A site-specific propagation prediction for UWB impulse signals within indoor non-line-of-sight (NLOS) environment is addressed in this paper. The transmitter and the receiver are placed in two rooms respectively with a concrete wall blocking the line-of-sight (LOS) propagation path. The simulation utilizes the principles of geometrical optics (GO) for reflected and transmitting paths' tracing and time domain technique for describing the transient electromagnetic field transmitting through as well as reflected from the wall, the floor, the ceiling, and the objects. Based on the simulation results, the time dispersion as well as antenna effect for the received impulse signals within the NLOS environment is analysed.