Low Complexity Rate Splitting Using Hierarchical User Grouping

Rate splitting multiple access (RSMA) is a powerful multiple access technique that exploits the advantages of both space-division multiple access (SDMA) and nonorthogonal multiple access (NOMA), making it suitable for heterogeneity of quality of service and high throughput requirements of future networks. RSMA is based on rate splitting at the transmitter and successive interference cancellation (SIC) at the receiver. The number of transmitted streams and the number of possible decoding orders grows exponentially with the number of users. In this paper, a low-complexity rate splitting approach is proposed that finds a subset of all possible common streams by hierarchical user grouping and assigning a common stream to each group. Since the hierarchical user grouping approach creates groups of users of the same size that do not have a user in common, this approach does not require user ordering. To implement this low-complexity RSMA, alternating max-min precoding design and power allocation is used. The beamforming vectors are designed to cancel the multiuser interference. As a result, the beamforming vectors can be designed in multiple levels. Simulation results show that the proposed scheme is able to achieve attractive performance vs. complexity tradeoffs compared to several others in the literature.