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We analyze a cellular communication system in which a base station or access point equipped with several antennas transmits packet data to several mobile data users by means of a TDMA scheme. A scheduler in the base station uses knowledge about the instantaneous channel quality (signal-to-noise ratio, SNR) of all users to allocate to the user the channel resource which maximizes a certain metric. By this means, the system exploits multiuser diversity, which increases system throughput (with constant quality of service) particularly if the rate of variation of the users' SNR is fast. In cases when the underlying physical channel dynamic is insufficient, time-varying beamforming is employed in order to increase the effective channel dynamic. We analyze the implication of time-varying beamforming on coherent channel estimation, which is performed by all users for coherent detection, and discuss fundamental tradeoffs between system (and user) throughput and estimation performance.