Analytical Framework of Hybrid Beamforming in Multi-Cell Millimeter-Wave Systems

Multi-cell wireless systems usually encounter both intra-cell and inter-cell interference, which can be mitigated via base station coordination. Previous works on multi-cell analysis in the microwave band generally consider fully digital beamforming, requiring a complete radio-frequency chain behind each antenna. This is practically infeasible for millimeter-wave (mmWave) systems where large amounts of antennas are necessary to provide sufficient gain and to enable transmission/reception of multiple streams to/from a user. This article provides a general methodology to analytically compute the expected per-cell spectral efficiency of a mmWave multi-cell single-stream system using phase-shifter-based analog beamforming and regularized zero-forcing digital beamforming. Four analog-digital hybrid beamforming techniques for multi-cell multi-stream mmWave communication are proposed, assuming that base stations in different cells share channel state information to cooperatively transmit signals to their home-cell users. Spectral efficiency of the proposed hybrid beamforming approaches are investigated and compared using two channel models suitable for fifth-generation cellular systems, namely the 3rd Generation Partnership Project model and NYUSIM model. Numerical results show that the benefits of base station coordination (as opposed to the no-coordination case) are governed by the underlying propagation model, and the aggregate interference levels proportional to the cell radius and the number of users per-cell.