## 5 Conclusions
### Future Work  
Finally, RIS has great potential for development in the future, and it is crucial to obtain accurate CSI to guarantee the efficient functioning of RIS. With the increase in cascaded links in multi-RIS systems, the channel estimation complexity increases further and the channel estimation method of single RIS may no longer be applicable. How to integrate more efficiently with RIS in higher frequency bands, such as the THz band, is also an issue of concern. Deploying RIS in complex and changing environments, such as cities with tall buildings, moving vehicles or satellites, requires more appropriate channel estimation methods to guarantee effective communication. The idea of fusing deep learning algorithms with conventional schemes and using the powerful data fitting capability of deep learning to improve the performance of conventional algorithms has a broad development prospect. In summary, RIS has great potential and deserves further exploration and research. We hope that this review can provide sufficient information and promote the development of RIS, regarding which the method of RIS channel estimation is still an evolving process. It is hoped that this research and analysis of RIS for wireless communication channel estimation will stimulate further research into the next-generation wireless communication technologies and their practical application in future wireless communication.  
[PERSON], [PERSON] and [PERSON] chose the topic and designed the structure of the paper. [PERSON] and [PERSON] sorted out and analyzed RIS channel estimation in different frequency bands. [PERSON] and [PERSON] reviewed the signal processing algorithms. [PERSON], [PERSON] and [PERSON] classified the involved algorithms and analyzed the data. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] performed investigation and revised the manuscript. [PERSON], [PERSON], [PERSON] and [PERSON] performed funding acquisition. [PERSON] performed supervision and project administration. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the Shandong Provincial Natural Science Foundation (Nos. ZR2022 MF256, ZR2020 MF008), in part by the Joint Fund of Shandong Provincial Natural Science Foundation (No. ZR2021 LZH003), in part by the Shandong Province Science and Technology Small and Medium-Sized Enterprises Innovation Ability Enhancement Project (No. 2023 TSGC0702), in part by the Scientific Research Project of Shandong University-Weihai Research Institute of Industrial Technology (No. 000620221002011), in part by the Science and Technology Development Plan Project of Weihai Municipality (No. 2022 DXGJ13), in part by the Shandong University Graduate Education Quality Curriculum Construction Project (No. 2022038), in part by the Education and Teaching Reform Research Project of Shandong University, Weihai (No. Y2023024), in part by the 17 th Student Research Training Program (SRTP) at Shandong University, Weihai (Nos. A22298, A22085), and in part by the 18 th Student Research Training Program (SRTP) at Shandong University, Weihai (Nos. A23247, A23249).  
Not applicable.  
The author declare no conflict of interest.