You’re right about the acceptable BER range (10^-9 or 10^-12 sometimes). I don’t know why even a back-to-back system in your implementation has 0.5 error, but I used your FSO channel in Optisystem sample (as attached) and then by using amplifiers in between the 1 km distance, the way you can see in my implementation), you can get better results. I did it this way because of the high attenuation of your line.