Hybrid Beamfocusing Design for RSMA-Enhanced Near-Field Secure Communications

TL;DR

This paper introduces a novel RSMA-based secure transmission scheme for near-field communications using hybrid antenna architectures, enhancing physical-layer security by joint optimization of beamfocusing and secrecy rate allocation.

Contribution

It proposes a new secure transmit scheme that combines RSMA with near-field beamfocusing under hybrid architectures, optimizing secrecy rates with a penalty-based alternating algorithm.

Findings

Approaches fully digital beamfocusing performance with fewer RF chains.

Outperforms traditional beamfocusing-only and far-field security schemes.

Maintains high secrecy rates without significant rate loss.

Abstract

Near-field spherical wavefronts enable spotlight-like beam focusing to mitigate unintended energy leakage, creating new opportunities for physical-layer security (PLS). However, under hybrid analog-digital (HAD) antenna architectures, beamfocusing alone may not provide foolproof privacy protection due to reduced focusing precision. To address this issue, this paper proposes a rate-splitting multiple access (RSMA)-enhanced secure transmit scheme for near-field communications with fully-connected or sub-connected HAD architectures. In the proposed scheme, the common stream is designed for dual purposes, delivering the desired message for legitimate users while acting as artificial noise to disrupt eavesdropping. The primary objective is to maximize the minimum secrecy rate by jointly optimizing the analog beamfocuser, digital beamfocuser, and common secrecy rate allocation. To solve the formulated non-convex problem, we develop a penalty-based alternating optimization algorithm. Specifically, the variables are partitioned into three blocks, where one block is solved via a surrogate optimization method, while the others are updated in closed form. Simulation results reveal that our transmit scheme: (1) approaches fully digital beamfocusing with substantially fewer radio frequency chains, (2) outperforms conventional beamfocusing-only and far-field security schemes, and (3) preserves secrecy without significantly compromising communication rates.