Efficient Interference Management Design for NTN/TN Co-Existence in HAP-Based 6G Networks

High-altitude platforms (HAPs) are critical for 6th generation (6G) networks, particularly in autonomous flying ad-hoc networks (FANETs). However, their extensive coverage areas cause severe co-channel interference (CCI) when overlapping with terrestrial networks (TNs) that share the same frequency bands as mandated by 3rd Generation Partnership Project (3GPP). To address this, we propose a novel three-phase interference mitigation design that operates without requiring non-terrestrial network (NTN)/TN coordination. First, subspace-based blind channel estimation extracts the channel state information (CSI) of the aggressor base station (BS). Next, an optimized sparse codebook is designed using the estimated victim channel to suppress interference and enhance spectral efficiency. Finally, the codebook is utilized for pilot-assisted data transmission, ensuring reliable communication despite CCI. Mathematical analysis and numerical results validate the proposed method, demonstrating significant reductions in CCI and inter-carrier interference (ICI), maintained similar level of peak-to-average power ratio (PAPR), and enhanced signal-to-interference-plus-noise ratio (SINR), making it a scalable and efficient solution for non-terrestrial network (NTN)/terrestrial network (TN) co-existence in 6G networks.