Semi-Static Hybrid Beamforming for O-RAN mmWave Massive MIMO Systems

This work proposes a novel semi-static hybrid beamforming strategy for open radio access network (O-RAN) millimeter wave (mmWave) massive multiple-input multiple-output systems, addressing the limited O-RAN wireless fronthaul (O-WFH) capacity and short wavelength of mmWave bands. Statistical analog precoders are designed at O-RAN radio units to maximize the average signal-to-leakage-plus-noise ratio using statistical channel state information under non-grid-of-beams (non-GoB) constant-modulus constraints. A joint optimization problem of dynamic digital precoders and hybrid combiners is then performed at O-RAN distributed units via a multi-block coordinate descent (MBCD) method. The joint optimization maximizes sum-spectral efficiency (SE) under constraints on transmit power, users' quality-of-service, O-WFH capacity, and non-GoB constant-modulus constraints. The digital processing subproblems are solved using a successive convex approximation with second-order cone programming, whereas the analog combining subproblem employs an MBCD-based Riemannian manifold optimization. Notably, the resulting hybrid combiners are forwarded to users for further processing and service applications. Numerical results reveal that the proposed framework offers superior user SE performance compared to practical analog and digital benchmark schemes, while significantly reducing O-WFH information exchange compared to a theoretical benchmark scheme, with only a small user SE performance loss.