Simple-DF versus selective-DF relaying over Rayleigh turbulence-induced FSO fading channels

In this paper, we consider the problem of relay-assisted free-space optical (FSO) transmissions. We evaluate the diversity orders that can be achieved by simple-DF and selective-DF relaying protocols over quantum-limited FSO systems that are subject to Rayleigh fading. We prove that for a N
r
-relay system, selective-DF captures the full spatial diversity order of N
r
+1 while simple-DF achieves a reduced order of ⌈N
r
/2⌉+1 making this scheme highly suboptimal for FSO communications.