Cell-Free Integrated Sensing and Communication: Principles, Advances, and Future Directions

TL;DR

This paper provides a comprehensive survey of cell-free integrated sensing and communication (CF-ISAC), discussing principles, recent advances, challenges, and future research directions in next-generation wireless systems.

Contribution

It offers the first thorough review of CF-ISAC, covering system principles, state-of-the-art developments, challenges, and emerging trends in a unified framework.

Findings

CF-ISAC enhances spectral and energy efficiency.

Multi-static sensing improves environmental awareness.

Key challenges include synchronization and interference management.

Abstract

Cell-free (CF) integrated sensing and communication (ISAC) combines CF architecture with ISAC. CF employs distributed access points, eliminates cell boundaries, and enhances coverage, spectral efficiency, and reliability. ISAC unifies radar sensing and communication, enabling simultaneous data transmission and environmental sensing within shared spectral and hardware resources. CF-ISAC leverages these strengths to improve spectral and energy efficiency while enhancing sensing in wireless networks. As a promising candidate for next-generation wireless systems, CF-ISAC supports robust multi-user communication, distributed multi-static sensing, and seamless resource optimization. However, a comprehensive survey on CF-ISAC has been lacking. This paper fills that gap by first revisiting CF and ISAC principles, covering cooperative transmission, radar cross-section, target parameter estimation, ISAC integration levels, sensing metrics, and applications. It then explores CF-ISAC systems, emphasizing their unique features and the benefits of multi-static sensing. State-of-the-art developments are categorized into performance analysis, resource allocation, security, and user/target-centric designs, offering a thorough literature review and case studies. Finally, the paper identifies key challenges such as synchronization, multi-target detection, interference management, and fronthaul capacity and latency. Emerging trends, including next-generation antenna technologies, network-assisted systems, near-field CF-ISAC, integration with other technologies, and machine learning approaches, are highlighted to outline the future trajectory of CF-ISAC research.