Three-Stage Precoding Scheme Based on Nullforming and Subarray Partitioning for Aerial–Terrestrial Spectrum Sharing

Non-terrestrial networks have been recognized as a key technology for providing a ubiquitous coverage in 5G, 6G, and beyond, where high-altitude platform stations (HAPSs) are an indispensable node. Spectrum sharing in integrated HAPS-terrestrial networks is an important task for achieving high spectral efficiency. This study proposes a novel three-stage precoding scheme for HAPSs to suppress the interference with terrestrial users. Specifically, an outer precoder reduces the interference with neighboring service areas based on nullforming and subarray partitioning. A middle precoder mitigates the interference with terrestrial cells, and an inner precoder forms user-specific beams toward HAPS users. In addition, three precoders with different subarray structures are developed based on the proposed scheme and compared with conventional precoders under the coexistence scenario of seven HAPSs with multiple terrestrial cells. For the case of 12 spatially-multiplexed HAPS users with increasing number of terrestrial cells, the results demonstrate that the proposed precoders reduce the interference with other service areas by over 15 dB, which improves the signal-to-interference-plus-noise ratio (SINR) of HAPS users by at most 7 dB. Moreover, the degradation in the SINR of terrestrial cell users caused by the proposed precoders is only 2.8 dB. Consequently, the total capacity of a HAPS and terrestrial cells improves by 61%–119%, compared with the conventional schemes. Additional evaluation results are provided to verify the effectiveness of the proposed scheme, focusing on the increasing HAPS users, performance with imperfect channels, and computational complexity. This study shows that the significant capacity enhancement and coverage extension achieved by the proposed scheme contribute to successful aerial-terrestrial spectrum sharing.