Secure Transmission via Sub-Symbol Precoding and Port Switching in Fluid Antenna System

The fluid antenna (FA) has emerged as a promising technology for enhancing wireless network performance due to its ability to dynamically adjust antenna positions and mitigate poor channel conditions. In this paper, we propose an FA-aided secure transmission scheme where the transmitter, equipped with an FA, intends to send confidential information to a legitimate receiver under the eavesdropping of multiple illegitimate receivers. To overcome the limited beamforming dimensionality, each symbol duration is divided into several sub-symbol periods, during which the FA activates different ports corresponding to varying legitimate and eavesdropping channel conditions. This dynamic switching enhances spatial degrees of freedom (DoF). During each sub-symbol period, the FA transmits a superposition of useful sig- nal and random noise with fine-tuned precoding factors, ensuring that the noise degrades eavesdropping channels while minimally affecting the legitimate channel. We consider a practical scenario where imperfect eavesdropping channel state information (ECSI) is available. A worst-case secrecy rate maximization problem is formulated via jointly optimizing the precoding factors, power allocation, and the receive beamforming vector. Numerical results demonstrate that the proposed scheme significantly improves secrecy performance compared to traditional secure schemes.