HAPS-Based Interference Suppression Through Null Broadening With Directivity Control in Space-Air-Ground Integrated Networks

Space-air-ground integrated networks (SAGINs) are attracting increasing attention for extending wireless communication coverage. Furthermore, high-altitude platform stations (HAPSs) can provide low-latency internet access to areas where terrestrial networks do not operate as unmanned flying base stations. However, as more frequency bands are used by wireless communication systems in terrestrial and non-terrestrial networks, it is necessary to make effective use of the finite frequency bands for multiple systems. Therefore, this study focuses on frequency sharing between the uplink from HAPS to satellite and the downlink from HAPS to ground users for efficient use of finite frequency bands in a SAGIN. In such frequency sharing, the back lobe toward the rear of the HAPS antenna for uplink communication causes interference when receiving the downlink signal from HAPS. Therefore, the zero-forcing (ZF) method is applied to the directivity control of the HAPS-mounted uplink antenna, which helps direct the nulls with significantly low antenna gain toward ground users. However, the coverage of the nulls due to the above control alone is extremely insignificant, which cannot sufficiently guarantee interference suppression for diverse user distributions. Therefore, this study proposes a HAPS-based null broadening approach with directivity control according to the user distribution for interference suppression in a SAGIN. The effectiveness of our proposal is evaluated by comparing it with the existing covariance matrix tapper(CMT)-based null broadening, ZF-based nulling, and no nulling for the HAPS-mounted uplink antenna using numerical analysis.