UltraStar: A Lightweight Simulator of Ultra-Dense LEO Satellite Constellation Networking for 6G Xiaoyu Liu, Student Member, IEEE, Ting Ma, Member, IEEE, Zhixuan Tang, Member, IEEE, Xiaohan Qin, Student Member, IEEE, Haibo Zhou, Senior Member, IEEE, and Xuemin (Sherman) Shen, Fellow, IEEE Abstract—The mega-constellation network has gained signifi- cant attention recently due to its great potential in providing ubiquitous and high-capacity connectivity in sixth-generation (6G) wireless communication systems. However, the high dynam- ics of network topology and large scale of mega-constellation pose new challenges to the constellation simulation and performance evaluation. In this paper, we introduce UltraStar, a lightweight network simulator, which aims to facilitate the complicated simu- lation for the emerging mega-constellation of unprecedented scale. Particularly, a systematic and extensible architecture is proposed, where the joint requirement for network simulation, quantitative evaluation, data statistics and visualization is fully considered. For characterizing the network, we make lightweight abstractions of physical entities and models, which contain basic representatives of networking nodes, structures and protocol stacks. Then, to consider the high dynamics of Walker constellations, we give a two-stage topology maintenance method for constellation initial- ization and orbit prediction. Further, based on the discrete event simulation (DES) theory, a new set of discrete events is specifi- cally designed for basic network processes, so as to maintain net- work state changes over time. Finally, taking the first-generation Starlink of 11 927 low earth orbit (LEO) satellites as an example, we use UltraStar to fully evaluate its network performance for different deployment stages, such as characteristics of constella- tion topology, performance of end-to-end service and effects of network-wide traffic interaction. The simulation results not only demonstrate its superior performance, but also verify the effec- tiveness of UltraStar. Index Terms—Discrete event simulation (DES), mega-constellation, network dynamics, performance evaluation, simulation architecture design.    I. Introduction T HE current terrestrial networks face significant chal- lenges to support seamless coverage, massive connectiv- ity, and diverse applications to meet exponentially growing data traffic [1]. Fortunately, recent developments in ultra- large-scale low earth orbit (LEO) satellite constellations may make up for the shortcomings of traditional terrestrial net- works [2]–[5]. An ultra-dense (UD) constellation formed by thousands of LEO satellites can supply ubiquitous coverage and provide reliable and low-latency services all over the world [6], [7]. Unlike traditional geosynchronous earth orbit (GEO) satellites, LEO satellites orbit the earth at an orbital altitude of less than 2000 kilometers. Such a low orbital alti- tude means that the communication delay between satellite and ground station is relatively low. Moreover, the faster speed of light in a vacuum and the avoidance of long and winding terrestrial optical fiber paths will make up for the connection overhead between satellite and ground station, which will enable the LEO satellite network to provide lower end-to-end communication delays for long-distance communi- cations than terrestrial optical fiber communication networks. Differently from the existing satellite network [8]–[10], which can only provide limited access, the emerging LEO satellite constellation can not only expand the network coverage to the most remote areas with broadband speed and low latency, but also compete with the terrestrial network for traditional traffic services in the current market. In order to achieve global cov- erage and provide sufficient access bandwidth for a larger tar- get user group, the number of satellites of the new system is unprecedented. Therefore, the name “mega-constellation” is created. New technologies such as the miniaturization of satel- lites and reusable rockets also make these emerging LEO satellite constellations possible [11], [12]. In recent years, more and more LEO satellites are launched into space. Since SpaceX [13] announced its project Starlink in 2014, it has developed into an LEO constellation system with the fastest transmission frequency and the largest number of satellites in orbit. Several other commercial enterprises have also announced their constellation projects, such as OneWeb [14] and Amazon [15]. Driven by the above exciting prospects of the future mega- constellation, researches on constellation simulation, perfor- mance evaluation and network optimization are urgently req- uisite. Fundamentally different from the traditional terrestrial network, the high mobility of a large number of satellites result in highly dynamic constellation topology and intermit- tent connectivity. Routers and switches in terrestrial wired networks are generally static, and even in terrestrial mobile cellular networks, the dynamics and connectivity of mobile Manuscript received August 27, 2022; accepted November 2, 2022. This work was supported in part by the National Key Research and Development Program of China (2020YFB1806104), the Natural Science Fund for Distinguished Young Scholars of Jiangsu Province (BK20220067), and the Natural Sciences and Engineering Research Council of Canada (NSERC). Recommended by Associate Editor Qing-Long Han. (Corresponding author: Haibo Zhou.) Citation: X. Y. Liu, T. Ma, Z. X. Tang, X. H. Qin, H. B. Zhou, and X. M. Shen, “UltraStar: A lightweight simulator of ultra-dense LEO satellite constellation networking for 6G,” IEEE/CAA J. Autom. Sinica, vol. 10, no. 3, pp. 632–645, Mar. 2023. X. Y. Liu, T. Ma, Z. X. Tang, X. H. Qin, and H. B. Zhou are with the School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China (e-mail: xyliu0119@163.com; majiawan27@163.com; zhi xuantang@smail.nju.edu.cn; xhderemail@smail.nju.edu.cn; haibozhou@nju. edu.cn). X. M. Shen is with the Department of Electrical and Computer Engineering, University of Waterloo, Waterloo N2L 3G1, Canada (e-mail: sshen@uwaterloo.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JAS.2023.123084 632 IEEE/CAA JOURNAL OF AUTOMATICA SINICA, VOL. 10, NO. 3, MARCH 2023