Joint precoding and power allocation in a FSO channel with signal-dependent noise

To enhance the throughput of visible light communication (VLC), this paper investigates precoding and power allocation jointly in a free space optical channel with signal-dependent noise (SDN-FSO). With the presence of SDN from a light source, a more realistic multi-user multiple-input single-output (MU-MISO) VLC channel model is introduced. First, based on the entropy power inequality, a capacity bound for the SDN-FSO channel is derived under a MU-MISO scenario. Second, due to the nonnegativity of SDN-FSO input, a zero forcing constraint is introduced to alleviate multi-user interference. Third, from the perspective of resource allocation, min-max and total power budget constraints are taken into consideration. A non-convex joint optimization problem is formulated and divided into two subproblems, i.e., the precoding subproblem and power allocation subproblem. Moreover, an iterative algorithm based on a concave and convex program and a simplified high signal-to-noise ratio approximation method are iteratively implemented in the precoding subproblem. For the power allocation subproblem, successive convex approximation is employed to ensure its optimality. Simulation results indicate that the proposed joint optimization methods can converge after about two iterations and accomplish a throughput improvement of up to 1∼3nats/s/Hz as compared with the benchmarks for four different spacing scenarios. When channel state information is interfered with noise, the proposed methods still demonstrate their superior performance with varying degrees of noise.