Index Modulation for Covert Transmission in Continuous-Trajectory Fluid Antenna Systems

Fluid antenna (FA), which exploits dynamic movement within a confined region to achieve unprecedented spatial flexibility, emerges as a compelling technique that enhances transmission by exploring the spatial degree of freedom. In this contribution, we propose an index modulation (IM) scheme for secure communication aided by continuous-trajectory FA (CTFA). Specifically, the movement capability of the CTFA is harnessed to embed covert information into the radiation element's trajectory for legitimate users, ensuring that eavesdroppers remain unaware of the covert transmission. Furthermore, theoretical bit error ratio (BER) performance is analyzed, and a message-passing detector is developed for the proposed system. Finally, simulation results validate the theoretical findings, showing that the proposed IM-CTFA scheme attains performance comparable to conventional IM while effectively concealing covert information from eavesdroppers.