Sum-rate Maximization under Unequal Rate Allocation in Downlink Q-RSMA

With the rapid advancement of the Internet of Things, forthcoming 6G wireless communications networks will demand higher data rates and enhanced spectral efficiency (SE) and energy efficiency (EE). Quadrature rate splitting multiple access (Q-RSMA) supports these requirements by enabling message transmission in two-dimensional spacing, thereby eliminating the need for successive interference cancellation operations. Previous work on downlink Q-RSMA has demonstrated its superiority over other multiple access schemes by assuming equal rate allocation among users. However, real-world scenarios often demand diverse rate requirements. The effectiveness of Q-RSMA under these practical conditions remains unexplored, particularly with varying rate demands among users. This paper addresses this gap by investigating the sum-rate optimization of Q-RSMA in scenarios with unequal rate allocation. Specifically, closed-form expressions for optimal rate and power allocation factors under unequal rate distributions are evaluated. Numerical results establish the superiority of Q-RSMA over conventional rate-splitting multiple access, non-orthogonal multiple access (NOMA), and quadrature-NOMA in terms of sum-rate, SE, and EE, particularly in lower power budget scenarios. Furthermore, this study provides a detailed analysis through demonstrations of sum-rate performance across varying rate requirements, underlining its potential to meet the SE and EE demands of next-generation wireless communication systems.