Evolution from NCR for 5G-Advanced to RIS for 6G: A Unified System Framework and Standardization Roadmap

As the 5G-Advanced landscape transitions toward 6G, mitigating severe path loss in mid-to-high frequency bands and expanding non-terrestrial network (NTN) coverage have become pivotal challenges. Network-Controlled Repeaters (NCR) and Reconfigurable Intelligent Surfaces (RIS) have emerged as flagship technologies for coverage enhancement; however, their technical boundaries and long-term standardization trajectories remain fragmented. This paper proposes a unified framework-"Homology, Heterogeneity, and Evolution"-to harmonize these two paradigms from the perspective of generalized electromagnetic radio frequency (RF) domain regulation. First, we establish the physical homology and model consistency among NCR, passive RIS, and active RIS, identifying their shared architectural essence characterized by baseband-less RF processing under a unified cascaded channel model. Second, we characterize their structural heterogeneity along with their spatial degrees of freedom (DoF). Third, we investigate the multifaceted standardization and engineering challenges during the transition from NCR to RIS, offering potential solutions to overcome these barriers. Finally, we conceptualize RIS as the advanced evolution path of NCR-termed "NCR Type-II (RIS)"-and propose a pragmatic standardization roadmap. By leveraging and extending the existing side control information (SCI) framework from 3GPP 5G Release 18 (Rel-18), we provide a pathway for the seamless integration of RIS into 6G architectures, supporting the broader vision of communicationsensing-computing integration.