Reduced-backhaul coordinated beamforming for massive MIMO heterogeneous networks

This paper studies coordinated beamforming design for massive MIMO heterogeneous network where the macro BS (MBS) is equipped with a large antenna array. To suppress the severe cross-layer interference with limited inter-BS communications, especially to avoid the exchange of instantaneous massive MIMO channels, we propose a two-stage coordinated beamforming scheme to realize space division interference mitigation through exchange of hybrid long-term and short-term channel information. In the first stage, we propose a space partitioning codebook for the macro cell with each codebook (beam) covers a certain area. Based on that we remove from the codebook the beams that cause severe interference to the lower-power cells on a statistical or long-term basis. The left beams are then used as the outer precoder at the MBS, which equivalently serves as a dimension reduction matrix and is able to eliminate the interference to the lower-power cells; While in the second stage, the MBS designs its inner precoder based on the reduced-dimensional instantaneous effective channel to improve the performance of its own users. Furthermore, we extend our approach to the case of three dimensional (3D) beamforming, by designing additional beams in the azimuth angel direction. Numerical results show that our proposal is particularly effective when the number of antennas at the MBS is sufficiently large.