Hi Debanjan,
I went through your project.
please notice that the parameter you changes is not the refractive index it is the “INDEX REFRACTION STRUCTURE”
the physical meaning of this parameter is related the VARIATION of the refractive index of the free space due to climate change (dust, rain, solar cells …etc) these change in the refractive index act like a series of lens that causes a fluctuation of the received power with distance especially at longer range (like what you have 10-15 km!!). you can understand that more, if you repeat the simulation many times, you would find different power fluctuations, as expected!!.
Hope that is clear now and beneficial.
Also, attached paper is a very useful and fundamental paper that you can read to widen your knowledge about FSO.
hope that helps,
best regards
Mohamed Abouseif