Muting-Based Partially Non-Orthogonal Block Diagonalization in Multiuser MIMO with Limited Channel State Information Feedback

This paper investigates a precoding method in downlink multiuser multiple-input multiple-output (MIMO) transmission employing multiple base station (BS) cooperation where the sets of user equipment basically feedback the instantaneous channel state information (CSI) only to the nearest BS, but the users near the cell edge additionally feedback the instantaneous CSI to the second nearest BS among the cooperating BSs. The proposed precoding method is categorized into multi-cell processing, in which the transmission information sent to a user is shared by the cooperating BSs in order to utilize fully the degrees of freedom of the spatial channel, and is based on block diagonalization of the channel matrix. However, since some elements of the channel matrix are unknown, we allow inter-user interference to users with limited instantaneous CSI feedback. More specifically, in the proposed method, the elements corresponding to the unknown instantaneous CSI in the overall channel matrix are muted by setting these elements to zero, and the standard generation procedure of the block diagonalization precoding matrix is performed on the muted channel matrix. The proposed method efficiently utilizes the degrees of freedom of the spatial channel compared to the case with full orthogonal transmission at the cost of increased inter-user interference. Simulation results show that the proposed method decreases the transmission power for a given capacity requirement compared to the case with full orthogonal transmission.