Cooperative FSO systems: Performance analysis and optimal power allocation

Abstract—In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-in-duced fading over free-space optical (FSO) links. In particular, we propose a novel cooperation strategy that is suitable for quantum-limited FSO systems with any number of relays and we derive closed-form expressions for the error performance of this strategy. In scenarios where the channel-state-information (CSI) is available at the different nodes, we propose an optimal power allocation strategy that satisfies the Karush-Kuhn-Tucker (KKT) conditions and that further boosts the performance of FSO networks. It turned out that this closed-form optimal solution corresponds to transmitting the entire optical power along the “strongest link ” between the source and the destination nodes. A simple procedure is proposed for selecting this link and for distributing the power among its different hops. Index Terms—Atmospheric turbulence, cooperative diversity, free-space optics, power allocation, spatial diversity. I