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A novel multiuser detection scheme for asynchronous ultra wideband (UWB) impulse radio systems is proposed. A block-spreading code-division multiple-access (BS-CDMA) system with zero correlation window (ZCW) is designed and applied in UWB systems. It is shown that multiple access interference is completely removed by maintaining code orthogonality, even with asynchronous reception. Furthermore, the UWB channel can be modelled as a block circulant matrix by re-ordering the received sequence. With the advantage of the circulant matrix, a fast Fourier transform (FFT) based minimum mean square error equalization scheme is applied to eliminate the ISI to such an extent that the system performance approaches the AWGN channel. As the large number order of the UWB channel, the FFT based equalization scheme also dramatically decreases the computational complexity.