Dual layer beamforming with limited feedback for full-dimension MIMO systems

Eigen-beamforming is an optimal beamforming scheme for full-dimension multi-input multi-output (FD-MIMO). However, it suffers from high complexity of the singular value decomposition (SVD) of the 3D channel matrix. In this paper, we propose a low complexity dual layer beamforming scheme to exploit both the horizontal and vertical degrees of freedom. The proposed scheme first segments the channel matrix into several sub-channel matrices. Then, by selecting the vertical and horizontal beamforming from the sub-channel matrices respectively, the proposed scheme can reduce the complexity, while achieving the performance approximate to the optimal eigen-beamforming scheme. Moreover, to enable practical implementation, a hybrid codebook design is proposed which exploits the structure of the dual layer beamforming to reduce feedback overhead. According to the different correlation characteristics in horizontal and vertical direction, we design different codebooks for the horizontal and vertical beamforming. Simulation results demonstrate that the proposed scheme can effectively improve the spectral efficiency of FD-MIMO systems with limited feedback, especially for the cell edge users.