Precoding Design and Sum Rate upper bound of RSMA Using Interference Nulling

Rate-splitting multiple access (RSMA) is a new multiple access technique that improves limitations of conventional multiple access techniques. RSMA manages interference by precoding at the transmitter and successive interference cancellation (SIC) at the receivers. Several precoding design approaches are considered in the literature that propose algorithms to solve a joint optimization problem. In this work, we focus on a special class of RSMA that nulls the interference of streams that must not be decoded, i.e., RSMA with interference nulling (RSMA-IN). We consider a sum rate maximization problem subject to a total power constraint and interference nulling (IN). To address the performance of precoding design algorithms relative to the optimal sum rate of RSMA-IN, we propose a convex upper bound problem for RSMA-IN sum rate and compare its performance with that of the precoding design algorithms. Further, to take advantage of the cases where the upper bound problem achieves the optimal sum rate, we propose an upper bound aided (UBA) algorithm to both enhance the performance and reduce the complexity of existing precoding design algorithms. Simulation results show that the performance of the UBA algorithm is close to the optimal sum rate of RSMA-IN.