Two-by-two user grouping for enhanced multipoint-to-multipoint Free-Space Optical communications with Pulse Position Modulation

In this paper, we consider the scenario of N collocated users communicating over Free-Space Optical (FSO) channels. We propose a novel cooperation scheme where the users are grouped together in groups of two for the sake of achieving enhanced diversity orders in a simple manner. In this context, instead of communicating independent binary Pulse Position Modulation (PPM) symbols over each of the N available links in a non-cooperative manner, the information from the different users will be combined and encoded in a joint manner. We propose a method for performing the joint encoding and we propose the structure of the 4-ary augmented constellation to be associated with the proposed cooperation scheme. A performance analysis is conducted over gamma-gamma channels reflecting the capability of the proposed scheme in enhancing the diversity order for all levels of background radiation. In particular, we prove that the diversity order is enhanced by the factor ⌊N2⌋+1.