# Performance Analysis of RIS-Assisted SatComs Based on a ZFBF and Co-Phasing Scheme
###### Abstract  
In recent high-throughput satellite communication (SatCom) systems, the use of reconfigurable intelligent surfaces (RISs) has emerged as a promising solution to improve spectral efficiency and extend coverage in areas with limited terrestrial network access. However, the RIS may amplify the inter-beam interference (IBI) caused by multibeam transmission at the satellite, and multiple RISs can also cause inter-RIS interference (IRI) to terrestrial users. In this paper, the performance of the RIS-assisted SatCom system is asymptotically analyzed for both full and partial channel state information (CSI) scenarios. In particular, zero-forcing beamforming is considered as the active beamforming for data transmission, while the co-phasing scheme is considered as the passive beamforming for RIS reflection. Based on the asymptotic analyses, deterministic active and passive beamforming techniques using partial CSI are proposed that can gradually eliminate both IBI and IRI, ultimately achieving ideal performance. Simulation results validate the accuracy of asymptotic analyses and demonstrate the superiority of deterministic active and passive beamforming techniques using partial CSI. The simulation results also confirm that the proposed beamforming can achieve approximately 92.8% of the ideal performance, even though it only requires partial CSI. active and passive beamforming; co-phasing technique; full frequency reuse (FFR); reconfigurable intelligent surface (RIS); satellite communication; zero-forcing beamforming (ZFBF)  
26 March 202418 April 2  
UAVs equipped with large-scale antenna arrays acted as relays between satellites and terrestrial networks. Additionally, in [13], a relay architecture for UAVs operating between the ground and satellites was investigated. The studies in [5; 6; 7] examined the concept of RISs deployed on man-made structures, such as buildings and walls, to reflect or refract signals from satellites to the destinations. However, the assumption of UAVs as active relays consuming additional energy for signal processing, as presented in [11; 12; 13], is impractical due to the limited battery capacity of UAVs and the need for energy-efficient operation. Moreover, the deployment of RISs on the surface of ground structures, as assumed in [5; 6; 7], may not adequately complement existing SatCom systems or fully harness the potential for enhancing network capacity. Meanwhile, to achieve high spectral efficiency and throughput in the upcoming multibeam HTS systems, full frequency reuse (FFR) is generally considered in SatCom systems [14]. This approach allows each beam to exploit all the available bandwidth, ensuring maximum resource utilization. However, the performance of FFR-based SatCom systems can be degraded by inter-beam interference (IBI) resulting from the overlapping side lobes between the multiple beam radiation patterns. In an RIS-assisted SatCom system, the FFR in the RIS-UE links may cause amplification of IBI, and multiple RISs may also result in inter-RIS interference (IRI) to terrestrial UEs. Moreover, acquiring full CSI for all RIS-related channels is challenging in practice [15]. Hence, integrating RIS into the SatCom system poses the inevitable challenge of mitigating IBI and IRI by relying on partial CSI of the desired channel. Recently, in [16], the performance of a two-layer RIS-assisted UAV communication system was analyzed. In [17], the authors jointly optimized user scheduling and phase shift in the simultaneously transmitting and reflecting (STAR)-RIS-assisted multicast SatCom systems. However, these studies did not consider the impact of IBI and IRI introduced by RIS on the performance of the considered system. In this paper, we conduct a performance analysis considering the impact of IBI and IRI, which are crucial factors in RIS-assisted SatCom systems. To overcome the aforementioned limitations, we propose a deterministic zero-forcing beamforming (ZFBF) combined with the co-phasing technique that only requires partial CSI of the desired channel.