Most of the current optical wireless communication (OWC) systems use the intensity modulation-direct detection (IM-DD) method to convey information. The existing results on the capacity analysis of OWC channels based on IM-DD cannot give good approximations to the capacity at finite signal-to-noise ratio (SNR). In consideration of OWC with non-negative input signals subject to an average optical power constraint, with duality capacity expression, by choosing an auxiliary output distribution, a new capacity upper bound on the capacity is derived. Also, by using the properties of discrete input entropy, combined with maximum entropy argument, a new capacity lower bound is derived. Compared with existing capacity bounds, numerical results show that the upper bound gives better approximation to the capacity at moderate and high SNR, and is asymptotically tight at high SNR. The lower bound significantly improves the performance of capacity approximation at low and moderate SNR. © 2024 Beijing University of Posts and Telecommunications. All rights reserved.
