Research On Multi-satellite And Multi-beam Cooperative Transmission Technology For Ultra-Dense Low Earth Orbit Satellite Networks

Low Earth orbit(LEO)satellite networks have attracted widespread attention both here and abroad due to the characteristics such as short satellite-to-ground transmission delay,low costs for the deployment of LEO satellites,and wide coverage.However,LEO satellite networks have many limitations in terms of available spectrum resources,power resources,and storage resources.Thanks to the unremitting development of satellite payload technology and the reduction of rocket launch costs,the number of satellites in orbit around the world has continued to increase,especially the number of LEO satellites has exploded.The scale of LEO satellite constellations has become dense and gigantic.The Ultra-dense Low Earth Orbit(UD-LEO)satellite network composed of a large number of LEO satellites has become the trend of competition and development in the field of satellite communications today.In the UD-LEO satellite network,the dense satellite topology facilitates the cooperation between satellites.The research on multi-satellite and multi-beam cooperative transmission aims to improve the transmission capacity and communication satisfaction of the UD-LEO satellite network and reduce the content delivery delay.It is of great significance to improve the service capability of the UD-LEO satellite network.This thesis studies the multi-satellite multi-beam cooperative transmission technology in the UD-LEO satellite network.The main research work is summarized as follows:First,for the downlink multi-group multicast transmission scenario in the UD-LEO satellite network,aiming at the problem that the multicast capacity is limited by the bottleneck user’s SINR and the complex interference of multi-satellite and multi-beam in the full frequency reuse mode.The beamforming and power allocation technology are studied.Based on the idea of multi-satellite cooperation,the multicast transmission problem is modelled as a constrained non-convex optimization problem with the goal of maximizing system capacity.Furthermore,multi-satellite cooperative beamforming and power allocation algorithm based on alternating optimization is proposed.The simulation results show that the proposed algorithm improves the system capacity of the UD-LEO satellite network.Second,for the downlink multi-group multicast transmission and caching scenarios in the UD-LEO satellite network,considering that the multicast capacity is limited by bottleneck users and the coupling relationship between satellite cache placement and multicast transmission.The multi-group multicast beamforming technology and on-board caching mechanism are studied.Then,a joint caching and transmission framework based on regional multi-satellite cooperation is proposed.Based on this framework,the joint caching and transmission problem is modelled as a constrained non-convex optimization problem aimed at minimizing the content delivery delay of each user.Furthermore,a transmission-aware and successive convex approximation based cooperative multicast beamforming and dynamic cache placement algorithm are proposed.The simulation results show that the proposed algorithm effectively reduces the content delivery delay of the UD-LEO satellite network.Finally,for the downlink multi-beam transmission scenario of spectrum sharing between the UD-LEO satellite network and the GEO satellite communication system,this thesis takes these problems into consideration:(1)The interference to the GEO satellite communication system during the transmission of the UD-LEO satellite network should not exceed the interference threshold;(2)The interference between GEO and UD-LEO satellite networks;(3)The interference in the UD-LEO satellite networks.Then,focusing on the beam association between multisatellite and terrestrial multi-beam in UD-LEO satellite network and multisatellite multi-beam power allocation problem,a cooperation beam association and power allocation algorithm based on matching game and successive convex approximation is proposed.Simulation results show that the proposed algorithm improves the overall communication satisfaction of the UD-LEO satellite network and ensures fairness among cells with multiple LEO beam cells.