Joint Active and Passive Beamforming Design for RIS-Assisted Cooperative ISAC Systems

Integrated sensing and communication (ISAC) is an emerging technology enhancing hardware and spectral efficiency by combining wireless communication and radar sensing functionalities. However, cooperative multi-static sensing performance in Cell-Free enabled multi-cell networks remains underexplored. To address this gap, we propose a reconfigurable intelligent surface (RIS)-assisted cooperative multi-static ISAC system comprising multiple collaborative ISAC transmitters and a centralized ISAC receiver. Specifically, the RIS introduces additional degrees of freedom (DoFs) to enhance performance by mitigating obstructions, creating virtual line-of-sight (LoS) paths, and aggregating sensing signals. To maximize sensing signal-to-noise ratio (SNR), we formulate a joint optimization problem involving radar receiver filter, active beamforming, and passive beamforming designs. Due to its non-convex nature, an alternating optimization (AO) algorithm is developed. Specifically, the radar receiver filter sub-problem is solved using a power iteration approach, while active beamforming is addressed via second-order cone programming (SOCP) and semi-definite relaxation (SDR) methods. Passive beam-forming is optimized by a successive convex approximation (SCA)-based algorithm with sequential rank-one constraint relaxation (SROCR). Simulation results confirm that: the proposed algorithm achieves significant radar SNR gain; RIS-assisted cooperative multi-static ISAC system consistently outperforms single-cell and cooperative mono-static ISAC systems; and our RIS-assisted sensing aggregation scheme significantly outperforms the scenario without RIS.