By transmitting unipolar positive signals, a novel association of Alamouti space-time coding to free-space optic(FSO) communication is presented. We examine intensity modulation (IM) based on electrical fast Fourier transform (FFT)design with Hermitian symmetry. The proposed technique aims at limiting signal fading due to scintillation. It maximizes the spatial diversity, increases the reliability, and makes it possible to use digital complex QAM modulations while preserving direct detection (DD).