A New Flexible HARQ Assisted by Rate Splitting Multiple Access

The escalating demand for massive connectivity poses formidable challenges to the sixth generation (6G) wireless communication systems. In addressing this pivotal issue, rate splitting multiple access (RSMA) emerges as a promising solution. By modifying both the decoding process and the power allocation, RSMA presents a new paradigm that allows for the smooth transition between well-known techniques including space division multiple access (SDMA) and non-orthogonal multiple access (NOMA). Expanding on this adaptability, and considering two different methods of hybrid automatic repeat requests (HARQ), new concepts like flexible HARQ can be developed to address the escalating demands. This work proposes flexible HARQ supported by RSMA features, detailing the overall characteristics, including channel aspects, delay, and imperfect channel state feedback. Two specific policies are proposed: one incorporates the NOMA and SDMA frame structures while the other utilizes the RSMA frame structure. In accordance with these policies, we are applying incremental redundancy (IR) and Chase combining (CC) for the re-transmission process. The throughput analysis of both policies is derived, the impact of imperfect channel feedback is investigated, delay analysis under imperfect channel feedback is conducted, and energy efficiency (EE) is calculated. Simulation results show that the proposed approach surpasses standard HARQ-RSMA in terms of the average number of transmissions, average waiting time, and EE.