## 1 Introduction  
### Background  
According to the report of Cisco, there is an ever-increasing number of devices that are wirelessly connected to the Internet, which will reach approximately 12.3 billion by 2023 [1]. The performance requirements of the next generation communication system are a peak data rate of 1000 Gbps (gigabits per second) and air latency of less than 100 microseconds (\(\upmu\)) [2]. Furthermore, NR over NTN is being specified in Release 17 [3]. Hence, this paper aims to stimulate future investigations by the research community on the next generation communication system. One of the research focuses of next-generation communication systems is to achieve 3D seamless coverage, thereby providing efficient, high-quality, low-cost, and global roaming broadband communication services [4]. 5G relies on the seamless integration of non-terrestrial communication systems (such as unmanned aerial vehicles and ultra-low orbit satellites) with cellular communication systems to achieve this goal [5]. Satellite communication systems have become the choice to achieve higher capacity, real-time communication, and wider coverage due to their abilities of seamless connectivity and high data rates [6]. Although the traditional satellite communication systems can cover the globe, it is difficult to meet the increasing demand for users due to their low spectrum utilization and limited satellite capacity. Compared with Geostationary Earth Orbit (GEO) and Medium Earth Orbit (MEO), Low Earth Orbit (LEO) satellites have many advantages, such as low cost, short delay, small path loss, and so on. There have been many mega-constellations of LEO satellites demonstrated by recent commercial corporations to provide global broadband internet access services, e.g., OneWeb, StarLink, Space X, and HongYan [7]. At the same time, CubeSats is also being studied which is cost-effective and can be built using commercial off-the-shelf components [8; 9]. Aiming at ubiquitous connection requirements of users, the next generation communication system puts forward higher requirements for multi-antenna multiple input multiple output (MIMO) technology. However, the small size and light weight make the onboard resource of LEO satellite severely restricted. It is difficult for a multi-antenna device on a single satellite to meet the requirements of MIMO technology for channel isolation, and it is difficult to implement the MIMO technology in a single satellite application scenario. The phased array antenna which can improve the Equivalent Isotropic Radiated Power (EIRP) and the spectral efficiency by building a virtual multi-antenna system is applied to multi-beam satellites. In addition, with the development of Orthogonal Frequency Division Multiplexing (OFDM) and multiplexing diversity technology, packets can be transmitted by coordinating multiple beams in multi-beam satellite communication systems. The demand for system capacity and reliability of the next-generation communication system to achieve global seamless ubiquitous access can be satisfied. In dense urban areas, the user equipment (UE) cannot receive satellite line-of-sight (LOS) signals, and the signal to interference noise ratio (SINR) of received non-line-of-sight (NLOS) signals is low, which makes it difficult to meet the demodulation threshold of communication signals. Therefore, satellites cannot be used to transmit signals in dense urban areas, resulting in a waste of radio resources. In multi-beam satellite communication systems, multi-beam satellites can provide multi-dimensional coverage for users according to the users' demands. Received NLOS signals are superimposed to enhance the SNR, which can meet the demodulation threshold of communication signals. With the development of various emerging applications and the promotion of new disruptive services and applications, the demand for network efficiency is constantly improving [10].