Transmit Power Minimization for IRS-Assisted Hierarchical Rate-Splitting Multiple Access Systems

Rate-splitting multiple access (RSMA) and intelligent reflecting surface (IRS) are powerful techniques for satisfying diverse quality of service (QoS) requirements in future wireless networks. In this letter, we propose an efficient hierarchical rate-splitting (HRS) transmission scheme tailored for IRS-assisted multiple-input single-output systems. Specifically, we formulate an optimization problem to minimize the transmit power by jointly designing the user clustering, common rate distribution, transmit beamforming and IRS passive beamforming. In particular, we propose a low-complexity user clustering method by taking into account the QoS diversity and channel similarity of users. Then, we decompose the original non-convex problem into two subproblems and adopt the alternating optimization algorithm. The subproblems for optimizing the beamformer, common rate allocation, and IRS phase shift, efficiently solved by using successive convex approximation and semi-definite relaxation algorithms. Simulation results show that the proposed scheme can significantly reduce the transmit power compared with various benchmarks.