Partial Dual-Relay Selection Protocols in Two-Way Relayed FSO Networks

In this paper, we present relay selection protocols for parallel relayed free-space optical (FSO) communication network, which consists of two non-line-of-sight terminal nodes and multiple two-way relays (TWR). The TWRs use the two time-phase decode-and-forward relaying protocol to establish a bidirectional link between the terminals. For the considered system, we propose a partial dual-relay selection (P-DRS) scheme, in which two TWRs are selected to participate in the communication process. By using two TWRs, the advantages of cooperative diversity are achieved. For the proposed relay selection scheme and assuming Gamma-Gamma fading channels with pointing errors and path loss, we derive an exact closed form expression for the system outage. Further, we derive an asymptotic expression for average error probability. The diversity order of the system is also presented. It is shown by numerical analysis that by using the proposed P-DRS scheme, the system performance can be significantly improved.