# Design and Evaluation of Dynamic Topology for Mega Constellation Networks
## 1 Introduction  
With the breakthrough of new technologies related to satellite communications, the promotion of the 5G era, and the development of smart cities, more than 10 domestic and foreign companies have proposed to build large satellite constellations, such as Globalstar, Iridium, and other new LEO constellations. However, there is no international standard for the number of giant constellations, and it is challenging to coordinate and manage a mega constellation system only from the national level. Generally, the emerging LEO mega-constellation program aims to provide broadband internet access services to users worldwide through hundreds to tens of thousands of LEO satellites. This program features comprehensive coverage, low latency, high resistance to destruction, and airspace integration, making it ideal for ubiquitous connectivity [1]. All aspects of our daily life are closely related to satellite communications. In the future, a star-ground-converged heterogeneous network will integrate wireless networks, the Internet, local area networks, cellular networks, and more to enhance collaboration between different networks [2]. The topology of a satellite constellation network refers to the connectivity relationships among the satellite nodes in the constellation, including mutual visibility, azimuth, elevation, inter-satellite distance length, and distance variation rate [3]. Variations in satellite topology often affect performance metrics such as ISL connectivity, communication terminal utilization, average end-to-end delay, average inter-satellite proximity distance, and the number of hops for packet forwarding [4]. In addition, the destructiveness of the satellite network itself is also closely related to the topology configuration. Therefore, areasonable network topology directly affects the routing communication, quality of service, and system complexity of the constellation. The high number of satellites in a mega constellation and their rapid and perpetual movement results in a complex satellite topology that requires immediate analysis, optimization, and evaluation. This paper employs the topological configuration as a basis and introduces dynamic links to investigate optimal network performance. Furthermore, we evaluate and compare the simulation results of the designed constellation network topology in three areas, highlighting the contributions of our research:  
* Based on the complexity of the number of satellites and the periodic and predictable nature of their movements within mega constellations, a virtual topology is constructed using equidistant time slicing to enable dynamic simulation and management of the topology;
* After introducing dynamic links, two different combinations were adopted for the three fixed links, and based on the dynamic link selection algorithm, connection thresholds were set for each satellite to obtain the corresponding dynamic topology during each time slice;
* A joint simulation platform established a call model based on global population density to perform network performance simulations and compare the designed static topology, two dynamic topologies, and a benchmark topology. The evaluation was focused on the performance of different topology networks in three aspects: average call distance, number of hops for packet forwarding, and total time delay. As shown in Section 2.1, two types of existing satellite network topologies are static and dynamic. Previous work has focused chiefly on the fixed mesh topology, but this type of topology could be more flexible and can limit network performance. To fully leverage a satellite's operation and obtain the variation law of topology during the operational cycle for optimal performance, it is imperative to optimize satellite topology [5]. A well-designed satellite network topology can reduce link switching caused by the high-speed operation of LEO satellites and maximize the use of limited link bandwidth to meet user requirements.