Cooperative System in Free-Space Optical Communications for Simultaneous Multiuser Transmission

Driven by the limitation that the intrinsic of conventional cooperative communication is utilizing the spare transceivers at the other nodes, we design a novel cooperative protocol in free-space optical communication. The designed protocol permits that the user with favorable channel conditions can receive its own message, and at the same time, it can be utilized as the relay for providing cooperative diversity for the others. A novel closed-form asymptotic expression of outage probability is derived, following a gamma-gamma model in the presence of pointing error and path loss. Based on the obtained result, the diversity gain is analyzed, which shows that the diversity gain is highly dependent on the location of users, and the proposed protocol can enhance the performance of users. Furthermore, the behavior of ergodic data rate is analyzed at large signal-to-noise ratio. All the results are corroborated by the Monte Carlo simulations, and the results show that the proposed system outperforms the conventional system in terms of ergodic sum rate.