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A random beamforming technique for MIMO systems that simultaneously obtains downlink multiuser diversity gain, spatial multiplexing gain, and array gain by feeding back only effective SNRs is described. In addition, power control using waterfilling is employed to improve the throughput of our method. In a slow fading channel, we prove that the throughput of the proposed method converges to that of eigen beamforming when many users are in a cell. The number of users required to achieve capacity bound increases with the number of antennas and SNR. However, the capacity bound is achieved even with a small number of users, e.g. 16 users is a cell, when the SNR is low, e.g., 0 dB, and the number of transmit and receive antenna is small, e.g., 2.