Abstract: Most of current optical wireless communication (OWC) systems use the intensity modulation-direct detection (IM-DD) method to convey information. Existing results on the capacity analysis of OWC channels based on IM-DD cannot give good approximations to the capacity at finite SNR. In the light of this, with duality capacity expression, by choosing an auxiliary output distribution, a new capacity upper bound on the capacity 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. Also, by using the properties of discrete input entropy, combined with maximum entropy argument, a new capacity lower bound is derived. Numerical results show that the lower bound significantly improves the performance of capacity approximation at low and moderate SNR.

Key words: optical wireless communication, channel capacity, intensity modulation-direct detection (IM-DD), information entropy