The arrival of 4G technology takes place simultaneously with the technology we have today (2G and 3G). Thus, a natural question is whether they will be able to coexist in the future or will present problems of coexistence. In order to answer these questions, it is necessary to search predicting if there is possibility of failure in the future. And that s how we explain interference, which is a problem that always exists and precludes good quality of service. In Brazil there is discussion about the use of 2.5 GHz frequency for 4G to better meet the users’needs. This will happened if in some way we use other frequencies as the case of 900 MHz band which is considered a hard fought in Europe. There are carriers that have begun to experiment with this band, and this makes us think that in any way we will live with different technologies in the same band. Thus, this work presents a simulation study of several cases of interference, in which each will affect the performance of 4G systems coexisting with older systems.