On the Optimization of Virtual RIS Partitioning for Grant-Free Non-Orthogonal Multiple Access

The integration of reconfigurable intelligent surfaces (RISs) and grant-free non-orthogonal multiple access (GF-NOMA) has emerged as a promising solution for enhancing spec-tral efficiency (SE) and massive connectivity in future wireless networks. This paper proposes a novel virtual RIS partitioning mechanism for GF-NOMA, where all user equipments (UEs) within a specific NOMA cluster have instantaneous access to resource blocks (RBs) without the need for grant acquisition and power control as in the traditional grant-based NOMA schemes. To achieve the required power disparity, RIS portions are allocated to the UEs in a manner that increases the reception power disparity. We derive closed-form equations for optimal RIS portions in two regimes of interest: 1) max-min fair regime and 2) maximum throughput regime. Simulation results demonstrate that the proposed RIS-assisted GF-NOMA yields a gain of 28% and 15% in terms of max-sum rate and max-min rate, respectively, outperforming existing grant-based approaches. The study highlights the potential of combining RIS with GF-NOMA as a powerful solution for future wireless communication systems.