A　bit　error　rate　(BER)　analysis　of　an　asynchronous　DS-BPSK　UWB　system　in　dense　multipath　channels　by　means　of　standard　Gaussian　approximation　(SGA)　and　simplified　improved　Gaussian　approximation　(SIGA)　is　presented.　The　standard　UWB　channel　(proposed　by　IEEE　802.15.3a　task　group)　is　employed　in　the　analysis　to　reflect　the　actual　channel　conditions.　Several　key　parameters　that　affect　the　BER　performance　are　obtained　by　means　of　SGA.　On　the　basis　of　SGA,　the　validity　of　improved　Gaussian　approximation　(IGA)　method　is　established　in　our　systems　and　the　accuracy　of　the　bit　error　rate　expression　obtained　via　SIGA　is　demonstrated　with　Monte-Carlo　simulations.　Besides,　the　impacts　of　several　different　kinds　of　transmitted　pulses　on　BER　performance　are　investigated　with　numerical　examples.