Beyond TR 38.811 for 6G NTN Channel Models

Abstract

As the 3rd Generation Partnership Project (3GPP) expands 6G non-terrestrial network (NTN) to S, Ku, and Ka bands for integration with existing terrestrial cellular networks, the necessary satellite channel model for link and network performance evaluation remains tied to TR 38.811, last revised in 2020. We identify issues on multiple fronts: inappropriate parameter specifications based on very limited site-specific environments considered, omission of satellite-induced Doppler spread and directional antenna effects and lack of a transparent channel model generation procedure. To address these limitations, we re-derive the channel parameters using ray tracing (RT) over urban and rural environments based on ITU-R building statistics. Our results show that TR 38.811 substantially overestimates the K-factor at low elevations and predicts a decreasing trend with elevation, which is inconsistent with RT. The Ka-band clutter loss (CL) also exceeds the RT estimates by more than 15 dB in urban cases. It also underestimates rural delay spread (DS) at low elevations and predicts an elevation trend inconsistent with RT. We further quantify the previously neglected satellite-induced Doppler spread. Finally, we demonstrate that the conventional TR 38.811 channel generator significantly underestimates the K-factor in the presence of directional antenna (beamforming), while our modified generator substantially reduces this discrepancy.