Reconfigurable Intelligent Surfaces (RISs) constitute a strong candidate physical-layer technology for the $6$-th Generation (6G) of wireless networks, offering new design degrees of freedom for efficiently addressing demanding performance objectives. In this paper, we consider a Multiple-Input Single-Output (MISO) physical-layer security system incorporating a reflective RIS to safeguard wireless communications between a legitimate transmitter and receiver under the presence of an eavesdropper. In contrast to current studies optimizing RISs for given positions of the legitimate and eavesdropping nodes, in this paper, we focus on devising RIS-enabled secrecy for given geographical areas of potential nodes' placement. We propose a novel secrecy metric, capturing the spatially averaged secrecy spectral efficiency, and present a joint design of the transmit digital beamforming and the RIS analog phase profile, which is realized via a combination of alternating optimization and minorization-maximization. The proposed framework bypasses the need for instantaneous knowledge of the eavesdropper's channel or position, and targets providing an RIS-boosted secure area of legitimate communications with a single configuration of the free parameters. Our simulation results showcase significant performance gains with the proposed secrecy scheme, even for cases where the eavesdropper shares similar pathloss attenuation with the legitimate receiver.

翻译：可重构智能表面（RISs）是第六代（6G）无线网络的一种极具潜力的物理层技术，它为有效应对严苛的性能目标提供了新的设计自由度。本文研究了一种集成反射式RIS的多输入单输出（MISO）物理层安全系统，旨在保护合法发射机与接收机之间无线通信免受窃听者的威胁。与当前针对固定合法节点和窃听节点位置优化RIS的研究不同，本文专注于为潜在节点的给定地理区域设计基于RIS的保密方案。我们提出了一种新颖的保密度量，用于捕捉空间平均的保密频谱效率，并给出了发射数字波束赋形与RIS模拟相位配置的联合设计方案，该方案通过交替优化与最小化-最大化技术的结合来实现。所提出的框架无需窃听者信道或位置的瞬时信息，旨在通过单次自由参数配置提供RIS增强的合法通信安全区域。仿真结果表明，即使当窃听者与合法接收者具有相似的路径损耗衰减时，所提保密方案仍能带来显著的性能增益。