| In recent years,the emerging mega-constellation plans,represented by Starlink and One Web,have promoted the new wave of Low Earth Orbit(LEO)mega-constellation networks(MCNs).MCNs plan to launch hundreds to thousands of satellites to provide ubiquitous broadband Internet access services for users all over the world.They have large scale,wide coverage,low latency,high bandwidth,and integrates the space and ground segments.The rapidly expanded scale not only improves the system capacity,but also greatly increases the system complexity,which brings many problems for network operation management and efficient data transmission.Based on the review of LEO mega-constellation(Mega-LEO)based integrated satellite-terrestrial networks(ISTNs),this paper studies topology dynamics management,inter-satellite relay hop-count evaluation,distributed routing algorithm design,and gateway placement optimization.The main contributions of this paper are as follows:1.Analysis and management of MCN topology dynamics.Firstly,the LEO constellation dynamics are introduced,and the topology dynamic characteristics of constellation network are summarized.Secondly,aiming at topology dynamics of polar orbit constellation,a Celestial Sphere Division-based Virtual Node(CSD-VN)method is proposed.The virtual node(VN)is bound with the fixed celestial region,and the topology dynamics in space segment are shielded by dynamic VN mapping.The static virtual network topology is generated and the issues of Earth rotation and seam movement are solved.To solve the VN switching asynchrony in polar regions,an improved inter-satellite link(ISL)configuration is proposed,and the minimization condition of inter-plane phase difference with non-zero phase offset is proved to maximize the available ISLs.Simulation results verify the dynamics management capability of CSD-VN and show it has advantages in ISL availability,system capacity,and network delay compared with traditional methods.2.Fast estimation method and analysis of ISL transmission hops in MCN.Based on the regularity and symmetry of constellation topology and subsatellite point moving model,according to the change of satellite phase and right ascension of ascending node(RAAN)caused by each forwarding,an explicit and analytical ISL hop-count estimation method is proposed,which can rapidly estimate the ISL hop-count with only users’ geographic location information.Based on the model,the spatial distribution symmetry of ISL hop-count is further studied,and it is proved that inter-satellite hop-count only depends on the user latitudes and the absolute value of longitude difference.The accuracy and applicability of the method in typical MCN are verified by simulations,and the intersatellite path characteristics in Starlink constellation are thoroughly analyzed,including the spatial distribution characteristics of ISL hop-count,the effects of constellation parameters on ISL path,and ISL path characteristics in specific regions.Finally,it is found that ascending or descending types of access satellites may cause significant variations in ISL hop-count and routing.3.Distributed congestion avoidance routing algorithm design in MCNs.Towards the Mega-LEO ISTN scenario,a distributed multi-hop routing algorithm with a congestion avoidance mechanism is proposed.CSD-VN method is used to shield the topology dynamics.The routing algorithm consists of three processes: i)Gateway assignment;ii)Path candidate region partitioning and iii)Optimal ISL path generation.Based on the 2-D Markov model,the congestion avoidance problem is modeled as a path survival problem.Longer Side Priority(LSP)strategy is proposed,aiming at maximizing the path searching space.By solving the path survival problem,LSP strategy is proved to perform better in congestion avoidance.The algorithm decides the next hop routing based on local network states,which is simple and easy to implement.Simulation results verify the advantages of the proposed algorithm in load balancing and packet loss rate in MCNs.4.Modeling and optimization of gateway placement in ISTNs.Two different models and methods are proposed to solve the gateway placement optimization(GPO)problem in Mega-LEO-based ISTNs.The network performance is evaluated by routing simulation and inter-satellite traffic model,respectively.The network characteristics of ISTN are thoroughly analyzed by simulation.1)By geographical grid division and gateway candidate selection,the optimal grid coding sequence is obtained by solving the traveling salesman problem,the gateway placement integer optimization model is established.The objective function is a combination of average network delay,satellite load,and gateway load balance,and is evaluated by network routing simulation.The optimal gateway placement is solved by genetic algorithm(GA).Based on Starlink,simulations analyze the impacts of gateway placement,amount,selection strategy,and antenna number on network performance,and obtains the popularity of gateway candidate regions.2)the ISL usage is converted into a traffic-weighted hop-count by inter-satellite traffic model and used as the objective function to establish a mixed integer optimization model for gateway placement.The problem is decoupled and simplified by deriving the optimal gateway assignment strategy,and is solved by an Improved Binary Discrete Particle Swarm Optimization(IBD-PSO)algorithm.Based on the hop-count model,an analytical objective evaluation approach is proposed,which can reduce the computation time by 80 % compared with the routing simulation method.Based on Starlink,the simulation analyzes the impacts of gateway placement,satellite ascending / descending type,user distribution model,and ISL capacity on network performance.The results of the two GPO studies are consistent and verify each other.This paper thoroughly studies the Mega-LEO-based ISTNs through both theoretical analysis and numerical simulation ways.The effects of constellation network topology characteristics,constellation design,inter-satellite routing algorithm,and ground station design on network performance are investigated.The research model,method,and results of this paper can provide significant theoretical and practical references for the design of MCNs. |
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