The iterative detector and decoder (IDD) in the serial concatenated coded and pre-transformed system is studied in this paper. The performance of the IDD in the single-carrier system over flat fading channel and the orthogonal frequency division multiplexing (OFDM) system over frequency-selective fading channel are thoroughly evaluated. Numerical results show that in the single-carrier system, IDD with 2 to 3 iterations over flat fading channel achieves performance only about 1dB away from that

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... over the additive white Gaussian noise (AWGN) channel. Our study also finds that for OFDM system, the IDD provides considerable performance improvement compared to the coded or pre-coded systems, even when the multipath diversity is low. In the meanwhile, the excellent performance of IDD does not rely on the optimal design of pre-transformation matrix.