Relaying techniques for free space optical communications

Abstract

Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading. Relay-assisted systems have been introduced as an effective method to extend coverage and mitigate the effects of fading in FSO links. In this chapter, we have analyzed and investigated the outage performance of relay-assisted FSO links with AF and DF relays. For serial relaying, it has been demonstrated that the outage probability is minimized when the consecutive nodes are placed equidistant along the path from the source to the destination. For parallel relaying, it has been shown that all of the relays should be located at the same place (along the direct link between the source and the destination) closer to the source and the exact location of this place depends on the system and channel parameters. Multi-hop parallel relaying which is the combined use of serial (multi-hop) and parallel relaying for FSO mesh networks has been also studied. Our analysis yields that multi-hop parallel relaying smartly exploits the distance dependency of the fading variance in FSO systems and bring substantial improvements with respect to standalone uses of multi-hop and parallel relaying. As an alternative way of implementation, all-optical relaying has been also considered. Unlike the earlier relaying schemes, the signals are processed in optical domain and therefore the requirement of OE and EO conversions is avoided. Comparisons between conventional and all-optical relaying demonstrate that the latter presents a favorable trade-off between complexity and performance and can be used as a low-complexity solution.