To analyze the time- hopping (TH) impulse radio Ultra- Wideband (UWB) transmission performance in the presence of multipath, a model for Bit Error Rate (BER) analysis of UWB systems with coherent demodulation in the presence of inter- symbol interference, intra- symbol multipath interference , and multiple access interference is proposed by studying the statistics of multipath channel models, which is defined by the IEEE 802.15.3a Task Group, and then through calculating the variance of intra- symbol interference, inter- symbol interference and multiuser interference, system BER formulation is obtained and verified in comparison with computer simulations. By analyzing the average multipath interval and comparing BERs with and without intra- symbol interference , the results show that intra- symbol interference should not be neglected. Finally, to satisfy BER requirement, in various channel environments, the maximum transmission data rate is analyzed by both considering and neglecting intra- symbol interference in single user system, and the maximum average transmission data rate is analyzed corresponding to different user numbers in multi- user system .