  Over the past decades, multiple antenna technologies have appeared in many
different forms, most notably as multiple-input multiple-output (MIMO), to
transform wireless communications for extraordinary diversity and multiplexing
gains. The variety of technologies has been based on placing a number of
antennas at fixed locations which dictates the fundamental limit on the
achievable performance. By contrast, this paper envisages the scenario where
the physical position of an antenna can be switched freely to one of the N
positions over a fixed-length line space to pick up the strongest signal in the
manner of traditional selection combining. We refer to this system as a fluid
antenna system (FAS) for tremendous flexibility in its possible shape and
position. The aim of this paper is to study the achievable performance of a
single-antenna FAS system with a fixed length and N in arbitrarily correlated
Rayleigh fading channels. Our contributions include exact and approximate
closed-form expressions for the outage probability of FAS. We also derive an
upper bound for the outage probability, from which it is shown that a
single-antenna FAS given any arbitrarily small space can outperform an
L-antenna maximum ratio combining (MRC) system if N is large enough. Our
analysis also reveals the minimum required size of the FAS, and how large N is
considered enough for the FAS to surpass MRC.
