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In this paper, a narrowband interference (NBI) mitigation scheme for ultrawide bandwidth (UWB) signals using multiple receive antennas is examined. The low spatial fading of UWB signals compared to NBI signals is exploited to provide "interference selection diversity (SD)." Whereas classical SD is designed to maximize the desired received signal power, the aim of interference SD is to minimize the effective NBI power. The resulting distribution of the signal-to-interference ratio at the receiver is derived for both Rayleigh and Ricean NBI fading scenarios. Expressions for the probability of error of the SD scheme are also derived for a perfect receiver (where the correlator template is matched to the received UWB pulse shape) as well as for a Rake receiver with a limited number of fingers. It is shown that doubling the number of antennas results in a 3-dB performance improvement for the Rayleigh fading case. Less substantial gains are observed under Ricean fading. The method provides a simple but effective technique to mitigate NBI effects in UWB systems.