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The paper deals with the problem of estimating the direction-of-arrival (DOA) of multiple digitally-modulated sources. This problem arises naturally when studying positioning applications in the context of cellular systems. The formulation of the optimal second-order tracker is found to depend on the transmitted symbols distribution for medium-to-high signal-to-noise ratios (SNR). Simulations show that the common Gaussian assumption yields significant losses for moderate SNRs if the sources impinge from similar angles and/or the aperture is short. In these circumstances the multiuser interference (MUI) plays a prominent role and MUI-resistant techniques are evaluated.