Thanks Alitsu and Sam sung for responding,i am using the 622 Mbps data rate, along with this I am not using any EDFA or dispersion compensation.
length of fiber was 10- km,of smf with dark current 5 nA, thermal noise coeffecient as 1.8 * e-23,0.25 attn,etc.
also i varied i/p power from 10 to 30 micro watt but bit error rate not improved ,the code i am using is MDW.
Thanks & regards.