RIS-Focus: On the Optimal Placement of the Focal Plane for Outdoor Beam Routing

Recently, reconfigurable/large intelligent surface (RIS/LIS) has been emerged as an essential key enabler for 6G communication scenarios. The surface can be programmed by software for forming and directing the electromagnetic (EM) wave in any desired direction. Furthermore, cooperative RISs can be dynamically aligned for routing the EM beams through some short line-of-sight (LOS) paths. This gives rise to the birth of a new physical layer networking (PHY0) concept. However, it is still limited to indoor scenarios where the whole walls and ceilings are coated with metasurface material. The routing hops are limited to short distances up to 10 meters per hop. In this paper, an extreme scenario of EM beam routing is explored under the restrictions imposed by the outdoor environment (small number of limited RISs). Specifically, the focusing functionality of RIS is employed. The inherent trade-off between the achieved beam waist and the focal length is leveraged for maximizing the routing hop. So, the placement of the focal plane will be adapted for confining the routed beam between RIS surfaces as being as a guided wave. Main equations, governing near-field focused RIS, are investigated for maximizing focusing gain. The routing problem is formulated as problem of finding a single-source shortest path (SSSP). Simulations are performed on outdoor scenarios under a size-limited RISs network. The impact of the relative orientation between RIS nodes and surface aperture is considered in our simulations. Compared to short indoor hops (limited to a few meters), the maximum hop between RIS nodes is enlarged up to
$47\;m$
and
$94\;m$
for RIS areas of
$2\;m^{2}$
and
$4\;m^{2}$
, respectively under optimal relative orientation.