CN114884565B - A Large-Scale LEO Satellite Network Topology Optimization Method Based on Communication Performance Constraints - Google Patents

Abstract

A large-scale low orbit satellite network topology optimization method based on communication performance constraint comprises the following steps of: representing a satellite communication system topology by a weighted undirected graph; step 2: performing satellite network topology design according to a repeatable unit (motif); step 3: analyzing and evaluating the communication performance of the system according to the communication time delay, the capacity, the survivability and the reliability; step 4: acquiring a satellite network topology optimization model based on communication performance constraint on the basis of the step 3; step 5: and screening the optimal satellite network topology structure according to the optimization model.

Description

A topology optimization method for large-scale low-orbit satellite networks based on communication performance constraints

Technical Field

The present invention belongs to the field of satellite communications, and in particular relates to a large-scale low-orbit satellite network topology optimization method based on communication performance constraints.

Background Art

Compared with traditional ground communication systems, satellite communication systems have the advantages of wide coverage and not being affected by terrain factors, and can be an effective supplement to ground communication systems. With the rise of microsatellite networks, low-orbit satellite constellations are expected to provide low-latency, high-capacity communication services around the world, so low-orbit satellite communication systems have become a hot spot for research and development.

Currently, the scale of low-orbit satellite constellations is expanding. For example, SpaceX's Starlink constellation is expected to have tens of thousands of satellites after deployment. In large-scale low-orbit satellite communication systems, the design of satellite network topology is of great research significance. First, the inter-satellite link connection method of the satellite network topology will have an important impact on the communication performance of the satellite system; second, the frequent switching and maintenance of the inter-satellite link will increase the system overhead; finally, the expansion of the constellation scale makes the constellation network design more complex.

After searching the existing literature, it is found that the satellite network topology based on the Grid model is widely used in the satellite network topology design. In this model, each satellite only establishes links with two adjacent satellites in the same orbital plane and two adjacent satellites in different orbital planes. However, in large-scale low-orbit satellite constellations, due to the increase in the number of visible satellites for each satellite, the satellite network design scheme based on the Grid model cannot guarantee the optimality of the system in terms of transmission hops.

After searching, it was also found that Debopam et al., in the paper "A. Network topology design at 27,000 km/hour", formed a satellite network topology by filling the satellite constellation with repeatable units (motifs), and selected the optimal motif based on the evaluation of end-to-end path distance stretching and transmission hop count to achieve satellite network topology design. This solution improves the limitations of the topology design method based on the Grid model in terms of system transmission hop count performance, but lacks consideration and analysis of other system performance.

The main problems of the above technologies are as follows: when multiple ground terminals have communication needs at the same time, the reasonable allocation of link bandwidth resources is not achieved through the topology structure design, and the system capacity is difficult to guarantee; when a satellite node fails, the impact of the failed node on the system communication quality is not minimized through the topology structure design, and the system anti-destruction performance is difficult to guarantee; when the satellite moves, the distance and pitch angle between satellites also change, and the corresponding inter-satellite link stability also changes with the change of its distance length and pitch angle. The existing topology design method does not fully consider the impact of the stability of the inter-satellite link on the reliability of the system transmission path. Therefore, a satellite network topology optimization method based on communication performance constraints is proposed, which is of great significance for improving system capacity, anti-destruction performance and reliability.

Summary of the invention

In view of the problems existing in the prior art, the present invention aims to provide a large-scale low-orbit satellite network topology optimization method based on communication performance constraints, aiming to improve the system communication performance through topology structure optimization.

The object of the present invention is achieved by providing a large-scale low-orbit satellite network topology optimization method based on communication performance constraints, comprising the following steps:

Step 1: Represent the satellite communication system topology through a weighted undirected graph.

Step 2: Design satellite network topology based on repeatable units (motifs).

Step 3: Analyze and evaluate the communication performance of the system based on communication delay, capacity, survivability, and reliability.

Step 4: Based on step 3, obtain a satellite network topology optimization model based on communication performance constraints.

Step 5: Select the optimal satellite network topology based on the optimization model.

Step 1 of the present invention comprises:

The satellite communication system topology is represented by a weighted undirected graph G = {V, E}, where V = {Sats, City} represents a node set consisting of a satellite node set Sats and a ground city terminal node set City; E = {E _isl ,E _udl } represents an edge set consisting of an inter-satellite link set E _isl and a satellite-to-ground link set E _udl , and the weight of the edge represents the distance length of the link.

The satellite network topology is represented by an undirected graph U = {Sats, E _isl }, which corresponds to the three parts of the satellite communication system: satellite constellation, ground city terminal and satellite-to-ground link. The satellite communication system topology is represented as

G＝{V,E}＝{U,City,E _udl }

Step 2 of the present invention comprises the following steps:

Step 2-1: Filter all repeatable unit motifs according to the visibility of the star and the ground to form a motif set Motifs

Step 2-2: Fill the constellation with all elements in the set Motifs to form the satellite network topology candidate set U _Motifs :

U _Motifs = {U _motif |motif∈Motifs}

Wherein U _motif represents the satellite network topology corresponding to the constellation after the motif fills the constellation.

In step 3 of the present invention, for different satellite network topologies U _motif ∈U _Motifs in step 2, the system performance is evaluated according to the following steps:

Step 3-1: According to the satellite communication system topology G _motif = {U _motif , City, _Eudl }, the transmission paths between all ground city terminals are obtained. The path between any two cities A, B∈City is expressed as

Path(A,B)＝{l ₁ (A,B),l ₂ (A,B),…,l _K (A,B)}

Where l _k (A, B), (k = 1, 2, ..., K) represents the Kth link that the path between the city pair (A, B) passes through.

Step 3-2: Evaluate the communication delay performance τ _motif of the system by stretching the path distance and the average number of transmission hops:

τ _motif = S _motif + H _motif

Where S _motif and H _motif represent the mean distance stretching and the mean number of transmission hops of the end-to-end path, respectively.

Where Path(A,B) represents the transmission path between any city pair (A,B), N _{city_pairs} represents the number of city pairs consisting of any two cities in the ground city set City, S(Path(A,B)) represents the distance stretch of the transmission path between the city pairs (A,B), that is, the ratio of the total distance of the transmission path between the city pairs to the great circle distance. The larger the stretch, the longer the transmission path distance and the longer the required propagation delay. H(Path(A,B)) represents the number of transmission hops between the city pairs (A,B). The more hops, the longer the required onboard data processing delay.

Step 3-3: When there is communication demand between all ground cities, the communication system capacity I _motif is evaluated by the total bandwidth of all transmission paths:

Where I(A,B) represents the transmission path bandwidth between the city pair (A,B), which is determined by the minimum bandwidth resources allocated to all links constituting the path, that is,

where

l _k (A,B)∈Path(A,B)

表示链路l_k(A,B)被分配到用于城市对(A,B)之间通信的最大带宽。in

Denotes the maximum bandwidth that link l _k (A,B) is allocated for communication between the city pair (A,B).

Step 3-4: Evaluate the system's invulnerability Q _motif by the uniformity of the distribution of the number of times the satellite node is passed through the transmission path:

表示卫星节点被传输路径经过次数的方差，

表示

的减函数。如果卫星s被过多城市对之间的最短路径经过，当该节点发生故障时，将会影响到所有经过该节点的城市对之间的通信质量。因此当

越小，卫星节点被传输路径经过次数越均匀，在卫星故障的越少的城市对之间的通信质量会受到影响，系统的抗毁性也就越强。in

represents the variance of the number of times the satellite node is passed by the transmission path,

express

If the shortest path between too many city pairs passes through satellite s, when the node fails, it will affect the communication quality between all city pairs passing through the node.

The smaller it is, the more evenly the satellite nodes are passed through the transmission path, the fewer satellite failures will affect the communication quality between city pairs, and the stronger the system's anti-destruction ability will be.

Step 3-5: Evaluate the system reliability R _motif by the reliability mean of the transmission path, that is,

Where R(A,B) represents the reliability of the path, which is the product of the reliabilities of all links in the path, that is,

表示链路l_k(A,B)的可靠性，由距离可靠性以及俯仰角可靠性确定，链路l的距离可靠性R_l(d)根据星间链路距离d获取in

The reliability of link l _k (A, B) is determined by the distance reliability and the pitch angle reliability. The distance reliability R _l (d) of link l is obtained according to the inter-satellite link distance d.

Where _{disl_max} represents the maximum distance of the intersatellite link, and the elevation angle reliability R _l (β) of link l is obtained according to the elevation angle β between satellites.

Where β _max represents the maximum scanning range of the satellite antenna. The reliability of link l is obtained based on the distance reliability and the elevation angle reliability.

Where m is the weight coefficient, which is between 0 and 1.

Step 4 of the present invention comprises:

Taking the communication delay index as the target cost function and the system capacity, survivability and reliability as constraints, the optimization model of satellite network topology design is obtained:

Among them, δ _I , δ _Q , δ _R represent the constraint thresholds of system capacity, invulnerability and reliability respectively. The system capacity, invulnerability and reliability corresponding to the designed satellite network topology structure are not less than δ _I , δ _Q , δ _R respectively, and at the same time have the minimum communication delay.

Step 5 of the present invention comprises the following steps:

Step 5-1: From the set U _Motifs obtained in step 2, remove the topological structures whose system performance does not satisfy the constraint conditions in step 4 to obtain a new set U' _Motifs .

实现卫星网络拓扑设计。Step 5-2: Select the topology with the minimum communication delay in the new set U' _Motifs

Implement satellite network topology design.

In summary, the beneficial effects of the present invention are as follows: static topology design is realized by selecting motifs, which effectively reduces the switching cost of the system for intersatellite links; in the process of topology optimization, the performance of the satellite communication system is analyzed and evaluated from four aspects: system communication delay, capacity, anti-destruction and reliability, and the satellite network topology structure is analyzed and optimized based on the above indicators. Compared with the prior art, the present invention can optimize the satellite network topology structure by constraining the communication performance, thereby improving the capacity, anti-destruction and reliability of the satellite communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flowchart of topology optimization of a large-scale low-orbit satellite network based on communication performance constraints.

Figure 2 is a block diagram of a large-scale low-orbit satellite communication system scenario.

Figure 3 is a schematic diagram of the motif structure.

FIG4 is a flow chart of motif structure selection.

FIG5 is a flow chart of satellite network topology design based on motif.

FIG6 is a diagram showing the evaluation results of the system path distance stretching.

FIG7 is a diagram showing the evaluation results of the number of transmission hops of the system path.

FIG8 is a diagram showing the evaluation results of system capacity.

FIG9 is a statistical diagram of the variance of the number of times a satellite node is passed by a path.

FIG10 is a diagram showing the evaluation results of system reliability.

Figure 11 is a schematic diagram of the motif structure corresponding to the optimal topology

Figure 12 is a comparison of satellite system performance under different topology schemes.

DETAILED DESCRIPTION

In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific examples described herein are only used to explain the present invention and are not used to limit the present invention.

Through analysis and evaluation of the system, the present invention proposes a large-scale low-orbit satellite network topology optimization method based on communication performance constraints. This method can improve the capacity, anti-destruction and reliability of the system through the satellite network topology design.

As shown in FIG1 , a large-scale low-orbit satellite network topology optimization method based on communication performance constraints includes the following steps:

S101: Represent the satellite communication system topology by a weighted undirected graph.

S102: Design satellite network topology based on a repeatable unit (motif).

S103: Analyze and evaluate the communication performance of the system based on communication delay, capacity, survivability and reliability.

S104: Based on step 3, a satellite network topology optimization model based on communication performance constraints is obtained.

S105: Selecting the optimal satellite network topology structure according to the optimization model.

The application principle of the present invention is described in detail below with reference to the accompanying drawings and examples.

As shown in FIG2 , the application scenario of the implementation example of the present invention is a satellite communication system based on a large-scale low-orbit constellation. The system consists of three parts: a satellite constellation, a city ground station, and a satellite-to-ground link. The satellite constellation is set to the satellite constellation deployed in the first phase of Starlink. The constellation has a total of 24 orbital planes, each with 66 satellites, an orbital inclination of 53°, an orbital altitude of 550km, and a phase factor of 1. The ground city station is set according to the top 100 cities with the largest population in the world. The satellite-to-ground link is determined according to the satellite-to-ground visibility. On the premise of satisfying the satellite-to-ground visibility, each city selects the satellite with the shortest distance from its satellite to establish an intersatellite link. On the premise of satisfying the intersatellite visibility, the maximum number of intersatellite links allowed to be established for each satellite is 4. When there is a communication demand, the system data transmission process is as follows: 1) The city first transmits the data to its access satellite through the satellite-to-ground link; 2) The access satellite of the sending city sends the data to the access satellite of the receiving city through the intersatellite link; 3) The access satellite of the receiving city transmits the data to the receiving city.

A large-scale low-orbit satellite network topology optimization method based on communication performance constraints provided by an embodiment of the present invention comprises the following steps:

Step 1: The satellite communication system topology is represented by a weighted undirected graph G = {V, E} = {U, City, E _udl }, where V = {Sats, City} represents a node set, which consists of a satellite node set Sats and a ground city terminal node set City. According to the number of orbits of the satellite constellation and the number of satellites in the orbital plane, the satellite node set is represented as

Sats＝{(m,n)|m＝1,…,24; n＝1,…,66}

Where (m,n) represents the nth satellite node in the mth orbit, and the city set is expressed as

City＝{city|city＝1,…,100}

Where city represents the city number. E = {E _isl ,E _udl } represents the edge set. The intersatellite link set E _isl is obtained through the satellite network topology design. The satellite-to-ground link set E _udl is determined according to the satellite-to-ground visibility. Under the premise of satisfying the satellite-to-ground visibility, each city selects the satellite with the shortest distance between it and the satellite to establish an intersatellite link. The edge weight represents the distance length of the link. U represents the satellite network topology.

Step 2: Select a set of repeatable units Motifs based on the inter-satellite visibility relationship, and obtain a set of satellite network topology design solutions U _Motifs based on the repeatable units:

Step 2-1: Filter out all repeatable unit motifs according to the star-ground visibility to form a motif set Motifs.

Referring to Figure 3, the motif structure consists of three satellites and two intersatellite links. In the motif structure, satellite A is the reference satellite, and satellite B and satellite C are any two visible satellites with satellite A, and there is an intersatellite link between satellite A and these two visible satellites. Therefore, the motif can be represented by the relative offset of the orbits of satellite B and satellite C and the relative offset on the orbital plane.

motif={(x _B ,y _B ),(x _C ,y _C )}

where (x _· , y _· ) represents the relative orbital offset of satellite · and the relative offset of satellites on the orbital plane.

Referring to Figure 4, when selecting a motif, first select a satellite A near the equator as the reference satellite, then obtain all visible satellites of satellite A; finally, select satellite B and satellite C from the visible satellites to establish an intersatellite link to form a motif topology, and during the selection process, ensure that the three satellites are not in the same orbital plane. In the Starlink scenario, according to visibility analysis, the maximum distance of the intersatellite link is 5014km.

Since the distance between satellites near the equator is the largest, the intersatellite links in the motifs obtained according to the above process always satisfy the intersatellite visibility relationship during the satellite operation cycle. All motifs selected according to the method described in Figure 4 constitute a repeatable unit set Motifs. In the example of the present invention, the set Motifs contains a total of 632 motifs.

Step 2-2: Fill the constellation with all elements in the set Motifs to form the satellite network topology candidate set U _Motifs :

U _Motifs = {U _motif |motif∈Motifs}

Wherein U _motif represents the satellite network topology corresponding to the constellation after the motif fills the constellation.

The motif-based satellite network topology design process refers to Figure 5. The specific process is to take each satellite in the constellation as a reference satellite in turn, and establish inter-satellite links according to the topological structure. Before establishing the inter-satellite link between satellite A and satellite B, the number of links of satellite A is first judged. If satellite A or satellite B already has 4 inter-satellite links, the link establishment process between satellite A and satellite B is skipped. Similarly, before establishing the inter-satellite link between satellite A and satellite C, the number of links of satellite A is first judged. If satellite A or satellite C already has 4 inter-satellite links, the link establishment process between satellite A and satellite C is skipped.

Step 3: For different satellite network topologies U _motif ∈U _Motifs in step 2, evaluate the communication delay, capacity, anti-destruction and reliability of the system:

Step 3-1: According to the satellite communication system topology G _motif = {U _motif , City, E _udl }, the shortest path between all ground city terminals is obtained by Dijsktra algorithm. The path between any two cities A, B∈City is expressed as

Path(A,B)＝{l ₁ (A,B),l ₂ (A,B),…,l _K (A,B)}

Where l _k (A, B), (k = 1, 2, ..., K) represents the Kth link that the path between the city pair (A, B) passes through.

Step 3-2: Evaluate the communication delay performance τ _motif of the system by stretching the path distance and the average number of transmission hops:

τ _motif = S _motif + H _motif

Where S _motif and H _motif represent the mean distance stretching and the mean number of transmission hops of the end-to-end path, respectively.

Where N _{city_pairs} represents the number of city pairs consisting of any two cities in the ground city set City, S(Path(A,B)) represents the distance stretch of the transmission path between the city pair (A,B); H(Path(A,B)) represents the number of transmission hops between the city pair (A,B).

The distance stretching evaluation result of the system transmission path is shown in Figure 6. The horizontal axis range of Figure 6 is 1, 2, ..., 632, corresponding to the number of motifs in the set Motifs. The value i on the horizontal axis represents the i-th motif in the set Motifs, and the value on the vertical axis represents the distance stretching evaluation index S _motif of the system transmission path when the motif is filled to form the satellite network topology U _motif .

The transmission hop count evaluation result of the system transmission path is shown in Figure 7. The horizontal axis range of Figure 7 is 1, 2, ..., 632, corresponding to the number of motifs in the set Motifs. The horizontal axis value i represents the i-th motif in the set Motifs, and the vertical axis value represents the hop count evaluation index H _motif of the system transmission path when the motif is filled to form the satellite network topology U _motif .

Step 3-3: When there is communication demand between all ground cities, the communication system capacity I _motif is evaluated by the total bandwidth of all transmission paths:

Where I(A,B) represents the transmission path bandwidth between the city pair (A,B), which is determined by the minimum bandwidth resources allocated to all links constituting the path, that is,

where

l _k (A,B)∈Path(A,B)

表示链路l_k(A,B)被分配到用于城市对(A,B)之间通信的最大带宽，实施方案中每条链路所能提供的传输带宽为5Gbps。in

It indicates that the link l _k (A, B) is allocated with the maximum bandwidth for communication between the city pair (A, B). In the implementation scheme, the transmission bandwidth that each link can provide is 5 Gbps.

The system capacity evaluation result is shown in Figure 8. The horizontal axis range of Figure 8 is 1, 2, ..., 632, corresponding to the number of motifs in the set Motifs. The horizontal axis value i represents the i-th motif in the set Motifs, and the vertical axis value represents the system capacity index I _motif when the motif is filled to form the satellite network topology U _motif .

Step 3-4: Evaluate the system's invulnerability Q _motif by the uniformity of the distribution of the number of times the satellite node is passed through the transmission path:

表示卫星节点被传输路径经过次数的方差。当

越小，卫星节点被传输路径经过次数越均匀，在卫星故障的越少的城市对之间的通信质量会受到影响，系统的抗毁性也就越强。in

represents the variance of the number of times the satellite node is passed by the transmission path.

The smaller it is, the more evenly the satellite nodes are passed through the transmission path, the fewer satellite failures will affect the communication quality between city pairs, and the stronger the system's anti-destruction ability will be.

的计算结果参考图9，图9横轴范围为1,2,……,632,对应集合Motifs中的motif个数，横轴取值i表示集合Motifs中的第i个motif，纵轴取值表示该motif填充形成卫星网络拓扑U_motif时卫星节点被传输路径经过次数的方差。System Invulnerability Assessment Basis

The calculation results refer to Figure 9. The horizontal axis range of Figure 9 is 1, 2, ..., 632, corresponding to the number of motifs in the set Motifs. The horizontal axis value i represents the i-th motif in the set Motifs, and the vertical axis value represents the variance of the number of times the satellite node is passed by the transmission path when the motif is filled to form the satellite network topology U _motif .

Step 3-5: Evaluate the system reliability R _motif by the reliability mean of the transmission path, that is,

Where R(A,B) represents the reliability of the path, which is the product of the reliabilities of all links in the path, that is,

表示链路l_k(A,B)的可靠性，由距离可靠性以及俯仰角可靠性确定，链路l的距离可靠性R_l(d)根据星间链路距离d获取in

The reliability of link l _k (A, B) is determined by the distance reliability and the pitch angle reliability. The distance reliability R _l (d) of link l is obtained according to the inter-satellite link distance d.

Where _{disl_max} represents the maximum distance of the intersatellite link, and the elevation angle reliability R _l (β) of link l is obtained according to the elevation angle β between satellites.

Where β _max represents the maximum scanning range of the satellite antenna. The reliability of link l is obtained based on the distance reliability and the elevation angle reliability.

The system reliability evaluation results refer to Figure 10. The horizontal axis range of Figure 10 is 1, 2, ..., 632, corresponding to the number of motifs in the set Motifs. The horizontal axis value i represents the i-th motif in the set Motifs, and the vertical axis value represents the system reliability index R _motif when the motif is filled to form the satellite network topology U _motif .

Taking the communication delay index as the target cost function and the system capacity, survivability and reliability as constraints, the optimization model of satellite network topology design is obtained:

Where δ _I , δ _Q , and δ _R represent the constraint thresholds of system capacity, survivability, and reliability, and their values are 1600 Gbps, -500, and 0.8, respectively.

Step 5: Select the optimal satellite network topology based on the optimization model.

Step 5-1: From the set U _Motifs obtained in step 2, remove the topological structures whose system performance does not satisfy the constraint conditions in step 4 to obtain a new set U' _Motifs .

实现卫星网络拓扑设计。Step 5-2: Select the topology with the minimum communication delay in the new set U' _Motifs

Implement satellite network topology design.

对应的motif^*可重复单元结构参考如图11所示，其结构表示为Satellite network topology obtained in step 5.2

The corresponding motif ^* repeatable unit structure is shown in Figure 11, and its structure is expressed as

motif ^* = {(1,-6),(2,2)}

Figure 12 compares the impact of the topological structures of the three schemes on communication performance: the satellite network topology design based on the Grid model (Scheme 1), the Motif-based topology design proposed by Debopam et al. (Scheme 2), and the large-scale low-orbit satellite network topology optimization based on communication performance constraints (Scheme 3) provided by this patent. Referring to Figure 12, compared with Scheme 1, Scheme 3 has significantly optimized other communication performance indicators except for the deterioration of system reliability and delay stretching performance; compared with Scheme 2, although Scheme 3 has deteriorated in delay performance by 1.3%, the system capacity is optimized by 24.4%, the system's anti-destruction performance is optimized by 25.5%, and the reliability is optimized by 6.25%. This section verifies that the large-scale low-orbit satellite network topology optimization based on communication performance constraints can effectively improve the capacity, anti-destruction and reliability of satellite communication systems.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (2)

1. A large-scale low-orbit satellite network topology optimization method based on communication performance constraints, characterized in that it includes the following steps: Step 1: Represent the satellite communication system topology through a weighted undirected graph; Step 2: Design satellite network topology based on repeatable units (motifs); Step 3: Analyze and evaluate the communication performance of the system based on communication delay, capacity, survivability and reliability; Step 4: Based on step 3, a satellite network topology optimization model based on communication performance constraints is obtained; Step 5: Select the optimal satellite network topology according to the optimization model; Step 1: The satellite communication system topology is represented by a weighted undirected graph G = {V, E}, where V = {Sats, City} represents a node set consisting of a satellite node set Sats and a ground city terminal node set City; E = {E _isl , E _udl } represents an edge set consisting of an inter-satellite link set E _isl and a satellite-to-ground link set E _udl , and the edge weight represents the distance length of the link; the satellite network topology is represented by an undirected graph U = {Sats, E _isl }, which corresponds to the three parts of the satellite constellation, the ground city terminal and the satellite-to-ground link in the satellite communication system. The satellite communication system topology is represented as G = {V, E} = {U, City, E _udl }; Step 2 includes the following sub-steps: Step 2-1: Filter all repeatable unit motifs according to the star-ground visibility to form a motif set Motifs; Step 2-2: Fill the constellation with all elements in the set Motifs to form the satellite network topology candidate set U _Motifs : U _Motifs = {U _motif |motif∈Motifs} Where U _motif represents the satellite network topology corresponding to the constellation after the motif fills the constellation; Step 3-1: According to the satellite communication system topology G _motif = {U _motif , City, _Eudl }, the transmission paths between all ground city terminals are obtained. The path between any two cities A, B∈City is expressed as Path(A,B)＝{l ₁ (A,B),l ₂ (A,B),…,l _K (A,B)} Where l _k (A, B), (k = 1, 2, ..., K) represents the Kth link that the path between the city pair (A, B) passes through; In step 3-2, the communication delay performance τ _motif of the system is evaluated by stretching the distance of the satellite-to-ground link path and the average number of transmission hops: τ _motif = S _motif + H _motif Where S _motif and H _motif represent the mean distance stretching and the mean number of transmission hops of the end-to-end path, respectively, as follows:

Where Path(A,B) represents the transmission path between any city pair (A,B), N _{city_pairs} represents the number of city pairs consisting of any two cities in the ground city set City, S(Path(A,B)) represents the distance stretch of the transmission path between the city pair (A,B), that is, the ratio of the total distance of the transmission path between the city pairs to the great circle distance; H(Path(A,B)) represents the number of transmission hops between the city pair (A,B); In step 3-3, the communication system capacity I _motif is evaluated by the total bandwidth of the transmission path when there is communication demand between all ground cities:

Where I(A,B) represents the transmission path bandwidth between the city pair (A,B), which is determined by the minimum bandwidth resources allocated to all links constituting the path, that is,

where l _k (A,B)∈Path(A,B) in

represents the maximum bandwidth that link l _k (A,B) is allocated for communication between city pair (A,B); In step 3-4, the system's invulnerability Q _motif is evaluated by the uniformity of the number of times the satellite node is passed through the transmission path:

in

represents the variance of the number of times the satellite node is passed by the transmission path,

express

The decreasing function of In step 3-5, the reliability R _motif of the system is evaluated by the reliability mean of the transmission path, that is,

Where R(A,B) represents the reliability of the path, which is the product of the reliabilities of all links in the path; that is,

in

The reliability of link l _k (A, B) is determined by the distance reliability and the pitch angle reliability. The distance reliability R _l (d) of link l is obtained according to the inter-satellite link distance d.

Where _{disl_max} represents the maximum distance of the intersatellite link, and the elevation angle reliability R _l (β) of link l is obtained according to the elevation angle β between satellites.

Where β _max represents the maximum scanning range of the satellite antenna; the reliability of link l is obtained based on the distance reliability and the pitch angle reliability

Where m is the weight coefficient, between 0 and 1; Step 4: Take the communication delay index as the target cost function, and the system capacity, survivability, and reliability as constraints to obtain the optimization model of satellite network topology design:

Among them, δ _I , δ _Q , δ _R represent the constraint thresholds of system capacity, invulnerability and reliability respectively. The system capacity, invulnerability and reliability corresponding to the designed satellite network topology structure are not less than δ _I , δ _Q , δ _R respectively, and at the same time have the minimum communication delay. 2. According to the large-scale low-orbit satellite network topology optimization method based on communication performance constraints according to claim 1, it is characterized in that: step 5 comprises the following steps: Step 5-1: In the set U _Motifs obtained in step 2, remove the topological structures whose system performance does not meet the constraints in step 4 to obtain a new set U'_Motifs; Step 5-2: Select the topology with the minimum communication delay in the new set U' _Motifs

Implement satellite network topology design.

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