Beamspace selection in hybrid architectures is an open problem

Determine an effective beamspace selection method for millimeter-wave massive MIMO hybrid beamforming architectures that achieves strong performance under hardware energy constraints that render fully digital architectures infeasible.

Background

Hybrid beamforming architectures reduce the number of required RF chains and data converters by performing part of the beamforming in the analog domain and the remainder digitally. Selecting an appropriate digital beamspace is crucial to exploit sparsity and reduce complexity, yet FFT-based selections can be suboptimal and SVD-based selections can be computationally prohibitive.

The paper highlights that, given the energy constraints of DAC/ADC hardware at mmWave, determining how to select the digital beamspace within a hybrid architecture remains an explicit open problem.

References

A fully digital architecture is not feasible in this case since the digital-to-analog converters consume too much energy, and the selection of beamspace in a hybrid architecture is an open problem.

— Virtual Sectorization to Enable Hybrid Beamforming in mm-Wave mMIMO  (2404.02161 - Bychkov et al., 2024) in Subsection "Hybrid beamforming" (within Introduction)

Prior work on mmWave systems and hybrid beamforming has established the importance of directional beam alignment and low-overhead beam training, but robust beam selection under predictive uncertainty remains a challenging open problem.

— BeamGuard: Risk-Aware Multimodal Beam Forecasting and Adaptive Virtual Beamwidth Control for 6G mmWave V2I Links  (2608.25433 - Orimogunje et al., 26 Aug 2026) in Section I, Introduction