Study Of Enhanced MU-MIMO Schemes In LTE-Advanced Downlink

To compete with WiMAX in Fourth Generation of cellular wireless networks, LTE has evolved into LTE-Advanced to achieve higher transmission rates, lower error rates, low latency and better Quality of Service. The precoding technology precodes the transmission signals according to channel information to do link adaptations and achieve better system performance. MU-MIMO(Multi-User MIMO) is one of the key technologies in LTE-Advanced. MU-MIMO in LTE-Advanced downlink denotes the MIMO technology by which multiple users receive multiple data streams from base stations with the same time-frequency resources. If it's combined with beamforming technology, then the interference among paired users can be effectively reduced and spectral efficiency can be increased, and correspondingly higher throughput can be achieved. This paper states the principles of MIMO and OFDM, depicts the characteristics and configurations of LTE-Advanced and its channel model. Three schemes are further proposed in LTE-Advanced downlink:the baseline scheme-Codebook-based Precoding SU-MIMO, the two enhanced scheme, e.g., MU-MIMO-based MU-CoMP(Coordinate Multiple Point) and non-cooperative MU-Beamforming schemes. The later two schemes depend on downlink channel information to get joint precoding matrix. In TDD system the base station can get downlink channel information by uplink reference signals from mobile station due to channel reciprocity between uplink and downlink channels, while in FDD system there is no such channel reciprocity, thus the downlink channel information can only be achieved by the downlink average covariance channel matrix in the wideband. Accordingly their scheduling and link adaptations are introduced and their performances are compared by simulation results.