HARQ-CC-aided slow fluid antenna multiple access with highly correlated ports: a Laplace–Stieltjes-transform-based performance analysis

Hybrid automatic repeat request with chase combining (HARQ-CC) improves the reliability of slow fluid antenna multiple access ( s FAMA) through multiround combining. However, tractable performance characterization of HARQ-aided s FAMA remains challenging because it must account for random interferer activity and signal-to-interference ratio (SIR) accumulation across multiple HARQ rounds. This study presents an analysis framework for downlink HARQ-CC-aided s FAMA with densely spaced and highly correlated fluid antenna system (FAS) ports. Under a spatial block correlation model, we constructed two admissible high-correlation approximations for the conditional per-round selected-port SIR cumulative distribution function (CDF): an endpoint-normalized Marcum- Q -kernel approximation and a lower-complexity step-threshold form. Then, CDF-based Laplace–Stieltjes transform (LST) representations were derived for the FAS approximations. The accumulated-SIR distribution was evaluated by SIR-domain Stieltjes convolution and numerical LST inversion to yield the outage probability, average number of transmissions, and payload throughput. The numerical results show close agreement between the two accumulation methods for the same corrected CDFs. Comparisons with the complete kernel under matched block parameters and Monte-Carlo simulations enabled assessments of the approximation accuracy and system-level trends, respectively. The FAS approximations were generally conservative while preserving the performance trends and receiver ordering. The FAS receiver consistently outperformed the block-representative antenna selection (BR-AS) and fixed-position antenna (FPA) benchmarks, whereas the throughput gains from additional HARQ rounds were marginal under severe multiuser interference.