Enhancing Communication Resilience through Fluid Antenna Index Modulation

Advanced anti-eavesdropping techniques play a crucial role in ensuring the security and reliability of 6 G communication systems. Index modulation (IM) has attracted attention due to its enhanced power efficiency achieved by the implicit index domain information, alongside explicit data symbol modulation. Given the distinct characteristics of bit transmission in IM, anti-eavesdropping methods specifically designed for novel IM schemes are required. This work proposes a secure fluid antenna index modulation (S-FAIM) framework to safeguard both index and data symbol transmissions against eavesdropping. Secure index transmission is achieved by randomizing the mapping between index bits and channel conditions, created by fluid antenna port positions, based on the channel state information of the legitimate link. Furthermore, data symbol transmission is protected by a combined configuration of non-orthogonal spectrally efficient frequency division multiplexing (SEFDM) waveforms and channel coding. In addition to constant error floors at 0.5 observed at the eavesdropper for both index and data symbol transmissions, the proposed framework enhances the reliability performance of the legitimate user. With a robust wavelet scattering neural network deployed for channel condition classification, data symbol transmission achieves error performance comparable to that of conventional non-IM SEFDM and orthogonal frequency division multiplexing (OFDM) systems. Moreover, additional index transmission, reliant on channel variations, demonstrates resilience against deep fades and exhibits improved error performance over data symbol transmission.