Hierarchical beamforming scheme based on virtual cell cluster in ultra dense network

Considering virtual cell clusters formed by several overlapping virtual cells in ultra dense network (UDN), virtual cells which are in the same virtual cell cluster coordinate transmissions to reduce inter-virtual cell interference. However, general beamforming scheme design for channel state information (CSI) demand still remains large, and system overhead for high-dimensional channel training and channel state information feedback is explosively-increased. To solve this problem, this paper focuses on hierarchical beamforming scheme based on virtual cell cluster. Specifically, based on perfect intra-cluster CSI, beamforming direction vector is designed by optimizing the achievable rate of each cluster with Zero-Forming block diagonalization method. Based on the large-scale inter-cluster CSI, the strength of beamforming can be devised by solving the sum rate maximization problem and mitigating the residual co-channel interference among different clusters with an iterative power allocation algorithm via a generalized weighted minimum mean square error (WMMSE) approach. Simulation results demonstrate that the proposed scheme achieves significant performance gain and requires less CSI than traditional schemes.