Relay selection strategy for parallel relayed FSO systems

This paper describes a coherent tree-space optical (FSO) communication system consisting of multiple parallel relays and two terminal nodes. When direct communication between the two terminal nodes of the network is not possible, one relay is selected to establish a dual-hop communication link between them. In order to best utilize the channel resources, the relay that has the best channel characteristics is selected by the terminal nodes. An atmospheric turbulent optical channel that is modeled by the Gamma-Gamma distribution and affected by misalignment errors and path loss is considered for the analysis. The closed-form expressions for the system outage probability and average bit error probability are derived. Furthermore, to provide a physical interpretation of the closed form expressions, we derive the asymptotic expressions and the diversity order of the system. It is observed, by employing relay selection a significant increase in diversity gain is achieved.