Do We Really Need OSTBCs for Free-Space Optical Communication with Direct Detection?

TLDR

The authors examine spatial diversity methods for free‑space optical links using intensity modulation and direct detection over log‑normal atmospheric turbulence channels. They compare orthogonal space‑time block codes and repetition codes, both recently suggested for such links. Analysis shows that while both schemes achieve full diversity, repetition codes consistently outperform OSTBCs, with the performance gap widening as the number of transmit apertures increases, indicating OSTBCs are unnecessary for FSO IM/DD links.

Abstract

In this letter, the authors 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.