Upper and Lower Bounds on the Capacity of Wireless Optical Intensity Channels

Improved upper and lower bounds on the capacity of wireless optical intensity channels under non-negativity and average optical power constraints are derived. We consider intensity modulated/direct detection (IM/DD) channels with pulse amplitude modulation (PAM). Utilizing the signal space geometry and a sphere packing argument, an upper bound is derived. Compared to previous work, the derived upper bound is tighter at low signal-to-noise ratios. In addition, a lower bound is derived based on source entropy maximization over discrete distributions. The proposed distribution provides a tighter lower bound compared to previous continuous distributions. The derived bounds asymptotically describe the capacity of PAM optical intensity channels at both low and high SNR.