Sample your points for multiple simulation runs and take the mean and variance to get the probability of error. This should be very easy in matlab (mean and var functions)
Actually, I am considering error only due to other interferering users, not physical noise; such as thermal, shot noises etc. I have only correlations between codes, not out data.
Is there any way to calculate error probability using MATLAB, that will not take too much time for large number of users??