Integrated Sensing and Communications with RISs

Reconfigurable metasurfaces, a.k.a. reconfigurable intelligent surfaces (RIS), have emerged as a revolutionary technique for sixth-generation (6G) networks, with their capability to intelligently manipulate wireless propagation environments. Meanwhile, integrated sensing and communication (ISAC) is also recognized as a pivotal technology for the evolution of 6G networks, owing to its substantial enhancements in spectral/hardware/energy efficiency. This chapter investigates the synergistic integration of RIS into ISAC in order to improve performance, expand coverage, and increase system flexibility. We first introduce the background and fundamentals of ISAC and describe the impact of RIS on sensing systems. Then, we examine the potential of RIS in ISAC systems by analyzing various RIS architectures, e.g., passive RIS, active RIS, and simultaneously transmitting and reflecting RIS (STAR-RIS). Some special functionalities of RIS in ISAC systems are also discussed with the aim of attaining greater integration gains. Joint beamforming design and performance improvement from a subspace perspective are presented to illustrate the fundamental principles of RIS-assisted ISAC systems and to verify the benefits of RIS deployment in ISAC systems. Finally, several prevailing challenges and prospective research directions are discussed, aiming to bridge the gap between theoretical studies and real-world implementations.