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The well-known vertical Bell Labs layered space-time (V-BLAST) ordering has been widely employed for tree search algorithms such as sphere decoder. The V-BLAST ordering was derived under the assumption that all transmitted symbols utilize the same modulation. In an interference-limited scenario, a cell-edge user probably receives serving and interfering signals with different modulations. In this case, the conventional V-BLAST ordering does not provide a satisfactory solution for complexity reduction. This letter shows how to incorporate the modulation information into the V-BLAST ordering to significantly reduce the complexity of the tree search algorithms. Simulation results show the new ordering based sphere decoder can provide a better trade-off between complexity (e.g., in terms of operations) and performance (e.g., in terms of throughput or error rate) in the interference-limited scenario.