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In this study, we propose a practical adaptive multiple-input multiple-output (MIMO) transmission approach for broadband wireless multiuser systems. The presented scheme adaptively switches between open-loop and closed-loop MIMO modes depending on the users' channel conditions and required rates as a means to enhance the system throughput and performance. Each mode is a combination of a transmission technique (space-time block coding and/or spatial multiplexing or beamforming) and a modulation/coding scheme. We first evaluate the spectral efficiency of each mode and provide a simple way to select adaptively the best combination of code rate and modulation scheme for open-loop and closed-loop transmission modes in a Rayleigh/composite fading environment. Based on these results, we propose a simple and practical switching criterion that is shown to yield significant spectral efficiency improvements over independent adaptive open-loop or closed-loop schemes for typical multiuser system scenarios. Simulations in an IEEE 802.16e framework are provided and illustrate significant improvements in throughput performance and service fairness.
Date of Publication: Sept. 5 2011