mobile communication system (5G) is a next-generation wireless mobile communication system that is being developed in order to meet the rapid spread of intelligent terminals and the rapid development of mobile Internet after 4G. It is a wireless mobile communication system for the needs of the human information society after 2020.
5G has become an active research in the field of mobile communication systems at home and abroad. In 2013, the 7th Framework Program, which was jointly undertaken by 29 participants including China’s Huawei Corporation, launched the mobile and wireless communications enablers for the 2020 information society (METIS) project5 for 5G research. China’s 863 Program also launched the first and second phase of 5G major projects in June 2013 and March 2014, respectively. At present, countries around the world are conducting extensive discussions on the development, application requirements, candidate frequency bands, and key technical indicators of 5G, and striving to reach a consensus at the 2015 World Radio Conference. And standardization process started in 2016.6
For the future vision and application of 5G, there have been relevant descriptions in academia and industry, from which we can summarize the technical needs of the future 5G. Compared with the traditional mobile communication network, 5G should have the following basic characteristics such as: (1) the data traffic is increased by 1000 times, (2) the number of networked devices is increased by 100 times, (3) the peak rate is at least 10Gbit/s, (4) the rate users can obtain is 10Mbit/s, and it can be up to 100 Mbit/s for special needs, (5) short delay and high reliability, and (6) high spectrum utilization and low network energy consumption.
At present, the key technologies of 5G are still in research. Technologies such as large-scale MIMO technology, beamforming technology, and cooperative wireless communication technology all have the possibility to become the key technologies of 5G.
MIMO technology can effectively improve the spectral efficiency of wireless communication and obtain receive diversity gain (RDG), which is recognized as the core technology of the next-generation mobile communication system. A typical M × N MIMO system is shown in Fig. 1.1.
Fig. 1.1. An M × N MIMO system.
Since each receiving antenna will receive a superimposed signal from all transmitting antennas, the received signal can be expressed as
where yn, hnm, sm, and nn, respectively represent the received signal of the nth receiving antenna, the channel gain from the mth transmitting antenna to the nth receiving antenna, the transmitted signal of the mth transmitting antenna, and the noise of the nth receiving antenna. It can be seen from Eq. (1.1) that each transmitted signal will have N backups at the receiving end, which is called reception diversity. However, signals from different transmitting antennas form interference at the receiving end. In order to detect the transmitted signal at the receiving end, signals from different transmitting antennas must be extracted. Therefore, the detection algorithm of the MIMO receiver is an indispensable component of the MIMO system.
Besides, beamforming is also a key technology to achieve space diversity gain. Beamforming technology is widely used in directional antenna array radar, sonar hydroacoustic positioning and classification, ultrasonic optical imaging, geophysical exploration, petroleum exploration, biomedical imaging, and wireless communication. At the transmitting end, beamforming technology is used to appropriately weight the signals transmitted by the corresponding antennas in the antenna array to generate a directional virtual beam. Thereby the purpose of enhancing the desired signal and suppressing interference and improving the communication capacity and quality can be achieved. At the receiving end, signals from different receiving antennas are combined in the receiver to achieve coherent superposition and improve the reception quality of the signals. Beamforming technology can be divided into two categories, namely array beamforming based on antenna array and multi-antenna beamforming based on signal pre-processing, which utilize the spatial correlation and independence of different antenna channels, respectively. Array beamforming technology utilizes the strong correlation of spatial channels and the principle of interference of electromagnetic waves. By weighting the correlation of the output signals of multiple antennas in terms of amplitude and phase, the signals will be superimposed in a certain direction and phase cancellation in other directions will be done to enhance the target signal and suppress interference. And for the multi-antenna beamforming technology, it utilizes the independence between different antenna channels to improve the space diversity gain of the system.
For a cellular communication system, when a user is at the cell edge, a signal from a neighboring cell base station will be received. The conventional method is to simply consider the signal from the neighboring cell base station as an interference signal. Because this competition-causing strategy will significantly reduce communication performance, coordinated multipoint (CoMP) technology has attracted widespread attention in 5G communication systems. By utilizing the interaction of mobile channel information and data information between adjacent base stations, the CoMP technology performs an interference avoidance strategy for the interfered users or joint transmission for mobile users by multiple base stations. Increasing the throughput of edge users and the coverage area of the high data transmission rate, reducing the interference of edge users, and improving the throughput of the cell can thus be realized. Downlink CoMP can be divided into two categories, joint processing (JP) and joint transmission (JT). For JP, the cooperative clusters not only share channel information but also share data information and perform joint preprocessing on user data to eliminate interference between base stations. For JT, a user terminal simultaneously receives data information transmitted by several transmission nodes and combines the information to improve the quality of the received signal.
In summary, the core technologies of 1G–5G are FDMA, TDMA, CDMA, OFDMA, and MIMO technologies, respectively, which utilize frequency, time, code element, space, and other resources to improve the spectrum efficiency of the system. Faced with the increasing communication demand in the future, the growing need for multimedia services and the rapid development of Internet technology, how to realize large-capacity data transmission at anytime and anywhere has become an important issue in wireless communication. Considering that there is still a broad prospect for the utilization of air-based and space-based wireless communication systems in the future, how to construct the space–air–ground integrated mobile Internet from the perspective of space–air–ground integration has become the main trend of the development of mobile communication networks. In the following, we will outline the space-based and air-based communication systems.
1.2The Overview of Air-Based Cooperative Transmission System
With the rapid development of wireless communication systems, higher spectrum utilization, larger system capacity, more flexible network coverage, and lower construction cost are increasingly required. However, the current wireless communication platforms are mainly ground platforms and satellite platforms, and each has its own drawbacks. For example, the ground platform requires large investment in large-scale coverage and high construction cost, and its configuration is inflexible. Serious channel fading happens in urban construction-intensive areas. The satellite platform has problems such as high terminal cost, difficulty in updating and repairing on-board equipment, and limited system capacity. Under this circumstance, the research of the new high-altitude communication platform has received increasing attention and become a research hotspot in the field of wireless communication.7
The air-based wireless communication system based on the high-altitude platform is a new type of communication system currently under research in the world. The
