This paper analyzes the receiver's ability to differentiate between multiple users in ultra-wideband (UWB) impulse radio (IR) during the initial synchronization phase after a cold start, i.e., when the receiver is not yet synchronized with the user of interest. Because of multiple access interference (MAI), the receiver may combine the pulses of other users in a partially coherent way. In this case, the receiver should be able to reject all the interfering users in order to synchronize with the user of interest. For pseudo-random time-hopping (TH) spreading codes, the expected performance of the receiver to differentiate between the user of interest and the interfering users is analyzed and some relations with the code properties are shown. Numerical results for a multipath propagation channel and MAI are provided.