What happens if we radiate 3G and 4G in same frequency? Will it cause interference to each other?
For example if you have 10MHz in total spectrum, usually you would allocate 5MHz for 3G and 5MHz for LTE so you should not see any interference. If you try to radiate both 3G and 4G across all 10MHz simultaneously, then I do not know. You would need to talk to your RAN vendor.
It all comes down to which carriers is the provider allowed (bought) to use.
Map the center frequencies, add 2.5 MHz to both ends (UMTS) and do the same for LTE (changing its length depending on the configured bandwidth) and see if they overlap.
To be absolutely sure that there’s no interference, remember to map both DL and UL carriers.