Spatial Non-Stationary Extremely Large-Scale MIMO with Sub-Connected Hybrid Beamforming: User-Subarray Pairing and Zero-Forcing Precoding

In this paper, we study the downlink transmission of spatial non-stationary extremely large-scale multiple-input multiple-output in the near-field. The line-of-sight channel with uniform spherical wave is assumed in typical mmWave/THz communications, and the sub-connected hybrid beamforming is employed to reduce the energy consumption. Based on these assumptions, we first study the user-subarray pairing. Concretely, the optimal allocation method to maximize the sum-rate is presented, and the signal-to-leakage-ratio based greedy method is presented to maximize the minimal rate. Furthermore, we present a heuristic two-stage hybrid BF optimization with emphasis on the digital beamforming calculation based on the zero-forcing principle. Finally, the effectiveness of the proposed method is verified by computer simulations.