A New Spatial Block-Correlation Model for Fluid Antenna Systems

Powered by position-flexible antennas, the emerging fluid antenna system (FAS) technology postulates as a key enabler for massive connectivity in 6G networks. The free movement of antenna elements within a given aperture allows several users to share the same radio channel at no additional cost and without the need of precoding. However, the true potential of FAS is still unknown due to the high spatial correlation of the wireless channel between very close-by antenna positions, which yields intractable analyses for conventional correlation models such as Jake's. Therefore, there is an outstanding need for simple yet accurate correlation models that allow to unveil the multiplexing capabilities of FAS, since realistic classical models are prohibitively complex while state-of-the-art approximations may be too simplistic. Aiming to fill this gap, we here propose a general framework to approximate spatial correlation by block-diagonal matrices, motivated by the well-known block fading assumption and by statistical results on large correlation matrices. The proposed block-correlation model makes the performance analysis possible, and tightly approximates the results obtained with realistic models such as Jake's and Clarke's. We finally leverage our framework to characterize slow-fluid antenna multiple access (slow-FAMA) systems, evaluating their performance for both one- and two-dimensional fluid antennas.