Non-Stationary THz Channel Characterization for S2A Communications: A Trajectory-Aware Doppler Analysis

Abstract

This letter investigates terahertz (THz) channel modeling for Satellite-to-Airplane (S2A) communications–a critical framework for achieving high-speed, cellular-equivalent in-flight connectivity in 6G networks. To capture the non-stationary nature of the S2A link, we model aircraft trajectory parameters, including speed, heading, and flight path angles, as first-order Gauss-Markov (GM) processes. This approach accounts for stochastic perturbations around nominal flight profiles across the following three distinct phases: climb, cruise, and descent. We characterize the airplane-induced Doppler shift as a Weighted Sum of GM (WSGM) process. To enhance the characterization of temporal dynamics, we propose a Modified WSGM (MWSGM) model that improves accuracy of the estimated coherence time. Using this framework, we derive a closed-form expression for the channel autocorrelation function (ACF), enabling a precise analytical determination of coherence time. The results of our simulations demonstrate the validity of our analytically derived ACF, which provides superior estimates of coherence time and Doppler spread across various flight modes as compared to WSGM and the Sum-of-Sinusoids (SoS) models.