On Fundamental Limits of Slow-Fluid Antenna Multiple Access for Unsourced Random Access

Massive connectivity in future networks demands fundamental advancements in multiple access techniques. Notably, unsourced random access (URA) and fluid antenna multiple access (FAMA) offer significant capacity improvements through coding gain and spatial diversity. In this work, we present the slow-FAMA (s-FAMA) URA model, along with detailed analyses of achievable bounds and performance floors for URA in a multi-input multi-output (MIMO) system. Closed-form expressions are provided for both the achievable rates and performance floors. Compared to existing MIMO URA bounds, the proposed approach significantly improves system capacity in both single-user and multi-user scenarios. Numerical results further demonstrate the performance gains enabled by the proposed s-FAMA URA model.