  Hybrid beamforming (HBF) is a key technology to enable mm-wave Massive
multiple-input multiple-output (mMIMO) receivers for future-generation wireless
communications. It combines beamforming in both analog (via phase shifters) and
digital domains, resulting in low power consumption and high spectral
efficiency. In practice, the problem of joint beamforming in multi-user
scenarios is still open because an analog beam can't cover all users
simultaneously. In this paper, we propose a hierarchical approach to divide
users into clusters. Each cluster consists of users inside a virtual sector
produced by the analog beamforming of an HBF-based mMIMO receiver. Thus, inside
each sector, a lower-cost digital beamforming serves a limited number of users
within the same cluster. Simulations with realistic non-line-of-sight scenarios
generated by the QuaDRiGa 2.0 demonstrate that our methods outperform standard
FFT-based alternatives and almost achieve SVD-based beamspace performance
bound.
