Beamforming Optimization for Extremely Large-Scale RIS-Aided Near-Field Secure Communications

TL;DR

This paper proposes an optimization framework for XL-RIS-assisted near-field secure communications, jointly optimizing precoding and reflection to maximize secrecy rate, incorporating artificial jamming and discrete phase adjustments, with algorithms validated through simulations.

Contribution

It introduces a novel joint optimization method for XL-RIS-aided secure communications, including algorithms that handle discrete phase shifts and improve practical applicability.

Findings

The proposed scheme maintains security even when eavesdropper is close and aligned with the legitimate user.

The algorithms effectively optimize beamforming and phase shifts in near-field scenarios.

Simulation results confirm the robustness and practicality of the proposed approach.

Abstract

This paper studies an extremely large-scale reconfigurable intelligent surface (XL-RIS)-aided near-field physical layer security (PLS) communication system, aiming to maximize the secrecy rate by jointly optimizing precoding vector at the BS and the reflection coefficient matrix at the XL-RIS. Artifi-cial jamming was introduced to further enhance communication security. To solve the non-convex secrecy rate problem, an alternate optimization-based algorithm is adopted to decompose it into two sub-problems. Specifically, when optimizing the transmit beamformer at the BS, the non-convex prob-lem is transformed into a convex one through the weighted minimum mean-square error and the successive convex approximation-based algorithms. For the optimization problem of the XL-RIS phase-shifting matrix, a low-complexity alternating direction method of multipliers-based algorithm is employed to enhance the flexibility of the design. The proposed algorithm is capable of accommodating discrete phase optimization for the XL-RIS, thus better aligning with practical system requirements. Simulation results demonstrate that when the eavesdropper reside in the same direction as the legitimate user and is located closer to the XL-RIS, the proposed scheme in this paper can still ensure the secure communication.