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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 and volume of the load: Compared to the ground mobile systems, the payload mass and volume of the high-altitude platform system are very limited. The limitation of the payload will limit the system capacity provided by the high-altitude platform, even if it covers a large geographical area.

(2)Power supply: Power supply is a common constraint for any aeronautical system. High-altitude platforms of the unmanned aerial vehicle (UAV) type mainly rely on fuel power. Then how the power system meets the needs of communication load is a big challenge for the high-altitude platform. Long-running airship-type high-altitude platform systems mainly rely on solar energy. During the day, the solar panels convert solar energy into electrical energy to maintain the stability and communication load of the high-altitude platform, and the excess power is stored for use at night. However, the currently available fuel cell technology is not mature enough, and the efficiency of photovoltaic panels needs to be improved.

In order to support the deployment and implementation of high-altitude platforms, the international telecommunication union (ITU) has adopted the high-altitude platform communication system as an alternative to the International Mobile Telecom System-2000 (IMT-2000) wireless communication service. The spectrum allocation of high-altitude platforms is listed in Table 1.1. It can be seen that the ITU has allocated the 48-GHz frequency band (worldwide) and the 31/28-GHz frequency band (selected countries) to the high-altitude platform communication system.^{12, 13} At the same time, the ITU also allocates the frequency bands used by the 3G system to the high-altitude platform system.¹⁴ Therefore, integrating high-altitude platforms into the network of the 3G communication deployment is an emerging and forward-looking task.

Table 1.1. High-altitude platform spectrum allocation table.

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.