Current Status and Future Development of Satellite Mobile Communications

btty038

Current Status and Future Development of Satellite Mobile Communications [Copy link]

Abstract : Satellite mobile communication systems have the advantages of wide coverage and insensitivity to ground conditions, and have become an important part of ground mobile communication, especially in places such as the air, ocean, and desert where ground wireless networks are difficult to cover. With the continuous advancement of science and technology, China's satellite communication technology has also made great achievements, but there is still a certain gap compared with other developed countries. This article briefly introduces satellite communication technology, describes the current development status of satellite communication technology, and looks forward to its future development trend.

0. Introduction

Satellite communication is defined as a communication method that uses satellites as relay stations to transmit or forward radio waves, and can achieve communication between two or more ground stations/handheld terminals and between spacecraft and ground stations. Compared with traditional ground communications, satellite communications can achieve wider seamless coverage at a lower cost, and the geographical environment does not constrain it. In addition, the available spectrum resources are very rich, and the carrier frequency band can range from very high frequency (VHF) to Ka band, and is developing towards higher frequency bands. In addition, satellite communications have been widely used in low-traffic areas such as islands and deserts, and areas where ground networks such as ships and aircraft are difficult to cover. The mobile communication services it provides have the advantages of large span, long distance, strong mobility, and flexible communication methods. It is a necessary supplement and extension of cellular mobile communications.

According to the communication orbit of the satellite, it can be divided into geostationary orbit satellite and non-geostationary orbit satellite. The altitude of geostationary orbit satellite is 35786km. Non-geostationary orbit satellite can be divided into low-orbit, medium-orbit and high-orbit satellite. The altitude of low-orbit satellite is generally 500-3000km, the altitude of medium-orbit satellite is 3000-10000km, and the high-orbit satellite belongs to the elliptical orbit, with the closest point to the earth's surface being 10000-21000km and the farthest point being 39500-50600km.

Recently, the rapid development of new satellite communication technologies and the growing demand for commercial communications have greatly promoted the innovative development of satellite communication services and communication models, making the current period one of the most active periods in the history of satellite communications. This article mainly introduces the composition of satellite communication systems and their satellite communication characteristics, and introduces some typical satellite communication systems from the geostationary orbit satellite communication system, medium orbit satellite communication system and low orbit satellite communication system. Finally, it describes the future development trend from the perspectives of 5G satellite communication integration, integrated space, land and sea communications and intelligent mobile communications.

1. Satellite communication system

1.1 Satellite communication system composition

Satellite communication refers to the use of satellites as relays to forward radio waves used for communication between mobile users or between mobile users and fixed users, so as to achieve mobile communication between two or more points. It includes space segment, ground segment and user segment. The composition of the satellite communication system is shown in Figure 1 below.

The space segment can be a geostationary orbit satellite or a medium or low orbit satellite, which acts as a communication relay station to provide connections between network users and gateway stations.

The ground segment usually includes a gateway, a network control center and a satellite control center, which are used to connect mobile users to the core network and control the normal operation of the entire communication network.

The user segment is composed of various user terminals, including handheld, vehicle-mounted, ship-mounted, and airborne terminals.

Figure 1 Satellite communication system diagram

1.2 Characteristics of satellite communications

1) The communication coverage area is large and the distance is long. Geosynchronous orbit (GEO) satellites only need one satellite to relay and forward to achieve long-distance communication of more than 10,000 kilometers. Three GEO satellites can cover the global surface except for areas above 76° latitude at the poles.

2) Flexible and mobile. Satellite communications are not restricted by geographical conditions and can be used anywhere, whether in large cities or remote mountainous areas or islands.

3) The communication frequency band is wide and has large capacity. Satellite communication channels are in the microwave frequency range, and the frequency resources are quite abundant and can be continuously developed.

4) Good channel quality and stable transmission performance. Satellite communication links are generally line-of-sight communications in free space. Transmission losses are very stable and can be accurately estimated, and multipath effects can generally be ignored.

5) Strong disaster tolerance: It can still provide stable communications when natural disasters such as earthquakes and typhoons occur.

6) The cost of communication equipment does not increase with the increase of communication distance, so it is particularly suitable for communication over long distances and in areas with sparse human activities.

Satellite communications also have some shortcomings and some aspects that should and can be gradually improved, mainly the following points:

1) The cost of satellite launch and onboard communication payload is high. Onboard components must use space-grade components that are resistant to strong radiation, and the lifespan of LEO and GEO satellites is generally only about 8 and 15 years respectively.

2) Satellite link transmission attenuation is very large, which requires the communication equipment on the ground and on the satellite to have high-power transmitters, high-sensitivity receivers and high-gain antennas.

3) Satellite link transmission delay is large. The round-trip transmission time between GEO satellite and the ground is 239~278 ms; in a star network system based on a central station, voice communication between small stations must be carried out through a double-hop link, so the transmission delay reaches 0.5 s, and the conversation process will feel unsmooth.

2. Current status of satellite communication development

The schematic diagram of satellite communication orbit is shown in the figure below2; According to the coverage area, there are regional systems and global systems. The more representative systems are as follows.

Figure 2 Schematic diagram of satellite communication orbit

2.1 Geostationary satellite communication system

The advantage of geostationary orbit communication satellites is that only three satellites are needed to cover the entire world except the two poles, and they have become an important tool for global intercontinental and long-distance communications. For regional mobile satellite communication systems, the use of geostationary orbit generally only requires one satellite, and the construction cost is relatively low, so they are widely used.

2.1.1 International Mobile Satellite System

2.1.2 ACeS System

2.1.3 Thuraya System

2.1.4 SkyTerra System

2.1.5 Tiantong-1 Satellite Communication System

(Due to limited space, each section is omitted here. For details, please see the complete PDF document of "Linfei Information Technology Port" on the computer)

2.2 Medium Orbit Satellite Communication System

Medium orbit satellites (MEO) are about 10,000 kilometers above the earth. The reduction of orbital altitude can reduce the shortcomings of high-orbit satellite communications and provide users with mobile terminal devices with smaller size, weight and power. A mobile communication system with global coverage can be formed with a smaller number of medium-orbit satellites. Medium-orbit mobile communication satellites generally use a mesh constellation, and the satellite orbit is an inclined orbit.

2.2.1 Odyssey System

2.2.2 IC O system

(Due to limited space, each section is omitted here. For details, please see the complete PDF document of "Linfei Information Technology Port" on the computer)

2.3 Low-orbit satellite communication system

In future space-to-ground mobile communications, low-orbit satellites will play an increasingly important role. Compared with ground communication systems, low-orbit satellites have a wider coverage area and are more suitable for global communications in uninhabited areas such as deserts, deep forests, and plateaus; compared with high-orbit satellite communication systems, low-orbit satellites have the advantages of small path attenuation, short transmission delay, short development cycle, and low launch cost.

2.3.1 Iridium system

The Iridium system constellation [6,7] consists of 66 low-orbit satellites with an orbital altitude of 780 km, as shown in Figure 4. It began commercial operation in November 1998. The system can achieve global coverage including the polar regions. The satellite uses a multi-spot beam phased array antenna and performs regeneration processing and switching. There are intersatellite links between the satellites, which is the most advanced low-orbit satellite communication system, as shown in Figure 3 below. The Iridium satellite system also launched the "Next Generation Iridium" (IridiumNEXT) plan in 2017. The maximum data rate for mobile users can reach 128kbps, data users can reach 1.5Mbps, and Ka-band fixed stations are not less than 8Mbps. Iridium Next mainly aims at IP broadband networking and scalability and upgradeability of payload capacity. These capabilities enable it to adapt to the complex needs of future space information applications. However, for the current increasing demand for mobile Internet, especially the advent of the 5G communication era, the data transmission capacity of the Iridium II system is still insufficient.

Figure 3 Low-orbit satellite communication system

Figure 4 Iridium constellation diagram

2.3.2 Globalstar system

The Globalstar system [6,8] was proposed by Loral Space Communications and Qualcomm in the United States at about the same time as the Iridium satellite system. The space segment satellites use an inclined orbit mesh constellation, including 48 satellites and 6 spare satellites, evenly distributed on 8 orbital planes with an inclination of 52°, an orbital altitude of 1,414 km, and an orbital period of 113 minutes, achieving global coverage between 70° north and south latitude. Users can simultaneously see 2 to 4 satellites, and each satellite can maintain communication with the user for 10 to 12 minutes before switching to another satellite through soft switching. The satellite uses transparent forwarding and multi-beam antennas, the user link uses the L/S band, and the feeder link uses the C/X band, providing users with paging, fax, short data and positioning services. User terminals include mobile terminals such as handheld, vehicle-mounted, airborne and ship-mounted, as well as semi-fixed and fixed terminals.

2.3.3 Orbcomm system

Orbcomm-1 satellites make up the majority of the satellites in the current constellation, with a total of 43 satellites, 35 of which have been launched, and the other 1 FM-29 has been rebuilt as TacSat-1 for use by the US military. Orbcomm-1 is a commercial system for global wireless data and messaging services, using the LEO constellation to provide inexpensive tracking, monitoring and messaging services anywhere in the world. The system can send and receive two-way text or digital data packets, such as two-way paging or E-mail, and its economy and short data characteristics can provide relatively economical data services to areas that cannot be covered by traditional communication systems.

2.3.4 OneWeb System

OneWeb satellites[9] are shown in Figure 5 below. The first generation of low-orbit constellation design includes 648 in-orbit satellites and 234 backup satellites, totaling 882 satellites. These satellites will be evenly placed on different polar orbital planes, about 1,200 km above the ground. The satellites move at high speeds, and different satellites appear alternately in the sky to ensure signal coverage in a certain area. The company is considering increasing the number of satellites to nearly 2,000. Once operational, the One Web constellation will not only cover the United States, but also rural and remote areas around the world that are not yet connected to the Internet. One Web's goal is to initially build a low-orbit satellite Internet system by 2022, and to establish a sound, global low-orbit satellite communication system by 2027, providing each mobile terminal with Internet access services at a rate of about 50Mbps.

Figure 5 Oneweb satellite image

2.3.5 Starlink System

In 2015, SpaceX submitted the Starlink plan to the Federal Communications Commission of the United States, as shown in Figure 6 below. It is planned to deploy 12,000 satellites, of which 4,425 medium-orbit satellites with an orbital altitude of 1,100 to 1,300 km will be launched in the first phase, and 7,518 low-orbit satellites with an altitude of no more than 346 km will be launched in the second phase. As the number of satellites increases, SpaceX will combine Ku/Ka dual-band chipsets and other supporting technologies to gradually turn to the use of Ka-band spectrum for gateway communications; as the system develops, phased array antennas will be gradually introduced. SpaceX expects to complete the deployment of 12,000 satellites in 2025, providing users on Earth with at least 1Gbps broadband services and ultra-high-speed broadband networks of up to 23Gbps, providing network speeds similar to fiber-optic networks, and greatly improving coverage areas. In addition, the entire system has great flexibility and can dynamically concentrate signals to where they are needed for specific areas, thereby providing high-quality network services.

Figure 6 Starlink constellation diagram

2.3.6 “Hongyan” system

The "Hongyan" global satellite communication system was proposed by Dongfanghong Satellite Communications Co., Ltd., a subsidiary of China Aerospace Science and Technology Corporation. The system will consist of 300 low-orbit small satellites and a global data business processing center. It has real-time two-way communication capabilities in all weather conditions and under complex terrain conditions, and can provide users with global real-time data communication and comprehensive information services. On December 29, 2018, the Long March 2D carrier rocket successfully sent the first test satellite of the "Hongyan" constellation into the predetermined orbit. The first launch satellite is a test satellite of the "Hongyan" constellation with L/Ka frequency band communication payloads, navigation enhancement payloads, and aviation surveillance payloads. It can realize the in-orbit test of the key technologies of the "Hongyan" constellation. At the same time, the ground system and terminal have been developed. After the satellite enters orbit, it can successively carry out satellite mobile communications, Internet of Things, hot information broadcasting, navigation enhancement, aviation surveillance and other functions. Test verification provides strong support for the subsequent comprehensive construction and operation of the "Hongyan" constellation.

Another important application of the "Hongyan" constellation is to provide aviation data services, which can support the safety communication services in the front cabin of the aircraft, provide reliable communication guarantee for aircraft tracking and emergency response, and support broadband Internet access services in the rear cabin.

2.3.7 “Hongyun” system

The "Hongyun" constellation is one of the "Five Clouds and One Car" (Feiyun, Kuaiyun, Xingyun, Hongyun, Tengyun and Flying Train) projects of China Aerospace Science and Industry Corporation to promote the development of commercial aerospace. It aims to build a low-orbit broadband communication satellite system covering the world. It plans to launch 156 satellites, which will be networked and operated in an orbit 1,000 km above the ground. Based on the space-based Internet access capability, it integrates low-orbit navigation enhancement and diversified remote sensing to achieve information integration of communication, navigation and remote control, build a satellite-borne broadband global mobile Internet network, and achieve global coverage without discrimination of the network.

The first satellite of the Hongyun Project applies millimeter-wave phased array technology to low-orbit broadband communication satellites for the first time, and can use dynamic beams to achieve a more flexible business model. In addition to the main communication payload, the first satellite of the Hongyun Project also carries a spectroscopic thermometer and 3S (AIS/ADS-B/DCS) payloads, which will realize the detection of upper atmospheric temperature and the collection of ship automatic identification system (AIS) information, aircraft automatic dependent surveillance-broadcast (ADS-B) information and sensor data information (DCS), realizing the integration of communication, navigation and remote information, and can be widely used in scientific research, environment, maritime affairs, air traffic control and other fields.

3. Future development trend of satellite communications

3.1 Integration of satellite communications and 5G

In response to the issue of satellite and terrestrial 5G integration, the International Telecommunication Union (ITU) proposed four application scenarios for satellite-terrestrial 5G integration, including relay to station, cell backhaul, mobile communication and hybrid multicast scenarios, and proposed key factors that must be considered to support these scenarios, including multicast support, intelligent routing support, dynamic cache management and adaptive flow support, latency, consistent service quality, NFV (Network Function Virtualization)/SDN (Software Defined Network) compatibility, and flexibility of business models.

In the technical report 22.822 released by 3GPP at the end of 2017, 3GPP working group SA1 evaluated the access network protocols and architectures related to satellites and planned to further conduct research on 5G-based access. In this report, three types of use cases for satellite access in 5G are defined, namely continuous services, ubiquitous services and extended services, and the requirements for new and existing services, the establishment, configuration and maintenance of satellite terminal features, and the switching between satellite networks and terrestrial networks are discussed.

In June 2017, 16 companies and research institutions including BT, Av anti, SES, and the University of Surrey jointly established the SaT5G (Satellite and Terrestrial Network for 5G) Alliance, planning to complete the seamless integration of satellite and 5G within 30 months and conduct trials. The entire project will complete the following six aspects of work: define and evaluate the network architecture that integrates satellite and terrestrial 5G; system architecture solutions; study the commercial value proposition of satellite and terrestrial 5G integration; define and develop relevant key technologies for satellite and terrestrial 5G integration; verify key technologies in a laboratory test environment; demonstrate the characteristics and use cases of satellite and terrestrial 5G integration; and promote the standardization of satellite and terrestrial 5G integration in 3GPP and ETSI.

3.2 Integrated communications between air, land, and sea

The goal of integrated air-space-ground-sea communication is to expand the breadth and depth of communication coverage, that is, to deeply integrate with satellite communication (non-terrestrial communication) and deep-sea ocean communication (underwater communication) on the basis of traditional cellular networks. The integrated air-space-ground-sea network is based on the ground network and extended by the space network, covering natural spaces such as space, air, land, and ocean, and providing information security infrastructure for the activities of various users such as space-based (satellite communication network), air-based (communication networks such as aircraft, hot air balloons, and drones), land-based (ground cellular network), and sea-based (networks composed of marine underwater wireless communication + offshore coastal wireless networks + ocean-going ships/floating islands, etc.). From the basic structure, the integrated air-space-ground-sea communication system can be composed of two subsystems: the integrated air-space subsystem that combines the land mobile communication network with the satellite communication network, and the deep-sea ocean (underwater communication) communication subsystem that combines the land mobile communication network with the deep-sea ocean communication network.

Figure 7: Air-space-ground integration

3.3 Integration of Multiple Functions

At present, satellite mobile communication systems mainly provide users with global or regional mobile communication services such as voice, SMS, and data. With the development of communications, satellite mobile communication systems will integrate navigation enhancement and diversified remote sensing to achieve information integration of communication, navigation, and remote sensing. In this way, satellite mobile communication system terminals can simultaneously support satellite mobile communications, the Internet of Things, hotspot information broadcasting, navigation enhancement, aviation surveillance, and other services. Therefore, the future satellite mobile communication system will inevitably expand its business scope and achieve the integrated development of multiple functions.

3.4 Higher frequency band, wider bandwidth

Satellite communications in the future will develop in the direction of laser links. This is mainly because the use of lasers for inter-satellite communications has opened up a new communication channel, greatly increasing the capacity of inter-satellite communications, while greatly reducing the size and weight of satellite communication equipment, and also increasing the confidentiality of satellite communications. Laser inter-satellite links between small satellite constellations are used to support high-speed data of large nodes or point-to-point transmission between international trunk lines. It can be foreseen that satellite optical communications will become the main way of ultra-large capacity satellite communications.

3.5 Intelligent Satellite Communications

6G stands for "artificial intelligence + ground communication + satellite network". Building a 6G network based on AI technology will be an inevitable choice. The use of intelligent dynamic spectrum sharing technology between ground communication and satellite communication can better improve spectrum efficiency. At the same time, the use of intelligent seamless switching technology and intelligent interference elimination technology can realize true intelligent communication between the sky, the ground and the sea. And "intelligence" will be the inherent feature of the 6G network, that is, the so-called "intelligent connection" can be manifested as the inherent full intelligence of the communication system: the intelligence of network elements and network architecture, the intelligence of connection objects (intelligent terminal equipment), and the intelligent services supported by the information carried.

It can be foreseen that if 6G is defined as intelligent mobile communication V1.0, then 7G will be intelligent mobile communication V2.0. At the same time, artificial intelligence will bring about a subversive revolution in 7G mobile communication. The OSI (Open System Interconnection) model divides the entire communication network into 7 layers, namely the physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer, in order to improve the compatibility of various protocols (ground and satellite), and then establish a powerful intelligent protocol system as a whole. Behavioral analysis based on deep learning can customize the corresponding neural network model according to the characteristics of each layer of the communication network, thereby improving the overall adaptability of the network, that is, improving the personal communication experience.

4. Conclusion

(slightly)

References

(slightly)

Author of this article: Gu Linhai, Aerospace Star Technology Co., Ltd., research direction: satellite mobile communications, intelligent wireless communications.