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In this work, we propose an adaptive linear interference suppression scheme based on single-carrier frequency domain equalization (SC-FDE) for multiuser direct-sequence ultra-wideband (DS-UWB) systems. In a block-by-block based transmission, by inserting the cyclic prefix (CP) between each data block, the inter-block interference (IBI) can be ignored and the Toeplitz channel matrix can be expressed as an equivalent circulant channel matrix. With this feature, we propose a simple signal model, in which only a vector formed adaptive filter needs to be trained in the frequency domain. Based on the new signal model, an adaptive block conjugate gradient (BCG) algorithm is developed. Complexity analysis shows that the proposed BCG algorithm is significantly simpler than the recursive least squares (RLS) algorithm. The bit-error rate (BER) performance of the proposed scheme is compared with the least-mean-square (LMS) and RLS algorithm in a number of scenarios and the simulation results show fast convergence speed and excellent performance of the proposed BCG algorithm.