We analyze the performance of ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems employing various modulation and multiple access (MA) schemes including time-hopping (TH) binary pulse-position modulation (BPPM), TH binary phase-shift keying (BPSK), and direct-sequence (DS-) BPSK. We quantify the performance merits of UWB space-time (ST) systems regardless of specific coding scheme. For each modulation technique, we introduce a framework that enables us to compare UWB-MIMO systems with conventional UWB single-input single-output (SISO) systems in terms of diversity and coding gains. We show that the combination of ST coding and RAKE receiver is capable of exploiting spatial diversity as well as multipath diversity, richly inherent in UWB environments. In addition, we adopt the real orthogonal design (ROD) as the engine code for UWB-ST codes. We find the upper bound of the average pairwise error probability (PEP) under the hypothesis of quasistatic Nakagami- m frequency-selective fading channels. The performance comparison of ROD-ST codes with different rates is also addressed. Finally, simulation results are presented to support the theoretical analysis.