Joint multi-user beamforming for hybrid beamforming remains open

Develop a joint analog–digital beamforming strategy for multi-user millimeter-wave massive MIMO hybrid beamforming receivers that can simultaneously serve spatially distributed users despite the constraint that a single analog beam cannot cover all users at once.

Background

Millimeter-wave massive MIMO systems commonly adopt hybrid beamforming to reduce hardware cost and energy consumption by combining analog beamforming via phase shifters with lower-dimensional digital processing. In multi-user scenarios, however, users are often distributed across different angles of arrival, and a single analog beam cannot provide high gain for all users simultaneously.

This limitation makes the joint design of analog and digital beamforming particularly challenging. The paper motivates this open problem and proposes a clustering-based virtual sectorization approach as a step toward addressing it, but the general multi-user joint beamforming problem remains explicitly stated as open.

References

In practice, the problem of joint beamforming in multi-user scenarios is still open because an analog beam can't cover all users simultaneously.

— Virtual Sectorization to Enable Hybrid Beamforming in mm-Wave mMIMO  (2404.02161 - Bychkov et al., 2024) in Abstract

We conjecture that this gap can be further shortened by designing new GNN architectures to explicitly model the inter-user interference. The exploration of this direction is left for future work.

— Efficient Graph Neural Networks for Multicarrier Wideband Hybrid Beamforming Optimization  (2609.09708 - Li et al., 9 Sep 2026) in Section 9, subsection 5, “Generalization over the Number of Users”