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In this paper, we propose to study the effect of the choice of the mapping on the performance of iterative receivers for flat fading channels. We use at the transmitter a BICM (Bit Interleaved Coded Modulation) based scheme obtained by the concatenation of a binary code and a modulator separated by an interleaver. Because of the interleaving, the optimal receiver is too complex. In order to achieve a good complexity/performance trade-off, we propose to use an iterative receiver. The basic idea of the iterative processing is to exchange extrinsic information between the different stages of the receiver to successively improve the performance during the iterations. We consider the case of flat fading channels where a turbo-demodulator is used. An optimization of the mapping is required to obtain a gain by using the iterative receivers. In order to make the mapping choice easier, we propose to perform an analysis based on the Gaussian approximation. This analysis relies on only one simulation of the demodulator.