Do we really need OSTBCs for free-space optical communication with direct detection?

In this letter, we investigate spatial diversity techniques for free-space optical (FSO) links with intensity modulation and direct detection (IM/DD) over log-normal atmospheric turbulence-induced fading channels. We restrict our attention to the deployment of orthogonal space-time block codes (OSTBCs) and repetition codes both of which have been recently proposed for FSO links. Our performance analysis demonstrates that, although both schemes are able to extract full diversity, repetition codes outperform OSTBCs. The performance gap increases with the increasing number of transmit apertures. Our findings clearly point out that deployment of OSTBCs is not necessary for a FSO IM/DD link.