The passive sensing method for the ISAC system with RIS. For more details, please refer our paper: Z. Chen, P. Chen, Z. Guo, Y. Zhang and X. Wang, "A RIS-Based Vehicle DOA Estimation Method With Integrated Sensing and Communication System," in IEEE Transactions on Intelligent Transportation Systems, doi: 10.1109/TITS.2023.3330172. https://ieeexplore.ieee.org/abstract/document/10315054