Signal Processing Technologies for RIS-assisted Wireless Communications

‍ ‍Reconfigurable intelligent surface （RIS）， as a revolutionary technology， can realize intelligent configuration of the wireless transmission environment by integrating numerous low-cost passive reflective elements on a flat surface， thus significantly improving the performance of wireless communication networks. Specifically， different reflection elements can independently reflect incident signals by controlling their amplitudes and/or phase shifts， thus cooperatively achieving three-dimensional passive beamforming for directional signal enhancement or weakening. In sharp contrast to existing technologies， RIS can provide a new degree of optimization to further improve the performance of wireless communication networks. Owing to its ability to actively change the wireless transmission channels through highly controllable intelligent signal reflections， RIS can enhance the desired signal power at the receiver or mitigate undesired signals such as channel interference. Moreover， owing to the specificity of their hardware structure， RISs are generally lightweight and small. Therefore， they can be easily installed and removed from surfaces such as walls， ceilings， buildings， and advertising panels， allowing their reuse. In addition， RISs are usually significantly cheaper than active small base stations and relays and， therefore， can be rapidly deployed at a low cost. RIS is a complementary device that can be deployed in existing wireless systems without changing the corresponding standards or hardware， as only necessary modifications to the communication protocols are required. Owing to these advantages， RISs have been extensively investigated in research areas such as transmission protocol designs， system capacity analysis， energy/spectral efficiency， physical layer security， and modulation/coding schemes. New challenges have also arisen in the design and implementation of RIS-aided wireless systems. According to existing studies， for RIS-assisted wireless communication systems， the joint active-passive beamforming of the base station and RIS is fundamental to ensuring system performance. However， because of their special structures， RISs cannot process incident signals but only passively reflect them. Moreover， because of the high number of RIS reflecting elements， the joint optimization of the active beamforming at the transmitter and the passive reflecting at the RIS can be highly complicated and time-consuming. Therefore， the design of RIS-assisted wireless communication systems should address the following issues： （1） how to design a low-complexity optimization method to achieve joint active and passive beamforming at the base station and RIS， and （2） how to design efficient collaborative transmission schemes in multi-RIS-assisted wireless communication systems to achieve optimal system performance. To address the aforementioned challenges， an in-depth study is conducted. Herein， we focus on the low-complexity signal processing techniques for RIS-assisted wireless communication systems， extend from single RIS-assisted wireless communication systems to multi-RIS-assisted collaborative communication systems， explore a series of advanced digital signal processing techniques， and design a low-complexity design methodology that is suitable for large-scale RIS-assisted communication systems to promote the deployment and application of RIS in practical applications.