and no significant increase of the SLL, confirms the feasibility of the proposed solution to realize a reconfigurable intelligent surface. At conference time results for a curved SES will also be presented.