The Key Techniques Of MIMO Research In Wireless Communication Systems

At present, with the integration of internet and multimedia applications in the wireless communications, the demand for wide-band high data rate communication services is growing. Both of high quality and high data rate will be required in next generation mobile communication system, but the performance of which is restricted by the fading effects of multi-path propagations, so it becomes very important to keep high quality and enhance the spectral efficiencies of wireless communication techniques. The multiple-input multiple-output (MIMO), which is the most promising techniques to cope with these problems, has been adopted by the Enhanced 3G stardands (such as 3GPP LTE) and drawn much attention in the 4G systems.In this dissertation, the MIMO system models, the principle of open-loop and close-loop MIMO technologies, the channel capacity of single-user and multi-user are introduced at first. And then, some key techniques for MIMO technology are researched, including the multi-user MIMO procoding technology, information feedback technology, inter-cell interference suppression technology as well as channel reciprocity technology of TDD MIMO system. Finally, after a short summary, an analysis of future research direction shows some of the interesting topics which can be explored in the future. Major contributions of these research works are summatized as following. For the MU-MIMO precoding system based on codebook, an adaptive transmitting mode selection scheme is proposed. In this scheme, based on maximum sum-rate, mobile stations determine their optimal mode and user set, and feedback to base station. According to the channel information from mobiles, base station analysis the CSI from users, and then uses this proposed scheme to select the optimal mode and user set. The sum rate ceiling can be avoided when high SNR and low user number, and the overall spectral efficiency of system is optimized. And this scheme achieves high throughput and gets unbounded capacity growth when large user number and high SNR.For the research of CSI feedback technique of the MIMO precoding system, some kinds of CQI feedback schemes are introdued and analyzed, and a novel feedback scheme combining HARQ with multiuser scheduling is proposed. HARQ ACK/NACK in this scheme is not only used to control (re)transmission in HARQ processes, but also involved in multiuser scheduling processes to help select appropriate users among multiple users. Base station uses the ACK/NACK information to analysis the channel condition and users pairing state, and determines when and how to feedback CQI/PMI and select users. This algorithm can provide considerable gain, reduce feedback load, decrease calculating complex and suppressing the CCI.For inter-cell interference suppression of MIMO system, we propose a new inter-cell interference mitigation solution which uses multi-cell collaborative transmission technology. The main BS communicate with the cell-edge user using closed-loop MIMO technique, while the coordinated BS communicate with the cell-edge user using open-loop MIMO technique. The biggest advantage is the complete elimination of inter-cell interference and the achievement of the open-loop gain with no additional feedback overhead at the same time. Moreover, it can increase the coverage of the network, and provide users with seamless connectivity.For channel reciprocity of TDD MIMO system, a method to use channel reciprocity is proposed. Based on the TDD LTE system, we analyze the frame architecture, system parameters and polit architecture, determine the factors of affecting when getting channel reciprocity, and then the specific channel reciprocity criterion is proposed. The interference encountered for the uplink and downlink is typically different from each other because a UE might have strong directional interference from neighboring eNB in downlink, so this method analyzes the asymmetric interference and alleviates the difference of UL and DL interference. Basing this method, we can using efficient channel reciprocity, and reduce CSI feedback load.