Spatial-mode multiplexing with mutually coherent channels in free space optical communications

In this paper, the spatial-mode multiplexing (SMM) free-space optical communication (FSO) systems with direct detection are considered. Unlike systems studied in several works in the literature where mutually incoherent channels are assumed, we investigate high-speed SMM FSO systems with mutually coherent channels where a single laser source with narrow linewidth is employed at the transmitter. An analytical model for such mutually coherent channels is developed and expression for aggregate achievable rate (AAR) is derived. Through numerical simulations, it is shown that an optimal transmitted mode set can be determined which results in the maximal asymptotic AAR at high transmitted power and for different turbulence conditions, the optimal mode set varies.