Guard bnd and interrerence

Hi i am a Masters student in USA ... i am working on my thesis which is on computing Guard band for Interference ... i am unable to find the exact relationship between the guard band and Interference


Lets say interference is some X db between two channels which are transmitting simultaneously then for that X amount of Interference what should be the guard band spectrum to be assigned ... I am looking for the exact formula ...

say like GB = Interference * some relation

Please help me .... its urgent