The high-altitude platform can provide a wide variety of services and applications for fixed or mobile, personal or group users, and therefore must comply with the existing wireless standard protocols or develop protocols that are consistent with them. Only in this way, more user terminals can use the high-altitude platform. At present, there is no established standard protocol for high-altitude platforms. The international telecommunication union radio communication group (ITU-R) stipulates that the high-altitude platform uses 2GHz when it provides communication services as a 3G base station. However, the actual broadband fixed access and mobile radio access bands have been increased to the millimeter band. More specifically, the frequencies are 31/28GHz and 48/47GHz. There are a number of candidate standards that can be adopted,¹⁵ in particular the IEEE 802 series of standards (IEEE 802.11, IEEE 802.16, and IEEE 802.20), the data over cable service interface specification (DOCSIS) which includes multichannel microware distribution system (MMDS) and Local multipoint distribution service (LMDS), and the digital video broadcasting (DVB) standards, such as DVB-S/S2 and DVB-RCS.

      At present, many countries have actively carried out research projects on high-altitude platforms, including the recently completed HeliNet project¹⁶ and the ongoing CAPANINA project.¹⁷ The HeliNet project began in January 2000 and ended in May 2003, and its outcomes had been presented to the fifth European Commission Framework Plan. Meanwhile, a large-scale project called Heliplat has also been carried out to implement three experimental applications: broadband communication, environmental monitoring, and remote sensing. This is also the first time in the history of the European Union funding has been provided for projects on high-altitude platforms. The CAPANINA project is funded by the European Commission to further develop wireless and optical broadband technologies for high-altitude platform systems. Its goal is to provide effective network coverage and low-cost broadband communication services for users in remote locations, users very long distance from ground communications facilities, and users on high-speed trains. At the same time, the project requires a transmission rate of 120Mbit/s within the coverage of 60 km. Millimeter wave technology and free space optical communication technology have become the research focus of the project.

      The high-altitude platform is to serve as a candidate technology for supporting and complementing the world’s two best communications systems, ground mobile communications system and satellite system. And thus, it requires that high-altitude platform systems have efficient spectrum multiplexing technology in order to ensure high spectral efficiency of the system. Therefore, the integration of high-altitude platforms into mobile cellular networks for frequency reuse is an actively studied area in high-altitude platforms research. In addition, the frequency bands used by the above-mentioned high-altitude platforms are also used by other systems. Therefore, some scholars have studied the sharing of spectrums between high-altitude platforms and other systems.¹⁸ It is worth emphasizing that array antennas are almost the best choice for high-altitude platforms. The stable coverage of multiple cells in the presence of random fluttering at high-altitude platforms can only be achieved by multi-beam pointing through the antenna array. Therefore, in order to provide communication services from high-altitude platforms for the ground, it is more important to rationally design multi-beam antenna arrays for high-altitude platforms and multi-cell planning based on antenna arrays. A little different from other systems, the high-altitude platform will suffer worse stability and aerial positioning, which requires a more precise design of the high-altitude platform and the ground receiving end to ensure that the beam of the antenna can maintain the correct orientation, thus maintaining a stable communication link.

      Compared with the ground mobile network, the most significant advantage of the high-altitude platform is that the cellular network it generates can periodically move within a certain area, and thus, its coverage is not subject to geographical conditions. Since the coverage area of the high-altitude platform is large, multiple cells can be sourced from the same high-altitude platform at the same time, which can effectively improve the utilization of communication resources. In addition, the coexistence systems of high-altitude platforms and ground wireless network will bring new issues such as radio network planning and avoiding inter-system interference. The network coverage of groundcellular systems is mainly affected by objects such as buildings, trees, and hills. However, the network coverage of high-altitude platforms is determined only by the direction of the antenna. Therefore, although the high-altitude platform can be used as an auxiliary communication system, it will also cause stronger interference to the ground cellular network. These problems have recently been extensively studied and discussed. The main solution is to use cognitive radio technology and dynamic spectrum sensing technology. Both technologies are highly promising solutions to avoid interference problems. Therefore, the research and development in this field will also promote the commercialization of high-altitude platform systems.

1.3The Overview of Space-Based Cooperative Transmission System

      The traditional space-based cooperative transmission system is a communication system taking satellite as the forwarding center. Since satellites are usually located at high altitudes away from the ground, space-based systems have incomparable advantages in terms of coverage. Satellite communication systems play an important role in data transmission and global information interaction, especially in maritime, earth observation, and all-weather surveillance. With the increasing demand for bandwidth, service providers and related agencies have to increase the number, bandwidth, and power of satellites. However, the lack of orbital positions for GEO satellites and the lack of available spectrum resources, as well as the increased complexity and increased operating costs caused by power improvements, make these improvements for satellite difficult to achieve. Considering these factors, the space-based cooperative transmission system is proposed to collaborate the multi-coorbital satellites and adopt the technical advantage of multi-antenna systems. Multi-coorbital satellites technology keeps multiple satellites with the same or similar functions in the same orbital position Synchronization and data exchange are achieved through satellite links to form a satellite group with cooperative transmission and forwarding capabilities. Therefore, the utilization rate of satellite orbit resources can be effectively improved, and the deficiencies of single-satellite platform load and power limitation can be compensated. For the multi-antenna technology,