Performance Analysis of Fluid Antenna System Aided OTFS Satellite Communications

Internet-of-Things (IoT) networks often rely on satellite links for rural coverage, but high mobility induces severe Doppler and delay spreads. Orthogonal time frequency space (OTFS) modulation can mitigate these impairments, while fluid antenna systems (FAS) offer mechanical simplicity and adaptability under space/energy constraints. We propose a FAS-aided OTFS (FAS-OTFS) framework for satellite IoT, deriving outage probability and ergodic capacity under a general channel model that accounts for aggregate interference, spatial correlation, and imperfect-CSI port selection. Analytical results are given as integrals or bounds, with closed forms for a single-path scenario using a compact correlation/CSI-aware interpolation. We benchmark against maximal-ratio combining (MRC) under identical single-RF constraints, highlighting FAS selection efficiency. Our numerical results demonstrate significant performance gains in terms of both the outage probability and capacity compared to conventional OTFS systems, with tight agreement between theory and Monte Carlo simulation, confirming the efficacy of FAS-OTFS in energy-constrained high-mobility environments.