Outage analysis of relay-assisted free-space optical communications

Despite their attractive features, free-space optical (FSO) communications suffer from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, multiple input multiple output (MIMO) and cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in radio frequency (RF) communications have greatly been investigated, there have not been remarkable researches on cooperative transmissions in FSO communications. In this study, the authors consider a relay-assisted FSO communication and investigate the outage performance of two well-known cooperative protocols, namely amplify-and-forward (AF) and decode-and-forward (DF), for this communication system. By a new definition of the order of diversity, the authors derive an analytical expression for the performance comparison of different cooperative and non-cooperative protocols. The authors also evaluate the performance of a FSO communication with two transmitter telescopes (MIMO systems) and compare its outage probability with those of cooperation strategies. The authors investigate the best place for the relay for both the AF and DF strategies and show that relay-assisted techniques can be used for FSO communication to achieve spatial diversity. Their performance can surpass that of the two-transmitter case by properly choosing the location of the relay.