  In this letter, we study the performance of a single-user fluid antenna
system (FAS) under arbitrary fading distributions, in which the fading channel
coefficients over the ports are correlated. We adopt copula theory to model the
structure of dependency between fading coefficients. Specifically, we first
derive an exact closed-from expression for the outage probability in the most
general case, i.e., for any arbitrary choice of fading distribution and copula.
Afterwards, for an important specific case, we analyze the performance of the
outage probability under correlated Nakagami-$m$ fading channels by exploiting
popular Archimedean copulas, namely, Frank, Clayton, and Gumbel. The results
demonstrate that FAS outperforms the conventional single fixed-antenna system
in terms of the outage probability. We also see that the spatial correlation
dependency structure for the FAS is a key factor to determine its performance,
which is natively captured through the choice of copula.
