Broadband LEO Satellite Communications: Architectures and Key Technologies

This article aims to provide a comprehensive overview for key issues in broadband LEO satellite communication systems. First of all, the network architecture is introduced, which is the basis of the whole system. The space-based LEO system with ISL, which requires a small number of ground gateways, is the focus. In this system, the satellite constellation design is important with impact on key system performances such as coverage. Two popular LEO constellations, the walker Delta and Star constellations, are introduced. Given satellite constellations, proper beam coverage schemes should be employed at satellites to provide seamless coverage all over the world. A hybrid wide and spot beam coverage scheme is presented, where the LEO provides a wide beam for large area coverage and several steering spot beams for highspeed data access. Moreover, special coverage schemes should be designed in broadband LEO systems for the interference coordination between LEO and GEO. To protect GEO communications, LEO satellites should be turned off if they cause interference to GEO. In this case, to provide services for users covered by the turned-off LEO satellites, a progressive pitch method and a coverage expanding method can be employed. Finally, the coverage performance of LEO is also closely related to resource management schemes. The global resource management for broadband LEO systems is complicated, involving a large amount of data, and a two-level management structure should be employed. Using this structure, an NMC with powerful storage and processing capabilities is employed to carry out the first-level management, making strategies based on all information collected from the whole system. Then satellite base stations with limited capabilities are employed to respond to the strategies generated by NMC in real time.

Introduction

Although communication and networking technologies are developing rapidly, there is still a long way to go to connect any user at any time and any place. According to “World Internet Development Report 2017” published at the 4th World Internet Conference, the global Internet penetration rate was only 51.7 percent, which means that half of the population on Earth is still not connected to the Internet. For instance, in remote regions, the North and South Poles, and underdeveloped villages, it is difficult to provide telecommunication services by terrestrial networks due to the high construction costs. Moreover, for those special scenarios such as high-speed trains and planes, the quality of service (QoS) provided by terrestrial wireless communication networks is poor, due to high call drop rate caused by high mobility and frequent handover between the cells. In these cases, satellite communications have attracted a lot of attention, thanks to its good performance in various aspects such as abundant radio frequency resources, large coverage areas, long communication distance, fast deployment, and little interference from the ground network. Satellite communications have played an indispensable role in television broadcasting, long-distance communication, and disaster relief [1], [2]. Recently, the Third Generation Partnership Project (3GPP) standards group has been codifying the use of satellite communication networks so that the future space and terrestrial integrated communication network can be used to support fifth generation (5G) networks [3]. In order to meet the demanding 5G requirements in terms of both large throughput and global connectivity, satellite communications provide valuable resources to extend and complement terrestrial networks [4]–[6]. There is no doubt that satellite communications will usher in a new upsurge for the next generation of wireless communications.