Passive vs. Active Reconfigurable Intelligent Surfaces for Integrated Sensing and Communication: Challenges and Opportunities

Reconfigurable intelligent surfaces (RISs) have emerged as a revolutionary technology for controlling wireless channels to improve the performance of wireless communication and radar systems. However, the performance improvement of these wireless systems is shown to be limited due to the multiplicative fading effect. Active RISs have been introduced to circumvent this effect while simultaneously increasing the degrees-of-freedom in the design of RIS-assisted networks. On a parallel theme, integrated sensing and communications (ISAC) has been identified as one of the promising 6G paradigms for enabling sensing and communication functionalities within an integrated platform, which allows more efficient use of energy and spectral resources at a reasonable cost, and paves the way to the birth of new use-cases and applications. This article investigates the roles of active and passive RISs in different ISAC scenarios. Indeed, while there is abundant literature covering the use of both passive and active RISs for wireless communications, there is still a shortage in studying their benefits for radar and ISAC scenarios. This paper presents an overview of the currently available studies on the use of active and passive RISs for ISAC purposes and provides numerical examples to show the superiority and greater flexibility of active RISs in the context of ISAC. Finally, possible use cases for active RISs in modern ISAC systems are explored, and research challenges are highlighted along with the relevant opportunities that RISs offer.