Robust Beamforming for Discrete RIS Enhanced RSMA-ISAC Systems

Integrated sensing and communication (ISAC), which relies on multi-antenna technology, has become a promising technology for sixth-generation (6 G) networks. However, space division multiple access (SDMA) is limited in many scenarios for enhancing ISAC performance. Therefore, we investigate the performance optimization of the discrete reconfigurable intelligent surfaces (RIS) enhanced rate-splitting multiple accessintegrated sensing and communication (RSMA-ISAC) systems to provide a near-line-of-sight link, where RSMA outperforms SDMA in interference management. Specifically, we propose a beam gain approximation method to improve the sensing beam gain while guaranteeing the communication user rate requirement. In particular, we employ an alternating optimization algorithm to decompose coupled variables, which solves the resulting precoding vector and discrete phase shift of the RIS non-convex optimization problem. Then, we develop a proximal distance algorithm to decompose complicated discrete constraints and achieve flexible solutions. Simulation results show that the proposed method can achieve a better sensing beam gain and validate the superiority of RSMA as an interference management strategy for ISAC systems. Index Terms-Integrated sensing and communication (ISAC), reconfigurable intelligent surfaces (RIS), rate-splitting multiple access (RSMA), discrete phase-shift.