Combined Vector Perturbation for Adaptive Modulation in MU-MIMO

We propose in this paper a combined vector perturbation (Comb-VP) precoding for multi-user multiple-input multiple-output (MU-MIMO) downlink systems. It takes into account different modulation coding schemes (MCSs) and enables an adaptive modulation scenario where users apply different modulation types. The search of the perturbation vector is combined for all users as in the conventional VP (Conv-VP), but different modulations are used simultaneously for different users. The performance of the combined VP is optimal compared to existing solutions. In this setting, we also propose the combined minimum mean square error (MMSE) VP which achieves better performance in terms of error rate by minimizing the MSE criterion. In addition, we suggest an user ordering according to their modulation size. Indeed, by starting with the highest modulation order in the search tree of the sphere encoder (SE) algorithm, this has the advantage of reducing the complexity by minimizing the size of the search tree in the average sense.