All-Active and Selective FSO Relaying: Do We Need Inter-Relay Cooperation?

In this paper, we study the impact of inter-relay cooperation on the outage performance of relay-assisted free-space optical (FSO) systems with two relays. Despite the fact that in realistic networks a FSO link might be available between the deployed relays, the additional advantages of exploiting such link were never investigated before. We explore this new dimension under the two strategies where either all relays are active or a single relay is selected. We evaluate the achievable diversity orders over gamma-gamma channels and we derive conditions under which inter-relay cooperation is advantageous. We study two variants of inter-relay cooperation; namely, one-way and two-way cooperation. We prove that based on the network structure, one of these variants, both or neither may be beneficial.