sir in the case of soa , it is shown that at a distance of 120 km , highest q factor is obtained. so q factor initially increases with length till 120 km and then it decreases. i am attaching the thesis as well as the paper i mentioned.the diagram is the one of optisystem
i wanted to thank you sir. i decreased the dispersion to 2 and i think i am now getting correct results. will update you soon.