Recent years have witnessed fast and tremendous growth in the wireless communication industry. Future wireless communication systems are expected to provide a wide variety of new services, from high-quality voice to high-definition video and wireless multimedia for anyone, anywhere at anytime. This results in ever increasing demands for high data rate through harsh wireless channels over scarce frequency spectrum and is the key challenge in wireless system design. In this work, new and novel downlink multi-user MIMO systems are described and investigated. The ultimate goal of this work is to 1) investigate the impact of various realistic considerations, e.g. spatial fading correlation, imperfect CSI, on system performance and develop adaptive algorithms to improve system performance under these considerations; 2) investigate methods to further improve the spectrum efficiency by exploiting the synergy between the PHY layer and the MAC layer. The investigation and analysis of this work may be helpful in defining MU-MIMO systems for different scenarios and should be especially useful to professionals in the wireless communications area.