Empirical 3-D Channel Modeling for Cellular-Connected UAVs: A Triple-Layer Machine Learning Approach

This work proposes an empirical air-to-ground (A2G) propagation model specifically designed for cellular-connected unmanned aerial vehicles (UAVs). An in-depth aerial drive test was carried out within an operating Long-Term Evolution (LTE) network, gathering thorough measurements of key network parameters. Rigid preprocessing and statistical analysis of these data produced a strong foundation for training a new triple-layer machine learning (ML) model. The proposed ML framework employs a systematic hierarchical approach. Accordingly, the first two layers, Stepwise Linear Regression (STW) and Ensemble of Bagged Trees (EBT) generate predictions independently; meanwhile, the third layer, Gaussian Process Regression (GPR), explicitly acts as an aggregation layer, refining these predictions to accurately estimate Key Performance Indicators (KPIs) such as Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Received Signal Strength (RSSI), and Path Loss (PL). Compared to traditional single-layer ML or computationally intensive ray-tracing approaches, the proposed triple-layer ML framework significantly improves predictive performance and robustness, achieving a coefficient of determination $(R^{2})$ values of approximately 0.99 in training and above 0.90 in testing while utilizing a minimal but effective feature set (log-transformed 3D and 2D propagation distances, azimuth, and elevation angles). This streamlined feature selection substantially reduces computing complexity, thus enhancing scalability across various operating environments. The proposed framework’s practicality and efficacy for real-world deployment in UAV-integrated cellular networks are further demonstrated by comparative analyses, which underscore its substantial improvement.