Multi-Functional RIS: Signal Modeling and Optimization

• Published in: IEEE transactions on vehicular technology Vol. 73; no. 4; pp. 5971 - 5976
• Main Authors: Zheng, Ailing, Ni, Wanli, Wang, Wen, Tian, Hui, Eldar, Yonina C., Niyato, Dusit
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.04.2024; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Amplitudes; Array signal processing; Beamforming; Budgets; Channel estimation; Coefficients; coupled amplitude and phase shift; Iterative algorithms; Mathematical models; Mathematical programming; Multi-functional reconfigurable intelligent surface (MF-RIS); NOMA; non-orthogonal multiple access; Nonorthogonal multiple access; Optimization; Phase shift; rate maximization; Reconfigurable intelligent surfaces; Reflection; Reflection coefficient; Refraction; Surface impedance
• ISSN: 0018-9545; 1939-9359
• DOI: 10.1109/TVT.2023.3331093

In this paper, a multi-functional reconfigurable intelligent surface (MF-RIS) assisted non-orthogonal multiple access multiuser network is investigated. In contrast to existing studies assuming that the amplitude and phase shift coefficients of the refraction and reflection can be adjusted independently, we propose a practical model for the MF-RIS, where the refraction and reflection coefficients are highly coupled. Then, we investigate a sum-rate maximization problem by jointly optimizing the transmit beamforming and MF-RIS coefficients, subject to the coupled amplitude and phase shift constraints. To address the formulated non-convex problem, we propose an efficient iterative algorithm based on the fractional programming theory. Finally, numerical results show that: 1) The coupled MF-RIS scheme is superior to the simultaneous transmitting and reflecting RIS (STAR-RIS) and single-functional RIS (SF-RIS) schemes under the same power budget; 2) The performance gap between the ideal MF-RIS and coupled MF-RIS decreases with the increase of the total power budget.