Binary signaling over the IEEE 802.15.3a ultra-wideband channel model is considered. For received waveforms observed during a finite time window, a formula is derived for the average bit error probability (BEP). The formula can be rapidly evaluated using Gaussian quadratures, and is used to study average BEP curves as a function of window size, which is then related to the percentage of energy captured. These techniques should be useful to system designers in assessing the performance impact of various signaling rates, guard times, etc.