On Fundamental Limits for Fluid Antenna-assisted Integrated Sensing and Communications for Unsourced Random Access

This paper explores the unsourced/uncoordinated random access (URA) problem for integrated sensing and communication (ISAC) systems. Recent findings indicate that conventional multiple access strategies, such as treating interference as noise (TIN) and time-division multiple access (TDMA), are often overwhelmed and fail to support the rapidly increasing number of active users. To address this, the unsourced ISAC (UNISAC) system model has emerged as a promising framework for future ISAC networks. In this work, we adopt a realistic channel model and propose the use of a fluid antenna system (FAS) for UNISAC. We derive the achievable performance bounds and floor for the proposed FAS-UNISAC to demonstrate its significant potential. For communication, the randomized overlapping of channel responses from different scattering paths yields substantial gains within a compact antenna space.We prove that the joint detection and decoding error is inversely proportional to the channel gain. For line-of-sight (LOS) only channel model, leveraging covariance information through the spatial diversity of FAS effectively expands the receiving aperture, thereby enhancing sensing resolution. A universal sensing upper bound is established as a benchmark for potential sensing methods. Additionally, we investigate the benefits of asynchronous transmission, which introduces an extra degree of freedom through resource multiplexing gains via randomized timing offsets among users. Our results show that the spatial diversity inherent in FAS significantly enhances the supported user volume, as well as the sensing and communication capabilities.