Broad Beam Reflection for RIS-Assisted MIMO Systems with Planar Arrays

TL;DR

This paper proposes a novel RIS-assisted MIMO system that uses dual-polarized RIS to create a broad beam for cell-specific downlink broadcasting, enhancing coverage in wide angular areas.

Contribution

It introduces a polarization-based phase configuration design for RIS to achieve broad beam radiation in MIMO systems, filling a gap in cell-specific transmission methods.

Findings

The RIS can radiate a spatially flat broad beam covering all user angles.

The broad beam gain scales with the RIS aperture size.

Numerical simulations validate the theoretical analysis.

Abstract

While reconfigurable intelligent surface (RIS)-aided user-specific beamforming has been vastly investigated, the aspect of utilizing RISs for assisting cell-specific transmission has been largely unattended. Aiming to fill this gap, we study a downlink broadcasting scenario where a base station (BS) sends a cell-specific signal to all the users located in a wide angular area with the assistance of a dual-polarized RIS. We utilize the polarization degree of freedom offered by this type of RIS and design the phase configurations in the two polarizations in such a way that the RIS can radiate a broad beam, thereby uniformly covering all azimuth and elevation angles where the users might reside. Specifically, the per-polarization configuration matrices are designed in such a way that the total power-domain array factor becomes spatially flat over all observation angles implying that the RIS can preserve the broad radiation pattern of a single element while boosting its gain proportionally to its aperture size. We validate the mathematical analyses via numerical simulations.