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

This letter investigates downlink (DL) physical layer security (PLS) in near-field (NF) extra-large MIMO (XL-MIMO) systems. To enhance secrecy rate (SR), null-space artificial noise (AN) is transmitted alongside confidential messages, remaining orthogonal to legitimate user equipment (LUE) channels. The goal is to maximize the minimum SR by jointly optimizing the NF beamfocusing matrix and power allocation between the signal and AN, under various channel state information (CSI) conditions and transmit power constraints. The proposed method combines successive convex approximation (SCA) for beam design and golden section search (GSS) for power allocation. It addresses two key questions: (i) Can AN improve SR with multiple LUEs and eavesdropping user equipments (EUEs)? (ii) Can null-space AN remain effective without EUE CSI? Both are answered affirmatively, and the algorithm shows clear gains over approaches without AN or using maximal-ratio transmission (MRT).