Multi-beam space-time coded systems for optical atmospheric channels

We consider a terrestrial free-space optical communication link affected by atmospheric turbulence. By means of numerical simulations we estimate the reduction in scintillation achieved by the combined effect of aperture averaging and four transmitters when mutually-incoherent sources and a realistic detector size are used. We evaluate two space-time coding schemes: the first scheme uses a repetition code operating on four transmitters, and the second scheme uses a size-four rate-one space-time block code, originally proposed for wireless radio-frequency links. Both schemes deliver large signal-to-noise ratio gains compare to a single-beam system. The first scheme gives the best performance in all cases because of the natural incoherence found among optical sources.