Demand Analysis in ATG Scenarios
Today is an era of high-speed information development, where mobile network technology has developed from traditional 2G/3G to high-speed 4G/5G internet. Mobile applications are extremely rich, and users are more dependent on mobile networks and terminal devices. However, network coverage in aviation scenarios is still in a stage of relatively slow development, which does not match the urgent needs of ground-to-air internet applications. At a standing meeting of the State Council chaired by Chinese Premier Li Keqiang in April 2018, the promotion of on-board internet access services was officially included in the work of increasing speeds and reducing fees. In the same year, the Civil Aviation Administration and the China Airlines Association issued implementation suggestions on internet access, clearly promoting air access to internet services and basically achieving full coverage of internet access for trunk airline passengers by 2022. The development of air internet market urgently needs a network access solution with low cost, large bandwidth and high performance.
Using mature ground mobile communication technologies such as 4G and 5G, the air to ground (ATG) system can develop high-speed movement and wide coverage features customized for aviation. Special base stations that can cover the sky are established on the ground. With these base stations, a dedicated ATG network with three-dimensional coverage can be built to effectively solve the problem of three-dimensional coverage at high altitude and realize high-speed data transmission between the ground and the air. Following the development of mobile communication technologies, the terrestrial base station solution enables high-bandwidth, high-traffic, and low-cost network that has great advantages in network deployment, upgrade and maintenance. The ATG service can provide airline passengers with on-board entertainment, on-board office and customized services. It has a wide range of industry application prospects, such as aviation medical rescue, flight operation, air weather, flight safety requirements, intelligent and digital aviation administration, and remote terrestrial industry control.

ATG Solution
The ATG system has an architecture similar to that of the terrestrial base station network. Terrestrial base station signals are introduced into the aircraft cabin through airborne antennas. After being received by a customer premise equipment (CPE), the signals inside the aircraft cabin are converted into WiFi signals to provide data services for aircraft cabin users. The ATG system architecture is shown in Fig. 1.

Ultra-High Speed Doppler Frequency Shift Compensation
The flight speed of an aircraft is usually 800 km/h and the highest speed is 1200 km/h. Ultra-high-speed flight may cause serious Doppler frequency shift and affect system performance. ZTE adopts its unique ultra-high-speed frequency offset compensation technology to reduce the impact of frequency offset on access performance. The airborne CPE pre-compensates at the uplink side for the frequency offset value estimated by the downlink channel. For gNB receiving signals, the clock frequency offset between CPE and gNB is twice as large as that between gNBs. The airborne CPE estimates and pre-compensates for the frequency offset, so that gNB receives uplink data without frequency offset. gNB also uses a similar frequency offset estimation method for frequency compensation. The use of ultra-high-speed frequency offset compensation technology can greatly reduce the impact on access performance and ensure the system performance of the air network.

Super-Large Cell Coverage Radius 
Due to the high flight speed of the aircraft, using an ordinary cell coverage radius will cause frequent handover and thus affect the performance and user experience of the ATG system. Therefore, the ATG system requires a super-large cell radius. In addition to avoiding handover, there are also requirements for airline coverage in the east coast of China and low-cost network construction in northwest areas with low traffic. ZTE's ATG system has a maximum cell radius of 300 km, which can meet the above air coverage requirements. The system uses ZTE's proprietary frame structure design, PRACH transmission timing adjustment, uplink/downlink HARQ, and interference suppression to ensure network access performance at a maximum cell radius of 300 km.

Differentiated QoS Guarantee 
As aviation scenarios have special requirements for service guarantee and monitoring, it is necessary to provide differentiated QoS protection for users. The ATG system supports different 5QI service priorities, such as signaling, flight data, version data, and service data. The mapping relationship between the 5QI and different data can be configured. As there is a Wi-Fi system in the cabin, the priority relation of 5QI can also be configured with the airborne server.

Customized High-Performance Antenna
Due to the special channel environment at high altitude, isolation from the ground environment, and anti-interference requirements, the ATG system needs specially customized antennas to enhance signals, suppress interference, increase signal gains, and achieve high-quality coverage from low altitude to high altitude.

Application Cases of ZTE ATG
The above scenario analysis shows that the ATG system involves complex algorithm customization and protocol modification. In particular, airborne equipment needs not only to meet aviation requirements, but also to be designed by manufacturers with rich experience in commercial deployment. Since 2008, ZTE has accumulated experience in R&D, ground testing, aerial testing, and deployment and construction for terrestrial base stations and airborne CPEs. It constructed the world's first and largest 3G ATG network for Gogo covering the whole United States in 2009, and built the world's first LTE FDD ATG network and completed ATG flight tests on Air China to provide 4G experience for passengers in 2014. It also delivered a live broadcast of the 2016 Spring Festival Gala at an altitude of 10,000 metres. At the MWC Shanghai 2019, ZTE won the Best Mobile Service for Connected Living in Asia Award by virtue of its ATG air broadband solution.
With the advent of the 5G era, ZTE has also launched the research and development and testing of 5G ATG, striving to achieve national 5G ATG coverage by the end of 2021 with industry partners, and ushering in a new era of 5G air high-speed Internet in China.