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 carriers of the high-altitude platform are mainly tethered balloons and airships. The former’s height is generally below 10 km, and the latter is generally located in the stratosphere with its height of 20–50 km. The stratosphere is located above the troposphere in the atmosphere, where the air is thin, the density is a few percent of that at the sea level, the buoyancy is small, but the airflow is relatively stable, and the wind is weak. It is an ideal airspace for deploying high-altitude hovering airships.8
The concept of stratospheric communication was proposed during the Second World War, and it began to attract the attention of scientists and technicians in the 1970s. With the breakthrough of several key technologies and the overall progress of technology level, research hotspots have been formed in recent years. Organizations such as NASA and companies such as SKYTOWER in the United States plan to deploy a stratospheric platform for security purpose with the support of the government. Japan uses the stratospheric platform for digital high-definition television broadcasting and IMT-2000 network construction, which is led by the Stratospheric Communication Platform Development Association. The European stratospheric communication projects are funded by the European Space Agency and governments to conduct research on stratospheric broadband communications. In 2004, the German Aerospace Center successfully implemented large data transmission from balloons floating in the stratosphere to the ground. South Korea and the United States also conducted similar studies. They divided the study of stratospheric communication into three phases and made rapid progress. In China, Tsinghua University used a helium airship to fly for 2 hours at a height of 300 meters to demonstrate the video conferencing system,9 and Peking University has established a professional organization studying the solar stratospheric suspension platform system. Although there have been many achievements in this field, no unified international standards have yet been put forward.
Compared with the communication satellite, the distance between the stratospheric platform and the ground is 1/1800 of the distance between the synchronous satellite and the ground. The free space attenuation and delay time are greatly reduced, which is conducive to miniaturization and broadbandization of the communication terminal. Besides, it is low in cost, fast in construction, can be recyclable, and is easy to maintain. Compared to ground-based cellular systems, the coverage of stratospheric platforms is much greater than that of groundcellular systems, and channel conditions (by Rice attenuation) are superior to ground systems (by Rayleigh attenuation). The stratospheric platform is not only suitable for urban use being an effective complement to ground mobile communication systems but also suitable for use in areas where ground mobile communication systems are inconvenient to deploy such as oceans and mountains. Stratospheric platforms can also be quickly transferred for use in battlefield areas or in the monitoring and communication in areas natural disaster occurred (such as floods). In the long run, the high-altitude platform communication system may also become the third wireless communication system in addition to the ground mobile communication system and the satellite communication system.
The current research of high-altitude platform mobile communication is generally based on the third-generation mobile communication (3G) technology,10 mainly using CDMA technology. The third generation of mobile communication still has many shortcomings in many issues such as air interface, system architecture, and openness. With the continuous increase of communication users and business volume and the increasing requirements for communication quality, it is of utmost importance to develop a new generation of mobile communication system with higher speed, larger capacity, a more complete and open system. The new generation of mobile communication systems is generally called beyond3G (B3G) or the fourth generation (4G).
Fig. 1.2. High-altitude platform communication scenario.
Figure 1.2 depicts a typical high-altitude platform communication scenario. The advantages of the high-altitude platform are detailed in the following points.11
(1)Larger coverage compared to ground mobile systems: Generally, the radius of the coverage of the high-altitude platform covers several tens of kilometers, but the radius of the range covered by the ground mobile system spans several kilometers.
(2)Flexibility to high-volume needs: Within its coverage, the high-altitude platform can centrally support the cellular system architecture and can flexibly perform frequency reuse and set the size of the cell. Therefore, in the high-altitude platform system, the process of reasonably allocating resources can be adopted to deal with the demand for large capacity of the system network.
(3)Lower cost compared to satellite systems: Compared with the constellation network composed of geostationary orbit satellites and low-orbit satellites, the cost of high-altitude platforms in network construction and platform launching will be greatly reduced. Meanwhile, for some ground mobile communication networks that need to build a large number of base station facilities, the cost of high-altitude platforms is also relatively low.
(4)Rapid deployment: The high-altitude platform can be launched and deployed quickly within a few days or even hours. This makes the high-altitude platform ideal for use in emergency and disaster-affected environments.
(5)The upgrade of platform and load: The high-altitude platform can be used in the stratosphere for several years, during which the platform can be lowered to the ground for maintenance and upgrades, and this is clearly difficult to realize in satellite systems.
However, the engineering realization and commercialization process of the high-altitude platform also face some difficulties and challenges as follows:
(1)Mass