Study Diversity Multi-user Wireless System

In multi-antenna transmission system, the spatial degrees of freedom of usingmulti-antenna reception and multipath propagation effects of radio channel is the emergingand development base of diversity technology. The diversity technology in MIMO systemincludes space-time coding techniques and Beam-forming technology, which can also be usedin collaborative system, forming distributed space-time coding techniques and distributedbeamforming technology. This article focuses on analysis and research on the classicalspace-time codes-Alamouti Codes in traditional MIMO system and distributed beamformingtechnology in collaborative system.It has a research on system gain of Alamouti code under different channels in traditionalMIMO system, and derives closed solution of system gain achieved by using theOrthogonality of Alamouti scheme under the condition that sending end dose not has thechannel state information. The article also make a comparison with major characteristic---Maximal Ratio Combining scheme under the condition that the sending end with the channelstate information, and the result shows the same diversity gain under the two circumstanceswhile the latter that under the condition of known channel information will get3dB array gainmore than the former.In Cooperative Communications, imperfect global channel state information (CSI) leadsto performance degradation. To address this issue, we introduce the beamforming algorithmwith dual constraints that accounts for uncertainties in the global CSI using worst-case designideas in amplify-and-forward relay networks. The semi-definite programs not easy to solvecan be converted into quasi-convex feasibility programs through the algorithm which uses theproperties of convex function and extended S-lemma. The optimal solution can be obtainedby bisection method. The domain of convergence is built by the method of importancesampling, and the search range can be reduced under the conditions that ensure the domain of convergence is a convex set. Simulation results reveal that the optimization algorithm that weintroduced is to improve the system gain through cooperative diversity and to resistattenuation effects effectively caused by CSI deviation on system performance.