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In this paper, we propose an improved pre-combining group detection in the presence of improper multi-access interference (MAI). We consider a transmission of K BPSK signals over M diversity channels in rich scattering environments. The different received signals are grouped at the receiver, and detected jointly in groups using maximum likelihood (ML) detection. The novelty of the proposed scheme consists in the proper use of the information contained in the MAI statistical properties. We show that the proposed receiver achieves higher diversity order gains than conventional group detectors. For example, a gain of the order of |G~|/2 is obtained for the group G compared with the conventional receiver, where |G~| is the interferers number. Particularly, when the group size is one, the proposed receiver is equivalent to a decorrelator that can out (K-1) interferers while providing a diversity order of (M- (K-1/2) for each signal. Exact pairwise-error probability along with bounds for the bit-error rate are derived for each group. Numerical results will also show that optimum performance is closely approached when the proposed architecture is used.