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In order to coexist with existing legacy wireless systems, the transmit power spectral density (PSD) of ultra-wideband (UWB) impulse radio systems is limited. The coverage range of the UWB systems is then confined within a few meters. One possible way to achieve a greater coverage is a short-range multihop relaying strategy. This paper proposes a two-hop UWB relay system using multiple detect-and-forward relays, each of which is equipped with multiple antennas. Each multiple-antenna relay performs spatial diversity reception with partial-Rake scheme in the first hop, and decides whether or not to perform spatial transmit diversity with partial-pre-Rake scheme in the second hop. This leads not only to low complexity at the receiver, but also to reduced error propagation. Moreover, we derive the outage probability of the proposed relay system based on the IEEE 802.15.3a channel model. The effects of using multiple relay antennas, relays, and Rake/pre-Rake fingers on the outage performance are demonstrated. The tradeoffs among the numbers of relay antennas, relays, and Rake/pre-Rake fingers are also investigated. Numerical results show that it is best to use multiple relay antennas to improve the outage performance of the proposed system. Furthermore, in general, increasing the number of relays is more beneficial than increasing the number of Rake/pre-Rake fingers.