Simulation at Baseband or Passband A naive question I guess. Would it make a difference in the results when a system is simulated at Baseband or at Passband. If I have a plot of a BER vs SNR curve that has been plotted at Baseband, will the same change if I perform the simulation at Passband with a carrier frequency of 2.4GHz? AFAIK, it should not make a difference, but I would like to know if what I am thinking is correct.

Another question is that if I have two signals from two transmitters and add these signals at the receiver, does it make a difference if these two signals are passband or baseband. Do the results vary? Since Passband simulation consumes a lot of time, would it be correct if I simulate the behavior at Baseband and assume that the result would be almost the same at the Passband?
The only thing I could see, with the information you gave, is that the way you get from passband to baseband and vice-versa could affect your performances. It should not, but it could. 

Let's say you have to down sample to get to baseband, and up sample to do the contrary and you use filters to do so. If your filters have variations of 1dB in their passband and a cutoff frequency that cuts some of your baseband signal, then your performances could be deteriorated. In an ideal situation it should not, but some considerations (restriction on the number of coefficients of your filters, for example) may imply some performance degradation.