Multi-beam Beamforming in RIS-aided MIMO Subject to Reradiation Mask Constraints - Optimization and Machine Learning Design

• Published in: IEEE transactions on communications p. 1
• Main Authors: Wang, Shumin, Hassani, Hajar El, Renzo, Marco Di, Poulakis, Marios
• Format: Journal Article
• Language: English
• Published: IEEE 2025
• Subjects: Array signal processing; Europe; MIMO; Minimax techniques; model-based neural networks; multi-beam design; Neural networks; Optimization; quadratic programming with quadratic constraints (QPQC); Reconfigurable intelligent surfaces; Reconfigurable intelligent surfaces (RISs); Reflection; reradiation masks; Training; Vectors
• ISSN: 0090-6778; 1558-0857
• DOI: 10.1109/TCOMM.2025.3592607

Reconfigurable intelligent surfaces (RISs) are an emerging technology for improving spectral efficiency and reducing power consumption in future wireless systems. This paper investigates the joint design of the transmit precoding matrices and the RIS phase shift vector in a multi-user RIS-aided multiple-input multiple-output (MIMO) communication system. We formulate a max-min optimization problem to maximize the minimum achievable rate while considering transmit power and reradiation mask constraints. The achievable rate is simplified using the Arimoto-Blahut algorithm, and the problem is broken into quadratic programs with quadratic constraints (QPQC) sub-problems using an alternating optimization approach. To improve efficiency, we develop a model-based neural network optimization that utilizes the one-hot encoding for the angles of incidence and reflection. We address practical RIS limitations by using a greedy search algorithm to solve the optimization problem for discrete phase shifts. Simulation results demonstrate that the proposed methods effectively shape the multi-beam radiation pattern towards desired directions while satisfying reradiation mask constraints. The neural network design reduces the execution time, and the discrete phase shift scheme performs well with a small reduction of the beamforming gain by using only four phase shift levels.