Transceiver Grouping: A Novel Diversity Method for Collaborative Multiuser FSO Communications

In this paper, we propose a novel multipoint-to-multipoint cooperative diversity scheme for multiuser free-space optical (FSO) communications. This scheme is motivated by the inherit immobility and directivity of FSO communications where several transceivers mounted on a given building can be easily interconnected for achieving enhanced diversity levels through joint encoding/decoding. In this case, the N users are clustered in a number of groups depending on the state of the FSO network. Users of a given group transmit the same encoded symbol comprising the information of all these users while maximum-ratio combining is implemented at the receive apertures. We present an asymptotic analysis of the bit error rate and derive the achievable diversity orders over the gamma-gamma turbulence-induced fading channels affected by pointing errors. Results show that while grouping all users in one group achieves the highest diversity order, this partitioning is suboptimal for practical ranges of the signal-to-noise ratio. In this context, the partitioning of the N users turns out to be of crucial importance where the optimal grouping outperforms the all-user grouping by several orders of magnitude.