Coherent vs IM/DD Transmissions for FSO Communications in the Presence of Atmospheric Turbulence

Abstract:
We investigate the performance of free-space optical communication systems in the presence of atmospheric turbulence with the purpose of assessing the advantages that a coherent communication system can bring with respect to a conventional intensity modulation/direct detection system. The perspective is an information theoretic one, hence we evaluate the mutual information and the corresponding outage probability of both channels, with traditional symbol constellations, as a pragmatic approximation to the capacity, or to the outage capacity, of those channels. In addition, we analyze non-uniform symbol constellations, in order to evaluate the possible shaping gain that can be achieved under different channel conditions.