Multiuser Diversity Scheduling in Free-Space Optical Communications

A point-to-multipoint system consisting of an optical transmitter (known as the central node) equipped with apertures, and users communicating over free-space optical (FSO) channels is considered. It is assumed that each link suffers from weak-turbulence conditions. The main goal of this paper is to analyze the performance of the system for various multiuser diversity scheduling schemes in terms of the turbulence strength, from the throughput and the latency perspectives. It is demonstrated that for the proposed system with low turbulence strength, the greedy scheduling scheme (i.e., the central node serves at each time slot the user with the best channel characteristics) achieves the maximum system throughput among various scheduling algorithms at the cost of a large latency. We extend the well-known RF concepts of multiuser diversity to FSO communications. In particular, we show that the Proportional Fair Scheduling with Exponential Rule (PFS/ER) scheme achieves the minimum latency. In addition, the results demonstrate that for point-to-multipoint FSO systems with low turbulence strengths, using the proportional fair scheduling (PFS) has no diversity gain over other schemes. Finally, two new scheduling algorithms are introduced to achieve a tradeoff between the throughput and the latency in FSO-based systems.