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A novel low complexity scheduling algorithm was proposed for multi-user MIMO downlink channels. Considering the scenario that zero-forcing beamforming is employed at the base station, the number of simultaneously supportable users is limited by the number of transmit antennas at base station and user receive antennas. Thus the base station may select a subset of users to serve. Exhaustive search for the optimal user set, however, is computationally prohibitive. Based on the orthogonal projection operators of subspaces, we proposed a sub-optimal user selection algorithm using the distance metric between subspaces spanned by the vectors of users' channels. Furthermore, an equivalent version of this algorithm was presented after proving the equivalence between the distance metric and the correlation metric. Simulation results show that this algorithm performs very close to the capacity-based algorithm with much lower computational complexity.