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For multiple input multiple output (MIMO) systems the link adaptation (LA) can significantly increase the system throughput. In the existing scheme of spatial multiplexing (SM), the link adaptation has been done by adaptive modulation and coding (AMC) on every sub-channel with QoS limit. However, the throughput loss occurs at lower SNR because the SM has no transmit diversity (TD). In this paper a novel LA strategy is proposed to solve the problem mentioned above by combining TD and SM over space. This combining scheme is adaptively to find the optimal spatial group scheme of the transmit antennas based on the derived post detection SNR and to do SM with them, which may use space-time block codes (STBC) with the QoS limit. The simulation results show that, with low complexity, this method can outperform the LA algorithm in conventional multiplexing scheme in the throughput performance and obtain higher transmit power utilization and decrease the number of invalid antennas.