Joint On-Ground and On-Board Beamforming for Multi-Gateway Multibeam Satellites: Two-Timescale Approach

This paper aims at designing a joint on-ground and on-board beamforming scheme for multi-gateway multibeam satellite systems. The proposed scheme exploits the degrees of freedom of on-board processing to mitigate feeder link interference and enhance satellite resource flexibility without requiring additional feeder link bandwidth, thereby improving the overall throughput. Based on the practical consideration of not escalating the complexity and real-time computational burden of the satellite payload, the two-timescale design is employed. Specifically, the long-term on-board beamforming network (BFN) is designed based on the statistical channel state information (S-CSI) of all links, while the short-term on-ground precoding is designed based on the instantaneous CSI (I-CSI) of the effective channel with optimized BFN. We formulate an average sum rate maximization problem that incorporates the lossless constraint of the BFN, as well as the average power constraints at the satellite. We propose an algorithm called two-timescale joint on-ground and on-board beamforming (TJGBB) to tackle this mixed-timescale non-convex stochastic optimization problem. Simulation results demonstrate the effectiveness of the proposed algorithm and its advantages over the benchmark schemes.