  This letter investigates a fluid antenna system (FAS) where multiple ports
can be activated for signal combining for enhanced receiver performance. Given
$M$ ports at the FAS, the best $K$ ports out of the $M$ available ports are
selected before maximum ratio combining (MRC) is used to combine the received
signals from the selected ports. The aim of this letter is to study the
achievable performance of FAS when more than one ports can be activated. We do
so by analyzing the outage probability of this setup in Rayleigh fading
channels through the utilization of Gauss-Chebyshev integration, lower bound
estimation, and high signal-to-noise ratio (SNR) asymptotic approximations. Our
analytical results demonstrate that FAS can harness rich spatial diversity,
which is confirmed by computer simulations.
