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In multiantenna downlink systems the optimization of the spatial precoding is severely hampered by the amount of feedback. An efficient solution consists in the opportunistic beamforming scheme (OB-MUX) that generates random orthogonal beams and schedules the user with the largest signal-to-interference-and- noise ratio (SINR) over each beam. In this paper we explore the effect of feedback quantization on the performance of OB-MUX scheme. We analytically assess the sum-rate of OB-MUX in terms of quantization strategy, number of users and number of antennas. Since the optimum quantization requires a numerical optimization, we design a practical quantization law that approaches the performance of the optimum scheme at reduced complexity. The proposed strategy permits to extract most of the multiuser diversity gain by using a few (in the limit of one) feedback bits.