Subband Precoding Optimization in OFDM–RSMA

Abstract

In this paper, we study the optimization of subband precoder for an orthogonal frequency division multiplexing rate-splitting multiple access (OFDM-RSMA) system in a multi-user multiple-input single-output (MU-MISO) downlink scenario. In conventional OFDM-space division multiple access (SDMA) systems, each subband consisting of multiple subcarriers is generally precoded using a single precoder. This can lead to a performance degradation if the channel is frequency selective within a subband. To address this issue, leveraging RSMA’s robustness to multi-user interference and imperfect channel state information (CSI), we introduce RSMA into the OFDM system where subband precoding is applied. For the proposed RSMA-based subband precoding system, the sum rate and minimum rate are maximized by jointly optimizing the subband precoders and the portion of the common rates using the successive convex approximation (SCA) technique. We also propose low-complexity algorithms that exploit the structure of the achievable rate expressions derived for proposed RSMA-based subband precoding system and leverage the characteristic of the subcarrier channels in OFDM systems. Simulation results show that the proposed RSMA-based subband precoding system outperforms the SDMA-based subband precoding system, verifying the robustness of RSMA. Furthermore, we demonstrate that the proposed low-complexity algorithms can achieve satisfactory performance compared to the proposed SCA-based algorithms and can significantly outperform the practical zero-forcing-based subband precoder with comparable complexity.