In that paper, the number of users is 5 and the code weight is 4. Having these numbers in mind, try to simulate the scheme depicted in figure 6 of the paper. Try reading the simulation part of the paper before implementing it. here’s another picture from another paper, with W=4 and N=2 which is close to your own simulation. FBGs are uniform. Take a look at this and its paper “Simulation of a SAC-OCDMA 10 User ×15 Gb/s System Using MD Code”, which expalains the same stuff again. try to make something out of these two.