Smart Environments and Integrated Sensing and Communication: Opportunities and Challenges from Link Design to Network Cooperation

Abstract: Smart environments and integrated sensing and communication (ISAC) are steering 6G from ''signal transmission'' toward ''collaborative environmental perception and proactive control''. Unlike conventional designs that treat the environment as an external random factor, smart environments leverage programmable metasurfaces, intelligent transmissive and reflective surfaces, vehicle-mounted surfaces, unmanned aerial vehicles (UAVs) and other autonomous platforms, reconfigurable antennas, and multi-base-station cooperative networks to transform propagation paths, array geometries, coverage topologies, and node cooperation into optimizable degrees of freedom. ISAC, through shared waveforms, spectrum, and infrastructure, provides real-time feedback on environmental understanding, target localization, blockage awareness, and state prediction, forming a closed loop where ''the environment empowers sensing and communication, and sensing and communication in turn refine the environment''. This paper systematically surveys the conceptual boundaries, core technologies, typical architectures, key gain mechanisms, and network-level cooperation modes of smart environments and ISAC, with emphasis on how fixed surfaces, target-mounted surfaces, hybrid sensing surfaces, UAV platforms, fluid antennas, and multi-base-station cooperative ISAC networks contribute to coverage extension, channel rank reshaping, target resolvability enhancement, multi-static sensing, and sensing-assisted communication. Furthermore, the paper identifies common design issues regarding performance metrics, optimization variables, and closed-loop system operation, and analyzes the intrinsic tensions between high-rank channel demands and line-of-sight (LoS) link preferences, as well as between communication decorrelation requirements and sensing geometric diversity needs. Finally, key challenges in typical scenarios, including vehicle-to-everything (V2X), industrial manufacturing, indoor spaces, and integrated space-air-ground networks, are summarized, along with promising directions for future research.