Performance of multi-hop M-ary PPM FSO systems with SIMO links over strong atmospheric turbulence channel and pointing errors

Strong atmospheric turbulence and pointing errors are major challenges in free-space optical communication (FSO) systems. In this paper, in order to overcome these challenges, we propose to employ multi-hop transmission with single-input multiple-output (SIMO) links (i.e., receiver diversity) in FSO systems. In addition, M-ary pulse-position modulation (PPM) and symbol detect-and-forward relaying scheme will be used to further improve the system performance. We formulate the closed-form expressions for bit error rate (BER) of multi-hop M-ary PPM FSO systems with equal gain combining (EGC) and maximum ratio combining (MRC) diversity techniques taking into account the effects of background noise and thermal noise. Numerical results indicate that the combination of multi-hop transmission and receiver diversity is a great solution for reducing effects of strong atmospheric turbulence and pointing errors.