Suboptimal Fluid Antenna Systems for Maximizing the Secrecy Rate with Untrusted Relays

This research examines the effectiveness of untrusted relays in fluid antenna-assisted communication systems subjected to arbitrary correlated fading channels. The transmitter is represented to share confidential information to a legitimate receiver,both utilizing a 1D Fluid Antenna System (FAS), while an untrusted relay, equipped with a fixed antenna, attempts to decrypt the intended message. In this case, cooperative jamming is adopted and we jointly optimize the power allocation of the transmitted jamming signal and the FAS beamforming weights to maximize the secrecy rate. Due to the non-convex nature of the optimization issue, we employ the Particle Swarm Optimization (PSO) approach to get the suboptimal secrecy rate. Ultimately, numerical results demonstrate that the use of FAS ensures more safe and reliable transmission, while an increase in FAS components produces more focused beams. This leads to improved alignment of the broadcast signal toward the legitimate receiver, increasing the necessary signal intensity while reducing leakage to unauthorized relays.