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This paper studies the effect of imperfect channel state information (CSIT) on the performance of multiple-antenna downlink scheduling for time-division-duplex system. The base station is equipped with nT transmit antennas and there are K mobiles in the system, each with single receive antenna. We propose an analytical framework to study the multiantenna downlink scheduling problem in the presence of imperfect CSIT at the base station. We found that with ideal CSIT, the space-time scheduler can achieve significant system capacity with respect to nT due to increased spatial channels. However, at moderate CSIT estimation error, the system performance using the ideal scheduling algorithm (designed for perfect CSI) saturates quickly with respect to nT and signal-to-noise ration (SNR) increases. This is because in the presence of imperfect CSIT, the scheduled data rate of a user may exceed the actual channel capacity and resulting in packet transmission outage. To realize the potential spatial multiplexing gain over nT and SNR, we propose an analytical framework to take into account of the packet transmission outage and propose scheduling solutions matching the imperfect CSIT situations. We found that, by considering the statistics of CSIT errors into the design, the proposed schemes provide significant system performance enhancement.