MMSE Precoding for Reliability Enhancement in Downlink MISO-RSMA Systems

Rate splitting multiple access (RSMA) has been regarded as one of the most promising technologies for the next-generation broadcasting and mobile communication systems. Many prior designs for RSMA systems focused on the capacity optimization from the information-theoretic analysis, while the reliability in realistic deployment is less considered. To this end, the linear precoder of downlink multiple-input single-output (MISO)-RSMA systems is elaborately designed in this paper by minimizing the mean square error (MSE) associated with the worst user equipment (UE). The optimization problem is first formulated by investigating the MSE for each UE from the view of signal processing, where the zero-forcing (ZF) precoding is utilized for the private messages. The minimum MSE (MMSE) precoding is then obtained by utilizing the semi-definite relaxation (SDR) for the common precoding vector and deriving the closed-form optimal power allocation coefficient between private and common messages. A heuristic closed-form solution is then developed to reduce the complexity caused by the semi-definite programming (SDP). Simulation results demonstrate the reliability superiority of the proposed schemes beyond space-division multiple access (SDMA) and conventional RSMA approaches based on the max-min fairness (MMF) rate optimization even though the near-optimal weighted MMSE algorithm can be deployed.