Hello Murtadha,
the reason is related to your choice of the TX and RX diameters in addition to the divergence angle and range. If you check equation 1 in the FSO Channel component datasheet there is a term relates all of these parameters that could give you gain or loss to the signal depending on the setup parameters. If the issue is still not clear please email me at ahmad.atieh@optiwave.com and I’ll provide more details.
Regards,
Ahmad