Interference Management with Nullforming and Resource Allocation for Coverage Expansion in Integrated HAPS–Terrestrial Networks

In direct air-ground communications, high-altitude platform stations (HAPSs) are attracting considerable attention for their potential to extend the coverage of cellular networks significantly. Nullforming schemes for HAPSs have been proposed, enabling spectrum sharing between HAPSs and terrestrial networks by reducing interference with terrestrial base stations (BSs). However, nulls toward terrestrial BSs significantly degrade the received power of nearby HAPS users, creating a coverage hole around every BS. To overcome this issue, this study proposes a resource-allocation-aided nullforming scheme. Here, a HAPS stops nullforming toward terrestrial BSs on specific time-frequency resources to remove coverage holes, and each terrestrial BS performs user scheduling considering the presence of nulls to mitigate the impact of HAPS' interference on signal-to-interference-plus-noise ratios (SINRs). Simulation results show that the proposed scheme outperforms the conventional nullforming scheme with respect to coverage and achieves an SINR of 0 dB in 99.9% of the target service area covered by a single HAPS.