On the Capacity of Free-Space Optical Intensity Channels

TL;DR

This paper derives new bounds on the capacity of free-space optical intensity channels, considering power constraints and noise, revealing capacity behavior at extreme power levels.

Contribution

It provides the first comprehensive bounds on channel capacity under combined average and peak power constraints, including asymptotic behaviors.

Findings

Bounds converge at high power levels

Bounds become tight at low power levels

Capacity ratios approach one or a constant depending on constraints

Abstract

New upper and lower bounds are presented on the capacity of the free-space optical intensity channel. This channel is characterized by inputs that are nonnegative (representing the transmitted optical intensity) and by outputs that are corrupted by additive white Gaussian noise (because in free space the disturbances arise from many independent sources). Due to battery and safety reasons the inputs are simultaneously constrained in both their average and peak power. For a fixed ratio of the average power to the peak power the difference between the upper and the lower bounds tends to zero as the average power tends to infinity, and the ratio of the upper and lower bounds tends to one as the average power tends to zero. The case where only an average-power constraint is imposed on the input is treated separately. In this case, the difference of the upper and lower bound tends to 0 as the average power tends to infinity, and their ratio tends to a constant as the power tends to zero.