A Variable Block-Correlation Model for Fluid Antenna Systems

Fluid antenna systems (FAS) achieve spatial diversity through multiple closely spaced antenna ports, but the spatial correlation among these ports complicates accurate performance analysis. Traditional block-correlation models with constant correlation coefficients often yield substantial approximation errors. This paper proposes a variable block-correlation model with optimal block-specific correlation coefficients. Our low-complexity algorithm reduces computational complexity from exponential to linear, while achieving significantly lower approximation error compared to constant correlation models. Applied to point-to-point FAS communications, our model provides tractable analytical expressions for channel statistics and outage probability. Numerical results show the proposed approach offers superior accuracy, particularly for systems with a limited number of ports.