Near-Field User Localization and Beamforming in Covert Communication

Covert communication ensures reliable communication with a legitimate user while preventing a warden from detecting the behavior of transmission. Most of the existing literature on covert communication focuses on far-field covert communication scenarios. However, the advancement of 6 G technologies, particularly the integration of extremely large-scale multiple-input multiple-output (XL-MIMO) systems and Terahertz (THz), highlights the emergence of near-field communications. Moreover, wideband XL-MIMO exacerbates the near-field beam-squint effect. Therefore, how to covertly determine the location of the target user and establish precise transmission in the near field remains a major challenge. In this work, we investigate near-field covert communication. Specifically, leveraging the emerging extremely large-scale antenna array (ELAA) architecture with true-time-delay lines (TTDs), a transmitter Alice employs an iterative search strategy based on near-field beam squint to achieve precise localization of the legitimate receiver Bob. Subsequently, the TTDs are adjusted to eliminate the near-field beam-squint effect, thereby enabling effective data transmission (DT). Considering a finite blocklength, we propose a novel design framework that jointly optimizes the number of subcarriers and the transmit power. This framework aims to maximize the effective covert rate (ECR) of the Alice-Bob link while ensuring that the communication remains covert against the warden Willie's detection. Theoretical analysis and numerical results demonstrate the effectiveness of our proposed method and confirm the existence of optimal values among the parameters under consideration. Furthermore, compared with far-field, the proposed near-field approach achieves more accurate beam focusing and lower detection probability, making it particularly suitable for covert communications.