UAV-RIS-Aided SAGIN Interference Alignment Design and Degrees-of-Freedom Analysis

In space-air-ground integrated networks (SAGIN), terminals face interference from various sources such as satellites and terrestrial transmitters. However, managing interference with traditional interference management schemes (IM) is challenging since different terminals have different channel state information (CSI). This paper introduces a UAV carrying an active RIS (UAV-RIS) to assist in the interference elimination process. Furthermore, a UAV-RIS-aided IM scheme is proposed, which takes into account the multiple types of CSIs present in SAGIN. In this scheme, the satellite, terrestrial transmitters, and UAV-RIS collaborate to design precoding matrices based on the specific type of CSI of each node. Additionally, the DoF gain obtained by the proposed IM scheme is thoroughly discussed for SAGIN configurations with different numbers of users and transceiver antennas. Simulation results demonstrate that the proposed IM scheme outperforms existing IM schemes without UAV-RIS for the same type of CSI. The results also showcase the capacity improvement of the network when the proposed IM scheme is adopted under different types of CSI.