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 coordinated multipoint (CoMP) transmission. 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 paper provides a general methodology to analytically compute the expected per-cell spectral efficiency of an 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 can share channel state information to cooperatively transmit signals to their home-cell users. Spectral efficiency of the proposed hybrid beamforming approaches is investigated and compared using two channel models suitable for fifth-generation cellular systems, namely the 3rd Generation Partnership Project model and the NYUSIM model. Numerical results show that the benefits of base station coordination (as compared with the non-CoMP case) are governed by the underlying propagation model, and the aggregate interference levels proportional to the cell radius and number of users per cell. We show that in sparse channels, non-CoMP approaches exceed CoMP (coordinated beamforming) performance.