Research On User Selection Scheme For Multi-user MIMO Downlink Systems

With the rapid development of wireless communications, the significant changes have occurred in lots of different fields. But the quickly increasing of communication operations and the number of users made the limited wireless resource a huge challenge. So, multiple-input multiple-output (MIMO) communication techniques have been an important area of focus for next-generation wireless systems because of their potential for high capacity, increased diversity, and interference suppression.Making the most of spatial resource, MIMO systems can do well in withstanding wireless channel eclipse, enhance the system capacity and improve the performance, but they don't use more system bandwidth and transmitting power. For applications such as wireless LANs and cellular telephony, MIMO systems will likely be deployed in environments where a single base must communicate with many users simultaneously. As a result, the study of multi-user MIMO systems has emerged recently as an important research topic.Multi-user MIMO systems can make use of space-division multiple access to achieve higher channel capacity compared with single-user systems, since no extra time slots and frequency resources are needed to support more users. However, the number of simultaneously supportable users is limited by the number of transmit and receive antennas. Therefore, the base station practically must select the available subset with some useful multi-user selection algorithms. Otherwise, in order to ensure capacity performance or to satisfy QoS requirement (SINR, fairness etc.), scheduling is required to manage the users' access to the resources.The hypothesis that base station can acquire full channel station information (CSI) is the precondition of the research above, but it is impossible in real systems. So in recent years, the more actual system models have attracted more and more research attentions, for example, the performance of multi-user MIMO systems and the algorithms to eliminate inter-user interference and so on. Considering both perfect CSI and partial CSI, the respective multi-user selection algorithms are studied in this dissertation. In general, this dissertation mainly discusses as follows:(1) In case of perfect CSI, by considering multi-user MIMO downlink systems, an upper bound of sum-rate capacity is given, and then a suboptimal adaptive user selection algorithm is proposed. Regard current capacity and the upper bound as the adaptive criterions. The performance of the proposed algorithm is near to the optimal method in different SNRs and with different number of users. Based on this method, a fair scheduling scheme is further given to ensure fair service to all users in the system. Users with poor channel condition can also be served while keeping relatively high data rate.(2) In case of partial CSI, an antenna feedback algorithm is proposed, so that, the multi-user MIMO systems transform to the multi-user multiple input single output (MISO) systems. In this equivalent system model, current capacity-based algorithm and channel correlation-based algorithm are used to select available user subset. The performance of the former one is enhanced obviously with the same computational complexity and the number of bits back to the base station. The latter one is a litter worse, but its computational complexity reduces a lot. The two different algorithms are conformable to different actual systems.