Contents I. Introduction
Compared with current fifth-generation (5G) wireless communication networks, the next-generation wireless communication networks is expected to provide uninterrupted and seamless connectivity for global users [1]. However, the cost of global interconnection through traditional terrestrial networks is extremely high, especially in remote areas [2]. In addition, due to geographical limitations, deploying high-density base stations (BSs) in areas such as high mountains and seas is difficult to achieve and unprofitable. Due to the advantages such as wide coverage, large communication capacity, and not limited by terrain, satellites can make up for the above shortcomings. But compared to terrestrial networks, it also has drawbacks such as severe fading and weak processing ability. Meanwhile, aerial networks have the characteristics of flexible deployment and low cost, which have been well developed and utilized. Therefore, combining satellite communication, aerial communication and terrestrial communication to form a multi-layer integrated network can meet the requirements of various users. In view of the above observations, the satellite-aerial-terrestrial (SATN) network is regarded as an important infrastructure of the sixth-generation (6G) networks [3], [4], [5], which can improve their flexibility and coverage [6]. In terms of composition, SATN includes many devices/platforms, which are mainly divided into three categories: Satellites, aerial platforms (APs), and terrestrial nodes. Due to the fact that SATN is a three-dimensional transmission system, it has advantages such as flexibility, mobility, and high adaptability, and each node can serve as a BS [7], [8], [9], relay [10], [11], [12], or receiving device [13], [14]. Compared to current mobile communication networks, SATN tends to provide additional and more reliable line-of-sight (LoS) links because satellite/aerial nodes have higher altitudes [15]. Therefore, SATN can provide a better channel environment than complex terrestrial fading channels. The channel state information (CSI) of the three-dimensional position is also easier to predict using the position information feedback from terrestrial equipment [16]. In addition to the above advantages, SATN in 6G will also face the following demands: 1) The connectivity density of the terminal will be 100 times compared to its 5G counterpart; 2) the peak rate requirements of either virtual reality or augmented reality users will reach the terabits per second level; 3) the energy efficiency (EE) and spectrum efficiency (SE) of 6G shall reach 100 times and 10 times of 5G, respectively. Before fulfilling these requirements, SATN still needs to address some key issues and specific challenges in the following.
Severe fading: Due to the long distance between satellite/aerial nodes and terrestrial users, it is inevitable that this will lead to signal delay and a decrease in communication quality. In addition, in future wireless communication, high-frequency bands and large bandwidth are the trends, such as millimeter waves (mmWave) and terahertz (THz), to solve the problems of insufficient throughput and spectrum shortage. However, fading of high-frequency signals is severe, and the effective transmission time is short. Besides, it is vulnerable to harsh environments.
Weak connectivity: Due to the mobility of high-altitude nodes, it is necessary to adjust the antenna pointing in a timely manner. In addition, in areas with high population density, such as cities, the wireless communication environment is relatively complex, which can lead to invisibility between APs and terrestrial users, resulting in the obscuration of LoS links and connectivity issues. For example, tall buildings, walls, and trees can all have a masking effect on signals. Therefore, not all users can establish a good quality communication link with satellite/APs.
Mutual interference: When terrestrial BSs transmit signals to aerial users, concentrated power is required for transmission to adapt to long-distance communication. However, strong beams cause serious interference to other aerial users in the same direction, especially for some users who require high communication quality, and lead to signal interruption. Moreover, the collaboration between multiple APs can also bring co-channel interference (CCI), and leading to a decrease in quality of service (QoS).
Power limitation: Energy supply is an important indicator of APs, which affects their weight, size, power, etc. Taking unmanned aerial vehicles (UAVs) as an example, due to the limitations of battery storage, it is difficult to achieve long-term operation, which is also one of the bottlenecks in the promotion of UAVs. Meanwhile, the onboard processing capability of UAVs is limited, making it difficult to execute high-complexity power allocation algorithms. In the complex wireless communication environment, fast and efficient signal processing is necessary, which also requires the system to provide sufficient energy.
Security risks: The satellite/APs in SATN will bring more potential safety risks. Specifically, the coverage area of satellite/APs is larger than the conventional terrestrial BS, thus the range of signal propagation is wider, making it more convenient for illegal users to eavesdrop on the legitimate signal.
Hardware constraints: In general, we can increase the spatial degree of freedom (DoF) by equipping more antennas to improve the communication performance of the system. However, due to hardware limitations of satellite/APs, such as the size of satellites and the upper bound of UAVs’ weight, many advanced technologies cannot be applied to satellite/APs, such as multi input multiple output (MIMO) technology.
