Channel making is simple in matlab. You can use built-in rayleigh functions in MATLAB. and then analyse them with the help of plot. Not sure what u mean by "its availability in different areas in cognitive radios"..
You can assume this by enforcing the assumption that the channel at your current location is vacant and it is in use at some other location. This could be one scenario.
Also you can do one more thing that is bit more complex and time consuming. You can generate a signal (a simple interfering signal, using BPSK) and use its statistics in your SINR (remember I is interference and it is from that simple BPSK signal). It will be absent when you want your channel vacant.
Again, the best and easiest way is to make assumption.