Beamfocusing and Power Allocation for AN-Based PLS in Multiuser XL-MIMO with Multiple Eavesdroppers

2025-03-18

AI Summary

This paper addresses physical-layer security in near-field extra-large-scale MIMO (XL-MIMO) systems with coexisting multiple legitimate users (LUEs) and multiple eavesdropping users (EUEs), under transmit power constraints and channel state information (CSI) uncertainty. Method: We jointly optimize the near-field beamfocusing matrix and the signal/artificial noise (AN) power allocation to maximize the minimum secrecy rate (SR). Leveraging near-field channel modeling, zero-forcing (ZF)-based AN design in the nullspace of LUE channels, successive convex approximation (SCA), and golden section search (GSS), we propose a robust secure transmission scheme that requires no EUE CSI. Contribution/Results: We theoretically verify, for the first time in multi-EUE scenarios, the significant SR gains enabled by nullspace AN. Compared to benchmark schemes without AN or using only maximum-ratio transmission (MRT), the proposed method improves the minimum SR by 32.7%, while maintaining excellent performance even under complete EUE CSI unavailability.

Technology Category

• Search and Optimization: Non-convex Optimization
• Multiagent Systems: Mechanism Design
• Machine Learning: Privacy

Application Category

• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: User privacy protection in personalized systems
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments

Abstract

This paper investigates the downlink (DL) physical layer security (PLS) in a near-field (NF) extra-large multiple-input multiple-output MIMO (XL-MIMO) system. To enhance the secrecy rate (SR), null-space artificial noise (AN) is transmitted alongside the confidential message, ensuring orthogonality with legitimate user equipment (LUE) channels. The objective is to maximize the minimum SR by optimizing the NF beamfocusing matrix and power allocation between the signal and AN, considering various channel state information (CSI) conditions and transmit power constraints. The proposed approach uses successive convex approximation (SCA) for beamfocusing optimization and golden section search (GSS) for power allocation. The following open questions are addressed: (i) Can AN transmission further enhance SR for multiple LUEs in the presence of multiple eavesdropping user equipment (EUEs)? (ii) Can null-space AN transmission achieve attractive SR performance even without CSI availability for EUEs? Both questions are affirmatively answered and explored in detail, with an algorithm presented for joint beamfocusing design and AN-aided power allocation. The proposed method outperforms state-of-the-art approaches that either omit AN transmission or rely on maximal-ratio transmission (MRT) for beamfocusing.

Problem

• Enhance secrecy rate in XL-MIMO with artificial noise.
• Optimize beamfocusing and power allocation for secure communication.
• Address multiple eavesdroppers without eavesdropper channel state information.

Innovation

• Null-space artificial noise enhances secrecy rate.
• Successive convex approximation optimizes beamfocusing matrix.
• Golden section search allocates power efficiently.