A Hierarchical Feedback Technique for Multiuser MIMO

This paper proposes a hierarchical feedback technique to solve the problem of massive feedback overhead in multiuser (MU-) multiple-input-multiple-output (MIMO) transmission, which is a serious obstacle to the application of MU-MIMO in real systems, especially with a joint operation of multiuser scheduling and preceding. Motivated by the observations that the multiuser scheduling phase dominates the feedback overhead and it is less sensitive to the accuracy of channel state information (CSI) than the preceding phase, the proposed technique adopts a two-stage feedback method to reduce the feedback overhead in the scheduling phase, meanwhile guarantee good performance of the preceding phase. In the scheduling phase, long-term CSI, such as mean and covariance channel matrixes, are used to reduce the overhead. In the preceding phase, on the other hand, more accurate short-term CSI is still used, since preceding is more sensitive to the CSI accuracy and also has less feedback overhead as only users selected in the scheduling phase are required to feed back their CSI. An angular information feedback technique is introduced to provide the long-term CSI for the transmitter in the scheduling phase, along with a low-complexity scheduling technique based on angular information. Simulation results have shown that the proposed hierarchical feedback technique can significantly reduce the feedback overhead of MU-MIMO transmission, while still achieve most of the performance gain.