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

TL;DR

This paper tackles secrecy leakage in near-field communications by leveraging RSMA with hybrid analog-digital beamforming. It formulates a nonconvex max-min secrecy-rate problem and resolves it via a penalty-based alternating optimization framework that decouples analog and digital beamformers through an auxiliary full-digital beamformer, with surrogate constructions for legitimate and eavesdropping rates. The proposed method yields closed-form updates for the digital beamformer and for the analog beamformer under both fully-connected and sub-connected HAD architectures, and it demonstrates that near-field beamfocusing combined with RSMA can nearly match fully digital performance while using far fewer RF chains. Numerical results show substantial secrecy gains over beamfocusing-only and far-field schemes, and the common RSMA stream acts as both data-bearing and artificial noise to thwart eavesdropping without sacrificing much throughput.

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.

Figures (6)

• Figure 1: The considered RSMA-enhanced secure NFC.
• Figure 2: Max-min secrecy rate versus the transmit power threshold.
• Figure 3: Max-min secrecy rate versus the number of legitimate users.
• Figure 4: Max-min secrecy rate versus the number of antennas.
• Figure 5: Max-min secrecy rate versus the number of RF chains.