Downlink AFDM-RSMA Scheme Based on Orthogonal Chirps and Sum-Rate Maximization

Rate splitting multiple access (RSMA) is an emerging multiple access mechanism that combines the advantages of various traditional techniques, such as non-orthogonal multiple access (NOMA) and space division multiple access (SDMA). However, RSMA struggles to maintain efficient transmission in high-mobility scenarios due to rapidly varying channels. In this paper, we propose a downlink transmission scheme termed AFDMRSMA to serve multiple high-speed users. Unlike traditional RSMA frameworks, the mechanism mitigates the interference among private messages. Specifically, in the proposed AFDMRSMA scheme, private streams of different users are mapped onto orthogonal chirps, while the common stream spans all chirps. To maximize the system sum-rate, we jointly optimize rate splitting, chirp allocation and power control. Due to the fact that this problem belongs to mixed integer programs (MIPs), we decompose the original problem into two subproblems. We derive the optimal number of chirps and common rate splitting in a closed-form solution under fixed power control. Then, we develop an efficient iterative algorithm to obtain a suboptimal power control solution. Simulation results demonstrate that the proposed AFDM-RSMA scheme outperforms the orthogonal multiple access (OMA)-based, NOMA-based, and orthogonal time-frequency-space (OTFS)-based schemes.