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In this paper, we present a turbo-detector for coded asynchronous DS-CDMA systems over frequency selective channels. In order to achieve good performance, the receiver has to accurately estimate the channel impulse response. The idea is to iteratively improve the initial channel estimate, obtained using training sequences, by exploiting the soft decisions provided by the decoders. To do this, we propose to use the EM (expectation-maximization) algorithm and to include it in our iterative receiver. Assuming that the receiver knows the channel statistics, we propose to iteratively estimate the channel according to the MAP (maximum a posteriori) criterion using the EM algorithm. Simulation results show that this yields a significant improvement in performance.