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The paper investigates the performance of adaptive space-division-multiplexing and bit-and-power allocation in multiuser multiple-input multiple-output (MIMO) flat fading broadcast channels. A previously proposed orthogonal space division multiplexing (OSDM) method is utilized to decompose jointly the broadcast channel into many single-user MIMO channels so that conventional bit-and-power allocation can be readily used for minimization of transmit power given the data-rate and bit-error-rate (BER) requirements of the users. The difficulty, however, is that OSDM cannot be done without prior knowledge of the number of the users' spatial channels, which itself has to be optimized. To this end, we begin by showing that the optimal number of operating spatial channels is insensitive to the channel state information (CSI), but greatly dependent on their data-rate requirements. We then develop a simple criterion that allows us to perform adaptive space-division-multiplexing at much-reduced complexity while maintaining the same performance as a brute force approach. The overall optimal allocation can be accomplished in two steps. Results demonstrate that there is a significant gap between performance with and without adaptive spatial multiplexing.