Fluid Antenna Grouping Index Modulation Design for MIMO Systems

The fluid antenna (FA)-enabled multiple-input multiple-output (MIMO) system based on index modulation (IM), referred to as FA-IM, significantly enhances spectral efficiency (SE) compared to the conventional FA-assisted MIMO system. To improve the performance in addressing the high spatial correlations between multiple activated ports, this paper proposes an innovative FA grouping-based IM (FAG-IM) system. Specifically, considering the characteristics of the FA two-dimensional (2D) surface structure and the spatially correlated channel model in FA-assisted MIMO systems, a block grouping method is adopted, where adjacent ports are assigned to the same group. Consequently, different groups independently perform port index selection and constellation symbol mapping, with only one port being activated within each group during each transmission interval. Then, a closed-form average bit error probability (ABEP) upper bound is derived for the proposed system. Numerical results show that, compared to state-of-the-art systems, the FAG-IM system consistently achieves substantial performance gains.