Interference at the radio receiver is a key source of degradation in quality of service of wireless communication systems. This paper presents a unified framework for OFDM/FBMC interference characterization and analysis in asynchronous environment. Multi-user interference is caused by the timing synchronization errors which lead to the destruction of the orthogonality between subcarriers. In this paper, we develop a theoretical analysis of the asynchronous interference considering the multi-path effects on the interference signal. We further propose an accurate model for interference that provides a useful computational tool in order to evaluate the performance of an OFDM/FBMC system in a frequency selective fading environment. Finally, simulation results confirmed the accuracy of the proposed model.