There are plenty of parameters whose changing causes the BER to change as well, within your system design. You can improve launched power by increasing laser power, decrease the number of users in some multiple access systems, decrease BER or/and channel length (at the cost of getting a smaller distance.bitrate product), … . But before doing any of these, you must know why you are designing the system. Are you making the simulation as a part of the university project? What is your objective?