Impact of Inter-Relay Co-Operation on the Performance of FSO Systems With Any Number of Relays

In this paper, we study the impact of inter-relay co-operation on the performance of decode-and-forward (DF) cooperative free space optical (FSO) communication systems with any number of relays. The idea of inter-relay co-operation (IRC) was introduced very recently where the relay-relay links are activated for further boosting the system performance. We evaluate the outage probability under forward and forward-backward IRC that constitute the two variants of this transmission strategy. We also derive the diversity orders that can be achieved over a composite channel model that takes both turbulence-induced fading and misalignment-induced fading into consideration. We present a comprehensive asymptotic analysis that is effective for tackling the usefulness of IRC with an arbitrary number of relays and for deriving the network conditions under which implementing IRC in any of its variants can be beneficial for enhancing the diversity order of the FSO system. The introduced framework answers the question on what is the optimal solution for a particular FSO network (among the parallel-relaying solution with no IRC, forward IRC or forward-backward IRC).