Hello Gurpreet Kaur
I Agree with the explanation given by the Abhishek,The paper and links given above are good.It is general trend in optical wireless channel that with increase in the size of the aperture diameter for both transmitter and receiver,the quality of system increase.However there is disadvantage of large aperture diameter which are cost of the system and designing the whole satellite .As there are many issues related to satellite design,if large the aperture diameter more bulkier the whole setup.Rest you are not considering that facts and simulate the system with upto 50 cm of aperture diameters.