Reduced Complexity Rate-Splitting Multiple Access Beamforming for Generalized Objectives

Rate splitting multiple access (RSMA) enables interference management trade-offs between space-division multiple access (SDMA) and non-orthogonal multiple access (NOMA) to serve multiple users in the multiple-input multiple-output (MIMO) broadcast channel. The design of RSMA beamforming, or precoding, to maximize typical rate performance is a non-convex optimization problem. In this paper, a parameterization of optimal beamforming directions for different performance maximization problems subject to a total power constraint is obtained for RSMA for systems with full column rank channel matrices. The performance evaluation function may be an arbitrary increasing function of split user rates. The proposed solution combines maximizing each stream's signal-to-noise-ratio (SNR) and reducing interference on other streams for different objectives. Using the derived beamforming directions, the design is completed by solving a power allocation problem. Simulation results reveal that the proposed approach is able to provide attractive performance/complexity trade-offs compared to existing schemes.