The performance of multihop parallel free- space optical (FSO) communication systems with decode-and-forward (DF) protocol over exponentiated Weibull (EW) fading channels has been investigated. The ABER and outage probability performance are analyzed under different turbulence conditions, receiver aperture sizes and structure parameters (R, C). The ABER and outage probability for FSO system is derived based on PSO. The ABER performance of the considered systems are investigated systematically combined with MC simulations. The comparison between EW fading model and PSO based EW fading channel demonstrates that performance of the both systems could be enhanced by large aperture sizes with the structure parameters R and C. With the particle swarm optimization (PSO) optimize path selection, fast average bit error rate (ABER) and outage probability reduction are achieved.