The　combination　of　forward　error　correction　(FEC)　and　interleaving　can　be　used　to　improve　free-space　optical　communication　systems.　Recent　research　has　optimised　the　codeword　length　and　interleaving　depth　under　the　assumption　of　a　fixed　buffering　size;　however,　how　the　buffering　size　influences　the　system　performance　remains　unsolved.　This　study　models　the　system　performance　as　a　function　of　buffering　size　and　FEC　recovery　threshold,　which　allows　system　designers　to　determine　optimum　parameters　in　consideration　of　the　overhead.　The　modelling　is　based　on　statistics　of　temporal　features　of　correct　data　reception　and　burst　error　length　through　the　measurement　of　the　channel　good　time　and　outage　time.　The　experimental　results　show　good　coherence　with　the　theoretical　values.　This　method　can　also　be　applied　in　other　channels　if　a　continuous-time-Markov-chain　model　of　the　channel　can　be　derived.