Geometric Shaping Non-Uniform Constellation Design in Fluid Antenna System

Fluid antenna systems (FAS) represent an emerging technology that enhances spatial diversity for wireless communications through reconfigurable antennas. Recently, coded modulation has been integrated into FAS. However, the system adopts uniform Gray-coded quadrature amplitude modulation (QAM) constellations, resulting in a gap between Bit-interleaved coded modulation (BICM) capacity and Shannon capacity. Geometric shaping non-uniform constellations (NUCs) is a promising approach for nearing Shannon capacity. In this paper, we derive the BICM capacity within the FAS system and use this metric to design NUCs for FAS. Simulation results confirm that the designed NUCs can achieve higher BICM capacity, approaching Shannon capacity, while providing improved error performance of 0.4 dB in link-level simulations.