Symbol Error Analysis for Fluid Antenna Systems with One- and Two-Dimensional Modulation Schemes

This paper considers a Fluid Antenna (FA) system comprising a single-antenna transmitter that communicates with a receiver equipped with an FA array with $N$ ports. The transmitter is assumed to deploy any of the modulation schemes: $\mathit{\text{i}}$) two-sided $M$-ary amplitude-shift keying, $\mathit{\text{ii}}$) $M$-ary phase-shift keying, iii) $M$-ary quadrature-amplitude modulation, and $\mathit{\text{iv}}$) binary frequency-shift keying, the channels between its antenna and the receiver ports are subjected to Rayleigh fading, and the receiver chooses the best $K$ out of its $N$ ports for symbol detection. Considering that the receiver combines the signals from the best $K$ ports using maximal-ratio combining, the optimal reception structures for all the considered signaling schemes are obtained. We also present novel exact closed-form expressions for the respective symbol error probabilities (SEPs) of the FA system, as well as asymptotic approximations valid at high signal-to-noise ratios. The presented analysis is corroborated through comparisons with simulation results, showcasing the critical role of various system parameters on the SEP performance.