Interference Suppression in HAPS-Based Space-Air-Ground Integrated Networks Using a Codebook-Based Approach

The forthcoming 6G technology aims to ensure widespread connectivity through Space-Air-Ground Integrated Networks (SAGINs), crucial for regions lacking proper terrestrial infrastructure. Utilizing High-Altitude Platform Stations (HAPSs) as aerial base stations, 6G facilitates low-latency internet access. However, the exponential growth in terrestrial wireless devices puts a strain on the available frequency bands shared by terrestrial and non-terrestrial networks. Therefore, in this research, we aim to optimize the resources between HAPS-to-satellite and HAPS-to-terrestrial uplinks. One challenge that arises is the interference caused by simultaneous uplink and downlink transmissions, especially from uplink back lobes disrupting ground user downlinks. To mitigate this interference, current methods like the Zero-Forcing (ZF) proposed a technique to steer nulls toward ground users. However, current methods randomly allocate limited nulls, potentially leaving many users outside null zones and experiencing interference. To address this issue, we propose a solution to design null directivity based on user locations. This is achieved by creating a codebook of antenna patterns that are linked to weight matrices. Selecting suitable antenna patterns from the codebook and adjusting weight matrices accordingly enables null formation at positions that maximize user communication satisfaction. Numerical analysis confirms the effectiveness of this approach, showing increased user satisfaction compared to existing methods.