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In single-user MIMO communication, the first-order throughput scaling is determined by the smaller of the number of transmit and receive antennas. This typically renders terminals the constraining bottleneck. In a multiuser downlink, this bottleneck can be bypassed by having the base station communicate with multiple terminals simultaneously, whereby the receive antennas at those terminals are effectively pooled in term of throughput scaling. This, however, requires that the base has instantaneous channel information. Without such information, the structure and statistics of the channel can be exploited to form multiple simultaneous beams towards the various users, but these beams are in general mutually interfering. Focusing on UMTS HSDPA (high-speed downlink packet access), the paper proposes the use of multiuser detection to discriminate the signals conveyed over interfering beams. This approach is formulated and experimentally evaluated on an HSDPA MIMO testbed that involves a commercial base station, multi-antenna terminals, and custom ASICs.