A Review on Rate Splitting Multiple Access: Challenges and Future Directions

Rate splitting multiple access (RSMA) has turned up as a popular multiple access (MA) for the optimization of non-orthogonal transmissions for 5G and beyond 5G wireless communications. By strategically, RSMA manages the interference levels by decoding a part of the other user's message in a pair and treating the rest of the message as noise, thereby, maintaining the secrecy of the message. RSMA bridges the connection between two extreme MA schemes such as Space division multiple access (SDMA) and Non-orthogonal multiple access (NOMA) with different operation areas of low and high interference levels. RSMA can be classified as a generalized MA scheme and conventional MA schemes like orthogonal multiple access (OMA), NOMA, SDMA, multicasting, etc., can be employed as special cases of it. RSMA performance is optimal for different transmission schemes from information and communication theoretical findings. RSMA offers an increase in energy efficiency (EE), spectral efficiency (SE), coverage improvement, latency reduction, and enhanced degrees-of-freedom in comparison to conventional MA schemes, for both underloaded and overloaded regimes. Through this review, the RSMA and conventional MA schemes are discussed along with their potential advantages and disadvantages. In the end, a proposal is given for RSMA pertaining to research gaps arising out of existing literature.