  To enable innovative applications and services, both industry and academia
are exploring new technologies for sixth generation (6G) communications. One of
the promising candidates is fluid antenna system (FAS). Unlike existing
systems, FAS is a novel communication technology where its antenna can freely
change its position and shape within a given space. Compared to the traditional
systems, this unique capability has the potential of providing higher diversity
and interference-free communications. Nevertheless, the performance limits of
FAS remain unclear as its system properties are difficult to analyze. To
address this, we approximate the outage probability and diversity gain of FAS
in closed-form expressions. We then propose a suboptimal FAS with $N^{*}$
ports, where a significant gain can be obtained over FAS with $N^{*}-1$ ports
whilst FAS with $N^{*}+1$ ports only yields marginal improvement over the
proposed suboptimal FAS. In this paper, we also provide analytical and
simulation results to unfold the key factors that affect the performance of
FAS. Limited to systems with one active radio frequency (RF)-chain, we show
that the proposed suboptimal FAS outperforms single-antenna (SISO) system and
selection combining (SC) system in terms of outage probability. Interestingly,
when the given space is $\frac{\lambda}{2}$, the outage probability of the
proposed suboptimal FAS with one active RF-chain achieves near to that of the
maximal ratio combining (MRC) system with multiple active RF-chains.
