Optimizing Beam Size in Multibeam LEO Satellite Networks: Addressing Interbeam Interference, Doppler Shift, and Frequency Reuse

Low Earth orbit (LEO) satellites offer a robust and promising solution for enhancing 6G connectivity by providing increased coverage and improved communication capabilities. In multibeam LEO satellite systems, the size of the spot beam plays a critical role in affecting interference and residual Doppler shift experienced by LEO users, thereby significantly impacting overall network performance. This article focuses on optimizing the spot beam radius in multibeam LEO systems by addressing key challenges in practical LEO deployments, such as Doppler shift, interbeam interference, and various frequency reuse schemes. We investigate three scenarios: 1) no frequency reuse; 2) full-frequency reuse; and 3) higher frequency reuse. An analytical framework is developed to design a multibeam LEO system with an optimal spot beam size and frequency reuse scheme that maximizes system capacity while minimizing interbeam interference. Corresponding optimization problems are formulated for each scenario. To address the nonconvex nature of these optimization problems, we employ the successive convex approximation method, converting them into simplified simplex forms. Through simulations, we determine the optimal beam radius and frequency reuse scheme for each scenario and analyze per-user capacity. Our results are benchmarked against a grid-based search method, demonstrating that our analytical approach provides optimal solutions with lower computational complexity. This study offers valuable insights into the design of multibeam LEO satellite systems that optimize capacity and reduce interference.