Multi-RIS-Aided Cell-Free mmWave Massive MIMO With Short-Packet uRLLC: Passive and Hybrid Active Beamforming

Simultaneously meeting high spectral efficiency (SE)-oriented enhanced mobile broadband (eMBB) and ultra-reliable and low-latency communication (uRLLC) remains a significant challenge. Given uRLLC requirements, this work investigates the problem of sum-SE maximization in multiple reconfigurable intelligent surfaces-assisted cell-free (CF) millimeter wave massive multiple-input multiple-output systems. The joint optimization problem of passive reflective beamforming (RBF) and hybrid active transceiver beamforming is tackled using a multi-block coordinate descent (MBCD) method, under constraints of transmit power, users’ quality-of-service, and constant-modulus characteristics. The subproblems involving analog transceiver processing and passive RBF, associated with constant-modulus constraints, are solved via MBCD-based Riemannian manifold optimization, while the subproblems involving digital transceiver beamforming are addressed using a successive convex approximation framework combined with semidefinite relaxation and Gaussian randomization. To further balance computational complexity, fronthaul overhead, and system performance, a user-centric access point selection scheme is adopted using statistical channel state information. Numerical results demonstrate that the proposed framework outperforms various practical benchmark schemes in terms of user SE. Furthermore, the user-centric CF design achieves a 99.29% reduction in computational complexity with only a 5.49% performance loss compared with full-association CF.