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Overview, problems and suggestions of inter satellite link in LEO communication constellation

(Summary description)Inter satellite link refers to the communication link established between satellites, also known as interstellar link or cross link. The inter satellite link can realize the information transmission and exchange between satellites. Multiple satellites can be connected together, and become a space communication network with satellites as switching nodes, which reduces the dependence of satellite communication system on ground network. With the advantages of inter satellite links, communication system can reduce the number of ground station settings, expand coverage area, realize global measurement and control, etc., and the attenuation caused by atmosphere and rain can be avoided effectively when the signal is transmitted in the inter satellite link, and a relatively independent communication constellation system or data relay system will be formed.

Overview, problems and suggestions of inter satellite link in LEO communication constellation

(Summary description)Inter satellite link refers to the communication link established between satellites, also known as interstellar link or cross link. The inter satellite link can realize the information transmission and exchange between satellites. Multiple satellites can be connected together, and become a space communication network with satellites as switching nodes, which reduces the dependence of satellite communication system on ground network. With the advantages of inter satellite links, communication system can reduce the number of ground station settings, expand coverage area, realize global measurement and control, etc., and the attenuation caused by atmosphere and rain can be avoided effectively when the signal is transmitted in the inter satellite link, and a relatively independent communication constellation system or data relay system will be formed.

Information
Inter satellite link refers to the communication link established between satellites, also known as interstellar link or cross link. The inter satellite link can realize the information transmission and exchange between satellites. Multiple satellites can be connected together, and become a space communication network with satellites as switching nodes, which reduces the dependence of satellite communication system on ground network. With the advantages of inter satellite links, communication system can reduce the number of ground station settings, expand coverage area, realize global measurement and control, etc., and the attenuation caused by atmosphere and rain can be avoided effectively when the signal is transmitted in the inter satellite link, and a relatively independent communication constellation system or data relay system will be formed.
 
In recent years, with the promotion of low orbit communication constellation with the characteristics of broadband, large capacity, low delay and global coverage, inter satellite links have become a hot topic. At present, the development momentum of Starlink, Leo sat, telesat, iridium next, O3b, oneweb and Globalstar projects is booming. According to the prediction, the communication capacity of high-throughput satellites will reach nearly 5tbps by 2020. With the completion of broadband and large-capacity communication constellation, the communication capacity will increase to more than 40tbps. Among these constellation, Starlink constellation in the United States will use laser inter satellite link to realize spatial networking, and achieve the goal of network optimization management and service continuity; Leo sat constellation will also use the laser inter satellite link to establish a spatial laser backbone network; Telesat in Canada also plans to set up laser inter satellite links; The iridium next constellation in the United States has Ka band inter satellite links. According to the information released, the constellation O3b, oneweb and Globalstar have no inter satellite links.
 
On January 12, 2018, Leo vantage 1, a Canadian satellite operator, successfully launched by means of an arrow of 31 stars on the "polar orbit satellite carrier" of India. At present, relevant in orbit tests are being carried out, which will provide technical verification for the construction of telesat constellation. On February 22, 2018, SpaceX successfully launched two pilot communication satellites (Tintin a/b, figure 1) of Starlink constellation by falcon-9 heavy rocket, which will provide the technical verification in orbit in the early stage for Starlink constellation construction. In the first half of 2018, iridium next constellation launched 15 satellites (i.e. 41-50 and 51-55 formations) in two groups, each with four microwave inter satellite links.
 
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According to the orbit of the satellite, the inter satellite links can be divided into inter satellite links between satellites of the same type of orbit (such as geo GEO, Leo Leo, etc.) and inter satellite links between different types of orbits (Leo GEO, etc.). From the orbit surface where the satellite is located, it can also be divided into the inter satellite link of the same orbit surface and the inter satellite link of the different orbit. For example, iridium constellation, each satellite has 4 Leo Leo inter satellite links, two of which are relatively fixed inter satellite links with adjacent satellites of the same orbit surface (for example, figure 2 satellites 1 and 2 and 3), and two other movable wave beam inter satellite links (such as Fig. 1 and 4 and 5 satellites) established with two satellites adjacent to the heteroorbital surface. Due to the limitation of the inter satellite link setting, there is no interstellar link between satellite 1 and satellites 6 and 7 on adjacent heteroorbiting surface, which reduces the complexity of inter satellite link design.
 
Beijing Ruifeng United Communication Equipment Co., Ltd
 
It is worth mentioning that, since the position relationship between satellites on the same orbit surface is fixed, such interstellar links are easier to maintain. However, the relative position relationship between satellites on the different orbit surface (such as link distance, link azimuth angle and link pitch angle) is time-varying. Not only does the antenna need to have certain tracking ability, but also the inter satellite link is difficult to maintain. Taking iridium system as an example, it needs to switch every 250 seconds.
 
In addition to the division of inter satellite links according to orbit, they can also be divided into microwave links (Ka band), millimeter wave links (some Ka band and q/v band), terahertz link (terahertz band) and laser link according to the working frequency. The frequency planning that can be used for inter satellite links is shown in Table 1; From the communication rate, it can be divided into narrow-band low-speed link and broadband high capacity link.
 
Table 1 frequency planning of inter satellite links
 
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The inter satellite microwave / millimeter wave link has the advantages of relatively mature technology, high reliability, relatively wide beam, easy tracking and acquisition. The advantage of inter satellite laser link is that the frequency band is wide, which can increase the communication capacity of the link; The power consumption, quality and volume of the equipment are small; The beam divergence angle is small, and has good anti-jamming and anti interception performance, and the system is safe. But the main disadvantage of the inter satellite laser link is that the aim, capture and tracking (PAT) system is complex due to the narrow beam. Therefore, pat is one of the key technologies of the laser inter satellite link.
 
The inter satellite terahertz link is between millimeter wave and far infrared wave, so terahertz link can have the advantages of millimeter wave communication and laser communication. Compared with millimeter wave communication, the inter satellite terahertz link has a larger communication capacity, which can reach the transmission rate of 10Gbps or more; The beam is narrower and directional than millimeter wave; THz wave can be absorbed by atmosphere and has better confidentiality; The equipment mass and volume are smaller than millimeter wave equipment. Compared with the laser link, terahertz communication has higher energy efficiency than optical communication; Terahertz wave has better ability to penetrate dust and smoke than laser, and has some advantages over laser communication in severe weather.
 
At present, the low orbit communication constellation is being constructed and designed. Different schemes are adopted for whether to set up inter satellite links. O3b and oneweb constellations do not set inter satellite links, while Starlink, Leo sat and telesat constellation choose to use laser inter satellite links, while iridium constellation adopts Ka band inter satellite links. However, the difficulty of constellation design will be increased by setting up inter satellite links, the technical complexity of satellites will be improved, and the technical problems such as routing on the satellite will also arise accordingly. The following is a review of possible problems in the use of inter satellite links:
 
(1) Frequency resources are limited. The available microwave frequency resources of inter satellite links are fixed and limited, which makes it difficult to meet the needs of broadband inter satellite links of multiple large constellation at the same time; The frequency resources of millimeter wave band are abundant, but the research and development of satellite borne equipment in this frequency band is difficult and the maturity of products is low; Although the bandwidth of laser link is wide, it is greatly affected by space illumination and other factors, which makes the availability of laser link decrease. Communication frequency and satellite orbit resources are the prerequisite for constellation design, and they must be solved first.
 
(2) Routing issues. For the large constellation, the routing of inter satellite links is bound to be a very complex problem. In addition, the relative position of low orbit satellites is changing all the time, so the routing table needs to be updated in time. If the work is completed on the satellite, the complexity and difficulty of the satellite development will be greatly increased, and the adaptability of the satellite to the technology upgrading and updating will be reduced; If the route assignment is completed by the ground and then uploaded to the satellite, the constellation will increase its dependence on the ground system. On the other hand, if routing is not the best solution, the data is transmitted many times in constellation, which makes the signal delay increase significantly, which reduces the advantage of low orbit constellation in delay. Therefore, the author thinks that routing is one of the most important problems in the design of the inter satellite link of LEO constellation.
 
(3) Data supervision issues. Since a communication network can be established outside the atmosphere through the inter satellite link, how to supervise the origin and departure of communication data, how to ensure the data reach the destination without being stolen, and how to accurately grasp the transmission mode of business information is also a problem that needs high attention.
 
(4) The complexity of inter satellite link design and the problem of link interference. The inter satellite links are relatively fixed for satellites in the same orbit plane, but the inter satellite links between satellites with different orbit surfaces and even different types of orbit satellites are more complex. It is necessary to solve the problems of the number of links, comprehensive coverage, chain direction angle, link distance and link switching frequency. In the operation of inter satellite links, it is necessary to avoid interference with other inter satellite links or star to ground links, so as to avoid the influence of communication links of all parties.
 
(5) The limitation of inter satellite link antenna. The antenna or laser terminal of inter satellite link needs complex capture, tracking, aiming servo mechanism and controller, so as to realize the establishment and switching of inter satellite link in time. However, it is difficult to install multiple inter satellite link antennas or terminals on the small satellite platform with low orbit, which limits the setting of single satellite satellite link. Although phased array antenna can be selected, the problem is that the expensive cost and service life of such antennas restrict the practical use value of large-scale constellations.
 
(6) Background noise control. The range of the antenna direction of the inter satellite link varies greatly, which leads to the dynamic change of background noise temperature of the inter satellite antenna / terminal, and the change range is large, which will affect the normal communication of the inter satellite link. If the antenna pointing control is not correct or the threshold setting is improper, the communication will be blocked or the link self excitation will be caused.
 
Although the above problems exist in the inter satellite link, it is suitable for the low and low orbit constellation which is not suitable for the global arrangement of ground station; Moreover, the inter satellite link has strong survivability, which can not rely on the ground independent network communication. Through constellation, the coverage of communication system can be expanded, and the roaming problem of ground cellular network can be solved to a certain extent. Therefore, the inter satellite link is still being paid attention to and applied. And with the technical advantages of communication and measurement and control, it can give full play to its potential through rational design and use.
 
In order to give full play to the role of inter satellite links, the following suggestions are put forward for the construction and use of inter satellite links:
 
(1) Actively develop new working frequency band, develop corresponding satellite borne equipment in new frequency band, including antenna, transponder and on-board processor. Q/v band (40/60ghz) and laser load equipment will be mature and available in the near future, which will help to improve the frequency resource shortage and link interference caused by the serious dependence on Ka band.
 
(2) Make full use of the existing frequency resources to make the overall planning of "beam time space" of inter satellite links in depth. The beam direction of inter satellite links of each constellation shall be managed according to the time and space records, so that the same frequency can be reused by as many inter satellite links as possible in the way of "Time-space separation", so as to avoid interference problems of the same frequency and direction as possible.
 
(3) Because the routing design of LEO constellation is different from GEO satellite network and ground communication network, it is suggested to optimize the topology of constellation network, strengthen the research on route selection design and data supervision, overcome the problems of large-scale network, dynamic topology, complex node relationship and rich business types, and realize the routing and exchange scheme applicable to LEO constellation.
 
(4) It is suggested that the inter satellite link be limited to avoid the establishment of inter satellite links at will, but only the inter satellite links with neighboring satellites, movable beams with limited rotation, and even fixed beams are used to establish inter satellite links. The optimal design of the whole constellation can be realized by limiting the inter satellite links, and the complexity of apt mechanism of satellite inter satellite link antenna is reduced, The background noise can be avoided.
 
(5) It is suggested that LEO / geo relay inter satellite link be adopted, and LEO satellite establishes relay relationship with GEO satellite through inter satellite link, and then transmits data to low and low latitude areas through GEO satellite to realize LEO satellite communication service outside the field of view of ground station, while LEO satellite focuses on communication work in responsible high latitude area.

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