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This paper discusses the channel capacity of multiantenna ultra-wideband (UWB) indoor wireless systems. It is shown first that for both single-input single-output (SISO) and multiple-input multiple-output (MIMO) cases, when the transmit SNR is lower than some value, say -20 dB, using optimal power spectrum allocation (OPSA) at the transmitter side can increase the reliable transmission rate considerably compared to the uniform power spectrum allocation scheme, while when the transmit signal-to-noise ratio (SNR) is higher than some value, say 10 dB, the benefit of OPSA is very limited. Therefore, measures such as the water-filling algorithm should be taken to make full use of the frequency-selectivity of UWB systems if the transmit SNR is low. Second, in the multiple-input single-output (MISO) case, the outage probability decreases with the number of transmit antennas when the communication rate is lower than some value, but increases instead when the rate is higher than another value. A critical transmission rate is identified. Third, in the cases of SIMO and MIMO, the communication rate supportable by the channel with a given outage probability increases approximately logarithmically and linearly, respectively, with the number of receive antennas.