Dear Jaafar,
please refer to the example “Division Demultiplexing (two modes) Amplified Transmission.osd” in OptiSystem example library on MDM designs and replace the MMF with FSO component. You need to start with short ranges (100m) and low attenuation depending on the data rate then increase the range.
I hope this example provides guidance to you to start your project.
Regards,
Ahmad