It is a simple WDM with 18 channels compensated with DCF and amplified with EDFA, depends on the bitrate starting from 50km up to 100km space between amplifiers, I have change all what you suggest but no results, for example for the length of 100km, Bitrate of 40Gbi/s, I can achieve Q factor of Q=16.5 for first output, Q=23 for the fifth output Q=19, for the ninth output Q=10.5 and the last one Q=6.8. Bandwidth of the Mux/Demux I have tried from 3*Bitrate, 2*Bitrate 1.5*Bitrate and same as bitrate to when I use it for the single user Q factor goes up to Q=80 but when it comes to multi users such as 18 channels I do not know what I can say. So please I need answer.