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Multiple access analysis is presented for pulsed direct sequence (DS) and hybrid DS time hopping (DS/TH) ultra-wideband (UWB) communications considering a Saleh-Valenzuela type clustered frequency selective Nakagami fading channel. A general analytical framework is developed for modeling the multiple access interference (MAI) in low-duty systems that is substantially complicated than analysis of more commonly known DS-code division multiple access (CDMA). Using Rake receiver with maximal ratio combining (MRC), we are able to produce a simple error rate expression for general number of combined paths. The theoretical results are then used for performance evaluation and comparison of the two systems. Results show that in multiple access environment, a DS/TH system may gain many fold performance improvement as compared to a DS system.