An Information-Theoretic Comparison between Coherent and IM/DD Transmissions for Free Space Optical Communications

We investigate the performance of free-space optical communication systems in the presence of atmospheric turbulence to assess the advantages that a coherent communication system can bring with respect to a conventional intensity modulation and direct detection (IM/DD) system. The perspective is an information-theoretic one, hence we evaluate the mutual information and the corresponding outage probability of both channels, with various 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 to evaluate the possible shaping gain that can be achieved under different channel conditions. We propose a method to quantify the gain that the coherent solution can achieve, in terms of signal-to-noise ratio (SNR), so that it can be compared, on a techno-economical basis, against the higher cost that it implies.