Energy-Efficient Hexagonal QAM for Fluid Antenna Systems: ASER Analysis Under Imperfect CSI

Abstract

Fluid antenna systems (FAS) have recently emerged as an effective solution for enhancing spatial diversity within compact and flexible wireless architectures. In practical communication systems, however, channel estimation errors are inevitable, which necessitates performance analysis under imperfect channel state information (CSI). This letter investigates the performance of an N-port FAS over Nakagami-m fading channels under imperfect CSI. Closed-form analytical expressions are derived for the cumulative distribution function, probability density function, outage probability, and ergodic capacity, along with high-SNR asymptotic approximations. The average symbol error rate of hexagonal QAM (HQAM) and rectangular QAM (RQAM) is also analyzed under the considered conditions. An asymptotic diversity study reveals that the achievable diversity order is jointly determined by the effective spatial rank of the FAS and the Nakagami fading parameter, while imperfect CSI degrades performance without affecting diversity scaling. Simulation results validate the analysis and demonstrate that HQAM outperforms conventional QAM, achieving approximately 2 dB and 0.8 dB SNR gains over 128-RQAM and 256-SQAM, respectively.