Fluid Antenna Systems: A DoF-Driven Paradigm for Future Multifunctional Networks

Fluid antenna systems (FASs) introduce antenna position as a reconfigurable spatial degree of freedom (DoF), enabling adaptive interaction with the propagation environment and efficient utilization of spatial resources. This survey provides a comprehensive review of DoF exploitation in FAS-enabled wireless communications, covering reliability enhancement, high-throughput transmission, and multi-access connectivity. Beyond these core functionalities, FASs also support a range of multifunctional capabilities, such as integrated sensing and communication (ISAC), simultaneous wireless information and power transfer (SWIPT), secure and covert transmission, and artificial intelligence (AI)-driven configuration. As a concrete example of this vision, we present a novel fluid antenna-enabled quadrature index modulation (FA-QIM) framework to illustrate how this spatial DoF can be architected to enable high-rate and robust ISAC. Collectively, these investigations illustrate the evolution of FAS from a diversity-enhancing technique to a core enabler of multifunctional networks, bridging theoretical DoF analysis with system-level design considerations. It concludes by outlining open challenges and potential avenues toward intelligent, multi-functional FAS architectures for next-generation networks.