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.