Free-space optical (FSO) communication suffers from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, spatial diversity based on MIMO techniques has been proposed for FSO systems. User Cooperation diversity is a new form of spatial diversity which is introduced to overcome some limitations of MIMO structures. Although the promising effects of cooperative transmission in RF communications have greatly been considered so far, there have been a few notable research on cooperative diversity in FSO. In this paper, we first consider a 3-way FSO communication setup, in which the cooperative protocol can be applied to achieve the spatial diversity without much increase in hardware, compared to their regular transmission. Then, we introduce different cooperative strategies and investigate their bit error rate (BER) performance in the presence of shot noise, using the photon-count method. We compare the results with those of the direct path link (non-cooperative scheme) and the two transmitters case, which are the upper and lower bounds on the BER of the cooperative scheme, respectively. The results illustrate the advantages of cooperation under a number of different scenarios for realistic SNRs.
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