Augmented PPM constellation for enhanced two-user FSO transmissions

In this paper, we consider the scenario of two collocated users communicating over Free-Space Optical (FSO) channels. We propose a novel cooperation scheme where the users jointly communicate symbols from an augmented constellation where each one of these symbols comprises two uncoded symbols each belonging to one of the users. We propose the structure of this 4-ary augmented constellation in the case where the users are initially employing binary Pulse Position Modulation (PPM). A performance analysis is then conducted over gamma-gamma channels reflecting the capability of the proposed scheme in doubling the diversity order for all levels of background radiation.