Sum Rate Maximization With Rate-Splitting Multiple Access for Hybrid Precoding Systems

In this paper, we propose a novel sum rate maximization algorithm with rate-splitting (RS) for a hybrid precoding downlink system. By utilizing the benefits of RS, we define a group RS (GRS) framework that serves multiple user equipments without being limited by the number of radio frequency chains. We then formulate and solve a sum rate maximization problem with a quality-of-service constraint to derive analog and digital precoders as well as rate-split. Through alternating optimization and a meticulous use of convex optimization techniques, we first derive the analog precoder utilizing the Riemannian conjugate gradient approach by defining a penalty factor and approximating the common rate. Next, the digital precoder and rate-split are derived by utilizing the successive convex approximation approach. To compensate for the excessive complexity of the proposed algorithm, we also derive a low complexity algorithm for the GRS framework that optimizes only the power of the digital beamformers and derives the directions of the analog and digital beamformers using linear operations. Simulation results verify the superiority of the GRS framework by comparing the performance with existing benchmarks. Furthermore, we corroborate via simulations that the low complexity algorithm achieves greater performance than the existing benchmarks even with low complexity.