This answer is tied to all your questions Alistu, Gurpreet and Samsung:
This gets deeper into signal theory, but the Layout Parameter Bit rate is tied to the Sample rate. Using the Samples per bit parameter in conjunction with the Bit rate the correct Sample rate is computed. Sampling is how the “real analog” signal is interpreted into a digital signal that the program can work with. Your sample rate has an effect on the largest frequency that the digital signal can represent, called the Nyquist rate. Since in our simulation our frequency signal is upconverted to the optical frequency you might say the sampling rate restricts the total bandwidth that can be represented.
What this means for this particular example is that while the bit rate per user is 200 Mbps, the bandwidth required to represent this signal is actually QUITE larger, since there are three users AND because of the format SAC-OCDMA being used. This is why lowered the bit rate changes the simulation, the sample rate is also reduced and now the signal cannot be represented entirely. However, you could lower the Bit rate layout parameter and increase the Samples per Bit parameter to balance it out.