A New Upper Bound for Free Space Optical ChannelCapacity Using a Simple Mathematical in Equality

In this paper, by using a simple mathematical inequality, we derive a $ new upper bound fkr the capacityof$ free space optical channel in coherent case. Then, by applying general fading distribution, we obtainan upper bound for mutual information in non-coherent case. Finally, we derive the correspondingoptimal input distributions for both coherent and non-coherent cases, compare the results with previousworks numerically and illustrate that our results subsume some of previous results in special cases