Digraph-based low earth orbit satellite switching path determination method and system

Abstract

The invention discloses a low earth orbit satellite switching path determination method and system based on a directed graph, and the method specifically comprises the steps: firstly, a gateway station obtains a task sequence of a user terminal according to real-time ephemeris information, and converts the task sequence into the directed graph; based on the directed graph, obtaining all path information between a starting point and an ending point in the directed graph by using an improved depth-first traversal algorithm; calculating three parameters of the hop count, the average signal quality and the average maximum load of each path, and removing the satellite coverage time of which the signal quality cannot meet the requirement and switching paths of satellites containing load congestion; and finally, calculating target functions of all the remaining satellite switching paths, and selecting the satellite switching path with the maximum target function as a final switching path. According to the invention, the switching satellite path selection problem of the user station is solved, the load balance is ensured, the user switching frequency is reduced, the problems of load imbalance and switching failure of user link switching under the condition that the satellite is congested are solved, and the user communication quality is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of low-orbit satellite terminals, and in particular to a method and system for determining a low-orbit satellite switching path based on a directed graph. Background Art

[0002] Satellite Internet is access to the Internet based on satellite communication technology, that is, the various functions of ground base stations are moved to satellite platforms in the air. Each satellite is a mobile base station in the sky, which can provide high-bandwidth, flexible and convenient Internet access services to users around the world. Satellite communication can provide a low-cost solution for network coverage in areas with weak infrastructure, and is also the best known solution for providing communications in natural areas such as oceans, deserts, and glaciers.

[0003] Like the terrestrial mobile communication system, the satellite Internet also uses terminal measurement and network judgment strategies to optimize the switching of the system, that is, the system core network optimizes the user switching beam, satellite and switching time to achieve the best system resources. Unlike the terrestrial mobile communication system, the satellite Internet is a dynamic resource scheduling system. How to ensure that system users can efficiently use system resources and achieve beam switching is a widely concerned issue in the satellite Internet system.

[0004] Invention CN 112911664 A discloses a low-orbit satellite switching method based on probability sorting, which obtains the optimal choice of low-orbit satellite switching by probabilistically sorting all satellites in the mobile user area covered by the low-orbit satellite; Invention CN115021799A discloses a low-orbit satellite switching method based on multi-agent collaboration, which realizes continuous autonomous optimization selection in the beam selection process and improves the accuracy of switching target beam selection. Invention CN 114449604A discloses a low-orbit satellite switching method and device based on graph theory and multi-attribute decision-making. The graph theory algorithm is used to save a lot of complex mathematical calculations, and user access and satellite switching are considered as a whole, so as to obtain the optimal switching plan more accurately. However, these methods do not consider the problems of load imbalance and switching failure of user link switching when the satellite is congested, and the quality of user communication needs to be improved. Summary of the invention

[0005] The object of the present invention is to provide a low-orbit satellite switching path determination method and system that can ensure load balancing, reduce the number of switching times, and improve signal quality.

[0006] The technical solution to achieve the purpose of the present invention is: a method for determining a low-orbit satellite switching path based on a directed graph, comprising the following steps:

[0007] Step 1: The gateway obtains the task sequence of the user terminal according to the real-time ephemeris information;

[0008] Step 2: Convert the task sequence into a directed graph;

[0009] Step 3: Based on the directed graph, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph;

[0010] Step 4: Calculate three parameters of each path, namely the number of hops, the average signal quality, and the average maximum load, and eliminate the satellite coverage time with signal quality not meeting the requirements and the handover paths containing satellites with load congestion;

[0011] Step 5: Calculate the objective function of all remaining satellite handover paths, and select the satellite handover path with the maximum objective function as the final handover path.

[0012] Furthermore, the gateway station described in Step 1 obtains the task sequence of the user terminal according to the real-time ephemeris information, specifically as follows:

[0013] The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information. The task sequence information includes the satellite numbers of all satellites passed by the user terminal within the time t of one revolution of the satellite around the earth, the starting time of passing a certain satellite, and the leaving time. The task sequence of the user terminal u i is represented by the following formula:

[0014]

[0015] where u i is the user terminal numbered i. The first column represents all satellites in the system. There are a total of N satellites in the system. The subscript of S represents the satellite number, and the value ranges from 1 to N; the second column represents the starting moment when the user terminal accesses the satellite corresponding to this row, t sij represents the starting moment when the user terminal u i accesses the satellite s j , and the sorting of the task sequence is in the order of the starting time of the user terminal passing the satellite from front to back, that is, in the task sequence, the second column has t si1 <t si2 <…<t siN ; the third column represents the moment when the user terminal is at the edge of the signal coverage range of the satellite corresponding to this row. t eij represents that after this moment, the satellite s j will drive away from the user terminal u i , and will no longer be within the coverage range of the satellite s j .

[0016] Furthermore, the conversion of the task sequence into a directed graph described in Step 2 is specifically as follows:

[0017] Compare satellites pairwise in the task sequence. When the coverage times of two satellites overlap, it indicates that the two satellites can switch. The satellite with an earlier start time switches to the satellite with a later start time. The former satellite is the head of the directed edge in the directed graph, and the latter satellite is the tail of the directed edge. After traversing all satellites, a directed graph of satellite switches is obtained.

[0018] Furthermore, based on the directed graph described in step 3, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph, specifically as follows:

[0019] Step 3.1: Set the number of paths LNUM to 1, set the current path number i to 1, and set the starting satellite begin as the current node of the current path.

[0020] Step 3.2: Determine whether the current path is the last path, that is, i = 1 or i < LNUM. If it is the last path, all path calculations are completed; if it is not the last path, then go to step 3.3.

[0021] Step 3.3: Determine whether the current node is the final node. If it is, then increment the current path number i by 1, and obtain the last node of path i as the current node, and return to step 3.1 to continue processing the next path; if not, then go to step 3.4.

[0022] Step 3.4: Determine whether the number of branches N of the current node is greater than 1. If the number of branches N of the current node > 1, then add N - 1 paths identical to this path, the number of paths LNUM = LNUM + N - 1, and add the 2nd to Nth adjacent nodes to the end nodes of the corresponding N - 1 paths, and enter step 3.5; if not, then directly enter step 3.5.

[0023] Step 3.5: Set the adjacent node as the current node, and then return to step 3.3 for the next loop until all path calculations are completed.

[0024] Furthermore, calculate the three parameters of the number of hops, average signal quality, and average maximum load for each path described in step 4, and eliminate the satellite coverage times with signal quality not meeting the requirements and the switching paths containing satellites with load congestion, specifically as follows:

[0025] Step 4.1: Obtain the number of hops J information for each path.

[0026] Step 4.2: In each handover path, there is a corresponding signal change sequence within the coverage time of each satellite. According to the estimation of the distance between the user's location and the center point of the satellite wide beam, when the user requests different services, there are certain requirements for signal quality. When the signal quality cannot meet the user's request, handover occurs. To reduce handover requests due to signal quality, when obtaining the satellite coverage signal quality, first eliminate the satellite coverage time with signal quality that does not meet the requirements.

[0027] The average signal quality of the remaining paths is calculated by summing the signal quality of each satellite starting from the second satellite. Then the average signal quality is defined as follows:

[0028]

[0029] where Q j is the signal quality of satellite S j .

[0030] Step 4.3: After obtaining the load function of each satellite, pre-calculate the load situation after the user accesses, and obtain the maximum load value of each satellite as the load parameter for the handover algorithm, specifically as follows:

[0031] Suppose user u i selects handover path pathj. Then the load change amount generated by user u i handover to satellite S j is added to the load change before accessing satellite S j to obtain the following formula:

[0032]

[0033] path i ={S j |j = u i is the number of the satellite on the handover path}

[0034] Suppose it is necessary to switch from satellite S1 to satellite S2 at this time. The kth handover moment of user u i is denoted as hottimei, and its definition is as follows:

[0035]

[0036] The average maximum load of satellite S j is defined as follows: is defined as follows:

[0037]

[0038] where S j ∈ path, and maxLoadj is the maximum load of satellite Sj The maximum load;

[0039] Mark the satellites that will experience load congestion, that is, during the period when the user accesses, there will be overloading situations, and eliminate the handover paths containing satellites with load congestion.

[0040] Furthermore, calculate the objective function of all remaining satellite handover paths in step 5, and select the satellite handover path with the maximum objective function as the final handover path, specifically as follows:

[0041] Step 5.1: Calculate the objective function of the satellite handover path according to the number of hops, average signal quality, and average maximum load of each path;

[0042] Step 5.2: Select the satellite handover path with the maximum objective function as the final handover path.

[0043] A low-earth orbit satellite handover path determination system based on a directed graph, which is used to implement the described low-earth orbit satellite handover path determination method based on a directed graph. The system includes a first module to a fifth module, and the functions of each module are as follows:

[0044] The first module: The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information;

[0045] The second module: Convert the task sequence into a directed graph;

[0046] The third module: Based on the directed graph, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph;

[0047] The fourth module: Calculate three parameters of the number of hops, average signal quality, and average maximum load of each path, and eliminate the satellite coverage time with signal quality not meeting the requirements and the handover paths containing satellites with load congestion;

[0048] The fifth module: Calculate the objective function of all remaining satellite handover paths, and select the satellite handover path with the maximum objective function as the final handover path.

[0049] A mobile terminal includes a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, it implements the described low-earth orbit satellite handover path determination method based on a directed graph.

[0050] Compared with the prior art, the remarkable advantages of the present invention are:

[0051] (1) The low-earth orbit satellite handover strategy based on a directed graph adds a handover strategy based on load balancing in addition to considering the least number of handovers and signal quality, and solves the problem of the satellite path selection for user station handover;

[0052] (2) Based on three parameters of the number of hops, average signal quality, and average maximum load of each path, by calculating the objective function, the satellite handover path with the maximum objective function is selected as the final handover path, overcoming the problems of load imbalance and handover failure in user link handover in the case of satellite congestion. Brief Description of the Drawings

[0053] Figure 1 is a schematic flowchart of a method for determining a low-earth orbit satellite handover path based on a directed graph according to the present invention.

[0054] Figure 2 is a schematic structural diagram of a low-earth orbit constellation satellite communication system in an embodiment of the present invention.

[0055] Figure 3 is a schematic data flow diagram of a method for determining a low-earth orbit satellite handover path in an embodiment of the present invention.

[0056] Figure 4 is a schematic structural diagram of a low-earth orbit satellite path handover model based on a matrix directed graph in an embodiment of the present invention.

[0057] Figure 5 is a binary graph of a handover model path based on a directed graph in an embodiment of the present invention.

[0058] Figure 6 is a schematic diagram of a directed graph and a data structure in an embodiment of the present invention.

[0059] Figure 7 is a schematic flowchart of obtaining all path information between a starting point and an ending point in a directed graph in an embodiment of the present invention. Detailed Embodiment

[0060] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0061] As Figure 1 shown, a method for determining a low-earth orbit satellite handover path based on a directed graph according to the present invention includes the following steps:

[0062] Step 1: The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information;

[0063] Step 2: Convert the task sequence into a directed graph;

[0064] Step 3: Based on the directed graph, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph;

[0065] Step 4: Calculate three parameters of the number of hops, average signal quality, and average maximum load of each path, and eliminate the satellite coverage time with signal quality not meeting the requirements and the handover paths containing load-congested satellites;

[0066] Step 5: Calculate the objective function of all remaining satellite handover paths, and select the satellite handover path with the maximum objective function as the final handover path.

[0067] Further, in step 1, the gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information, specifically as follows:

[0068] The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information. The task sequence information includes the satellite numbers of all satellites passed by the user terminal within the time t of one revolution of the satellite around the earth, the start time and the departure time of passing a certain satellite. The task sequence of the user terminal u i can be expressed by the following formula:

[0069]

[0070] where u i is the user terminal numbered i. The first column represents all satellites in the system. There are a total of N satellites in the system. The subscript of S represents the satellite number, and the value ranges from 1 to N; the second column represents the start time when the user terminal accesses the satellite corresponding to this row. t sij represents the user terminal u i can access the start time of satellite s j , and the task sequence is sorted in the order of the start time of the user terminal passing through the satellite from front to back, that is, t si1 <t si2 <…<t siN ; the third column represents the moment when the user terminal is at the edge of the signal coverage range of the satellite corresponding to this row. t eij represents satellite s j will leave the user terminal u i after this moment and will no longer be within the coverage range of satellite s j .

[0071] Further, in step 2, the task sequence is converted into a directed graph, specifically as follows:

[0072] The satellites in the task sequence are compared pairwise. When the coverage times of two satellites overlap, it indicates that the two satellites can be switched. The satellite with an earlier start time switches to the satellite with a later start time. The former satellite is the arc head of the directed edge in the directed graph, and the latter satellite is the arc tail of the directed edge; after traversing all satellites, the directed graph of satellite handover can be obtained.

[0073] Step 3: Based on the directed graph, use the improved depth-first traversal algorithm to obtain all path information between the start point and the end point in the directed graph, specifically as follows:

[0074] Step 3.1: Set the number of paths LNUM to 1, set the current path number i to 1, and set the starting satellite begin as the current node of the current path;

[0075] Step 3.2: Determine whether the current path is the last path, that is, i = 1 or i < LNUM. If it is the last path, the calculation of all paths is completed; if it is not the last path, go to Step 3.3;

[0076] Step 3.3: Determine whether the current node is the final node. If it is, increment the current path number i by 1, and obtain the last node of path i as the current node, then return to Step 3.1 to continue processing the next path; if not, go to Step 3.4;

[0077] Step 3.4: Determine whether the number of branches N of the current node is greater than 1. If the number of branches N of the current node > 1, add N - 1 paths identical to this path, the number of paths LNUM = LNUM + N - 1, and add the 2nd to Nth adjacent nodes to the last nodes of the corresponding N - 1 paths, then go to Step 3.5; if not, directly go to Step 3.5;

[0078] Step 3.5: Set the adjacent node as the current node, then return to Step 3.3 for the next loop until the calculation of all paths is completed.

[0079] Further, in Step 4, calculate three parameters of each path: the number of hops, the average signal quality, and the average maximum load. Eliminate the satellite coverage time with signal quality not meeting the requirements and the handover paths containing congested satellites with load, as follows:

[0080] Step 4.1: Obtain the hop count J information of each path;

[0081] Step 4.2: In each handover path, there is a signal change sequence corresponding to the coverage time of each satellite, which can be estimated according to the distance between the user's location and the center point of the satellite's wide beam. When the user requests different services, there are certain requirements for signal quality. When the signal quality cannot meet the user's request, a handover occurs; to reduce handover requests due to signal quality, first eliminate the satellite coverage time with signal quality not meeting the requirements when obtaining the satellite coverage signal quality;

[0082] The average signal quality of the remaining paths is calculated as the sum of the signal quality of each satellite starting from the second satellite, then the average signal quality is defined as follows:

[0083]

[0084] where Q j is the signal quality of satellite S j ;

[0085] Step 4.3: After obtaining the load function of each satellite, pre-calculate the load situation after the user accesses, and obtain the maximum load value of each satellite as the load parameter of the handover algorithm, which is specifically as follows:

[0086] Set user u i Select the handover path pathj, then user u i The handover to satellite S j The load change generated and the load change before accessing satellite S j Are added together to obtain the following formula:

[0087]

[0088] path i ={S j |j = u i The numbers of the satellites on the handover path}

[0089] Set that at this time, it is necessary to switch from satellite S1 to satellite S2. The k-th handover moment of user ui is denoted as hottimei, and its definition is as follows:

[0090]

[0091] The average maximum load of satellite S j Is defined as follows: Define as follows:

[0092]

[0093] Among them, S j ∈path, and maxLoadj is the maximum load of satellite S j ;

[0094] Mark the satellites that may experience load congestion, that is, the satellites that will be overloaded during the period when the user accesses, and eliminate the handover paths containing the satellites with load congestion.

[0095] Furthermore, in step 5, calculate the objective function of all the remaining satellite handover paths, and select the satellite handover path with the largest objective function as the final handover path, which is specifically as follows:

[0096] Step 5.1: Calculate the objective function of the satellite handover path according to the number of hops, average signal quality, and average maximum load of each path;

[0097] Step 5.2: Select the satellite handover path with the largest objective function as the final handover path.

[0098] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0099] Embodiment

[0100] In this embodiment, the low-earth orbit constellation satellite communication system is composed of a space segment, a ground segment, and an application segment, as Figure 2 shown below:

[0101] (1) Space segment

[0102] The space segment consists of a low-earth orbit satellite constellation. A satellite constellation is a collection of satellites that are launched into orbit and can operate normally. It is usually a satellite network composed of some satellite rings configured in a certain way. A low-earth orbit satellite constellation is a low-earth orbit satellite network composed of several low-earth orbit satellites.

[0103] (2) Ground segment

[0104] As an important part of the low-earth orbit constellation satellite communication system, the ground segment completes functions such as the management of satellite payloads, the service processing, network management, operation management, and cross-border business settlement of the low-earth orbit constellation satellite communication system. At the same time, it is responsible for the interconnection and interoperability between the low-earth orbit constellation satellite communication system and other systems. It mainly consists of an operation control center, a global operation service center, and gateway stations distributed around the world.

[0105] (3) Application segment

[0106] The application segment consists of various fixed and mobile terminals distributed within the beam coverage of the low-earth orbit constellation. The terminal is the gateway and application platform for users to access the low-earth orbit constellation satellite communication system, used to establish a data transmission link between the user and the satellite. Each terminal has the ability to switch between beams, satellites, and gateway stations, and can provide continuous business services for users.

[0107] A method for determining the handover path of low-earth orbit satellites based on a directed graph according to the present invention is based on a matrix directed graph model, combines a load balancing strategy, and pre-plans inter-satellite handovers using ephemeris information, user information, location, and requirements, etc. The algorithm regards each satellite beam as a separate cell with its own data and control channels and signals. By designing a satellite and cell conversion table, a handover scheme is provided for the user terminal. The data flow of this handover strategy is as Figure 3 shown.

[0108] The satellite and cell conversion table provides a list of satellites that the user terminal will choose to hand over to in the future, and can accurately indicate that the user terminal switches from one cell / beam of the current serving satellite to another cell / beam at a certain moment. For multiple satellites, the conversion table can indicate the cell / beam to be used for each satellite. For each cell / beam, the conversion table can indicate the frequency of the cell. The conversion table can also indicate the cell / beam ID of each cell / beam.

[0109] The algorithm designs the satellite / cell conversion table as defined in Table 1 as follows:

[0110] Table 1 Satellite and Cell Conversion Table

[0111]

[0112] The entries in Table 1 include satellite ID, beam ID, beam frequency, start time, and end time. TAbeam represents the tuning-off time from one beam to another beam on the same satellite. The user terminal tunes to satellite 1, beam 1 at frequency F11 from time a1 to time b1. Subsequently, the user terminal tunes to satellite 1, beam 2 at frequency F21 from time b1 + TAbeam to time c1, and so on. The user terminal coordinates to satellite 2, beam 1 at frequency F12 from time a2 to time b2, where a2 = TAsatellite + n1. Where TAsatellite represents the tuning-off time from satellite 1 to satellite 2. The gateway supports how the end user performs handover by sending the satellite and cell conversion table to the user terminal.

[0113] The tuning-off time is indicated by the capability information reported by the user terminal. The tuning-off time is divided into the tuning time between cells / beams and the tuning time between satellites. In actual use, the tuning time can be used as an upper limit, and the tuning time can indicate the maximum amount of time expected for the user terminal to tune from one cell / beam or satellite to another cell / beam or satellite.

[0114] Tuning time between cells / beams: It indicates the duration that the user terminal stops listening to one cell / beam and starts listening to another cell / beam on the same satellite, and also indicates how long it takes for the user terminal to tune from one cell / beam to another cell / beam.

[0115] Tuning time between satellites: It indicates the duration that the user terminal stops listening to a cell / beam on the current satellite and starts listening to a cell / beam on another satellite, and also indicates how long it takes for the user terminal to tune from one satellite to another satellite.

[0116] When the terminal has the measurement function, it can measure the channel conditions measured by the current serving satellite or the target satellite. If the current signal strength is too low, the user terminal can determine that it needs to send a measurement message to the gateway. In this case, the gateway can generate a new satellite and cell conversion table based on this measurement message. Subsequently, the terminal and the gateway will use this new satellite and cell conversion table to determine when to switch to the next cell / beam and / or satellite, as well as handover information such as the target satellite / beam and frequency of the handover. The gateway makes a decision on whether to instruct the user to interrupt the satellite handover based on the user's measurement information about the satellite.

[0117] When the terminal does not have a measurement function, such as when the current terminal does not support sensing of multiple cells / beams / satellites, it does not send measurement information to the gateway station. In this case, the terminal and the gateway station rely on the existing satellite and cell conversion table to determine when to switch to the next cell / beam / satellite, as well as switching information such as the target cell / beam / satellite and frequency. The gateway station makes a decision on whether to instruct the user to interrupt the satellite handover based on the user's measurement information about the satellite. Therefore, when designing the handover process, it is designed separately according to whether the user has a measurement function.

[0118] The satellite / cell conversion table is updated in real time according to the terminal measurement information reported by the user in real time. Whether the user reports measurement information depends on whether the user has a measurement function. The measurement function refers to the ability of the terminal to sense information of multiple beams or multiple satellites, and can sense the current signal strength of the beam or satellite.

[0119] Such as Figure 1 As shown, the method for determining the low-earth orbit satellite handover path based on a directed graph provided in this embodiment includes the following steps:

[0120] Step 1. The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information, specifically as follows:

[0121] First, the system calculates the satellite numbers of all satellites that the user passes through within the time of one revolution of the satellite around the earth according to the user's location, as well as the start time and end time of the coverage of each satellite. The signal quality of the user within the satellite coverage range is related to the distance from the satellite center. Therefore, the signal quality of the user under the coverage of each satellite can be estimated. The satellite number, start time, end time, and signal quality constitute the task sequence of the user within the time range of one revolution of the satellite around the earth. The satellites in the task sequence are compared pairwise. When the coverage times of two satellites overlap, it indicates that the two satellites can be switched. The satellite with an earlier start time switches to the satellite with a later start time. The previous satellite is the head of the directed edge in the directed graph, and the latter satellite is the tail of the directed edge. After traversing all satellites, a directed graph of satellite handover can be obtained.

[0122] The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information. The task sequence information includes the satellite numbers of all satellites that the user terminal passes through within the time t of one revolution of the satellite around the earth, the start time of passing through a certain satellite, and the leaving time. At the same time, within the period t of one revolution of the satellite around the earth, the user terminal cannot pass through the same satellite repeatedly;

[0123] User terminal u i The task sequence of can be expressed by the following formula:

[0124]

[0125] Where ui For the user terminal numbered i, the first column represents all the satellites in the system. There are a total of N satellites in the system, where the subscript of S represents the satellite number, and the value ranges from 1 to N; the second column represents the starting time when the user terminal accesses the satellite corresponding to this row, t sij represents the user terminal u i can access satellite s j at the starting time, and the tasks are sorted in ascending order of the starting time when the user terminal passes through the satellite. That is, in the second column of the task sequence, there is t si1 <t si2 <…<t siN ; the third column represents the time when the user terminal is at the edge of the signal coverage range of the satellite corresponding to this row, t eij represents satellite s j will leave the user terminal u after this time i and will no longer be within the coverage range of satellite s j .

[0126] Step 2: Convert the task sequence into a directed graph, specifically as follows:

[0127] Compare the satellites in the task sequence pairwise. When the coverage times of two satellites overlap, it indicates that the two satellites can switch. The satellite with an earlier starting time switches to the satellite with a later starting time. The previous satellite is the head of the directed edge in the directed graph, and the latter satellite is the tail of the directed edge; after traversing all satellites, a directed graph of satellite switching can be obtained;

[0128] Use the directed graph G(V, E) to represent the connectivity of the user terminal for switching between satellites. V is the set of satellite nodes in the graph, and the total number of nodes is N; the set of directed edges E between satellites represents that the user terminal can switch between satellites, and the state of E is related to the corresponding satellite switching state in the task sequence.

[0129] The switching model based on the matrix directed graph is as Figure 4 shown Figure 4 in (a), which refers to the residence time of all satellites passing through a certain user terminal within the time period t of a satellite orbiting the earth once. As shown in the figure, TS0, TS1, TS2,..., TS7 represent the available residence times of the user terminal in satellites numbered 0, 1, 2,..., 7. That is, in Figure 4 in (a), the line segment represents the time period when a certain satellite passes through the user terminal. The starting point of the line segment represents the starting time when the satellite passes through the user terminal, and the end point of the line segment represents the time when the satellite leaves the user terminal. All these contents can be obtained from the task sequence of the user terminal.

[0130] From Figure 4 in (a), we can obtain Figure 4(b) among them, where the circles represent satellite nodes, and the edges between satellite nodes represent that the user terminal can switch between two satellites, and the switching is carried out in the direction of the arrow of the edge. If Figure 4 there is a time overlap area between the two satellites in (a) among them, it means that the user terminal can switch between the two satellites, and it is reflected as there is a directed edge in the two corresponding satellite nodes in Figure 4 (b). All the obtained paths are as shown in Figure 5 .

[0131] Based on the directed graph, the algorithm uses an improved depth-first traversal algorithm to obtain all the path information between the starting point and the ending point in the directed graph. In each path of the directed graph, the fewer the number of hops, the better the signal quality, the more balanced the load, and the relatively fewer the switching frequencies. When obtaining the task sequence, a threshold is set for the signal quality, and only the coverage time greater than the threshold is the effective time. In the calculation of the load function, the satellites that are already fully loaded during the time period when the user is about to access cannot be accessed again. Therefore, in the process of obtaining the candidate paths, the paths that do not meet the load requirements are first excluded. Through the calculation of the paths in the directed graph, each candidate path has a number of hops, a signal quality sequence, and a load function sequence. By processing the three parameters, the objective function related to the three functions is calculated, and the path with the largest objective function is selected as the final switching path.

[0132] Step 3. The switching times, the corresponding signal quality, and the load conditions of each path in the directed graph are all different. Therefore, in order to select the path with the lowest possible switching frequency, it is first necessary to obtain all the path information between the starting satellite and the ending satellite in the directed graph.

[0133] To reduce the storage space, the directed graph uses the adjacency list storage method. The directed graph is as shown in Figure 6 (a), and the adjacency list storage is as shown in Figure 6 (b). For each vertex in the graph as shown in the left part of Figure 6 (b), a single linked list is established as shown in the right part of Figure 6 (b). In this way, all adjacent nodes can be obtained by traversing the linked list of each vertex.

[0134] Based on the directed graph, all the path information between the starting point and the ending point in the directed graph is obtained by using an improved depth-first traversal algorithm, as shown in Figure 7 . Specifically as follows:

[0135] Step 3.1. Set the number of paths LNUM to 1, set the current path number i to 1, and set the starting satellite begin as the current node of the current path;

[0136] Step 3.2: Determine whether the current path is the last path, i.e., i = 1 or i < LNUM. If it is the last path, the calculation of all paths is completed; if it is not the last path, go to Step 3.3;

[0137] Step 3.3: Determine whether the current node is the final node. If it is, increment the current path number i by 1, and obtain the last node of path i as the current node, then return to Step 3.1 to continue processing the next path; if it is not, go to Step 3.4;

[0138] Step 3.4: Determine whether the number of branches N of the current node is greater than 1. If the number of branches N of the current node > 1, add N - 1 paths identical to this path, the number of paths LNUM = LNUM + N - 1, and add the 2nd to Nth adjacent nodes to the last nodes of the corresponding N - 1 paths, then go to Step 3.5; if not, directly go to Step 3.5;

[0139] Step 3.5: Set the adjacent node as the current node, then return to Step 3.3 for the next loop until the calculation of all paths is completed.

[0140] According to the above path calculation algorithm, all path results can be obtained as shown in Table 2:

[0141] Table 2 Satellite handover path table

[0142] Path number Satellite number sequence 1 S0, S1, S2, S4, S5, S7 2 S0, S2, S4, S5, S7 3 S0, S1, S3, S4, S5, S7 4 S0, S1, S2, S4, S6, S7 5 S0, S2, S4, S6, S7 6 S0, S1, S3, S4, S6, S7

[0143] Step 4: Calculate three parameters for each path: the number of hops, the average signal quality, and the average maximum load. Eliminate the satellite coverage time with signal quality not meeting the requirements and the handover paths containing satellites with load congestion, specifically as follows:

[0144] Step 4.1: Obtain the number of hops J information for each path;

[0145] Step 4.2: In each handover path, there is a signal change sequence corresponding to the coverage time of each satellite. It can be estimated based on the distance between the user's location and the center point of the satellite's wide beam. When the user requests different services, there are certain requirements for signal quality. When the signal quality cannot meet the user's request, a handover occurs; to reduce handover requests due to signal quality, first eliminate the satellite coverage time with signal quality not meeting the requirements when obtaining the satellite coverage signal quality;

[0146] The average signal quality of the remaining paths is calculated by summing the signal quality of each satellite starting from the second satellite, then the average signal quality is defined as follows:

[0147]

[0148] Among them, Q j is the signal quality of satellite S j .

[0149] Step 4.3: To prevent the occurrence of load congestion, a load function is used to achieve load balancing. Load balancing in a computer network means that multiple equivalent servers use a certain load sharing technology to evenly distribute the requests sent from the outside to one of the servers in a symmetric structure, quickly obtain important data, and solve the problem of a large number of concurrent access services. Load balancing in a satellite communication system means using a load balancing strategy to evenly distribute the load users of the satellite system.

[0150] Load balancing means using a load balancing strategy to evenly distribute the load users of the satellite system. Load balancing algorithms mainly include the round-robin method, the random method, and the least-connection method. The round-robin method attempts to achieve absolute balance in request transfer, but to achieve absolute balance in request transfer, a considerable price must be paid; the random method is to randomly select one of them for access through the system random function according to the size value of the list of backend servers. According to probability theory, as the number of calls increases, its actual effect is getting closer and closer to evenly distributing the traffic to each backend server, that is, the effect of the round-robin algorithm; the least-connection number algorithm is more flexible and intelligent. Due to the different configurations of the backend servers, the processing of requests is fast or slow. According to the current connection situation of the backend servers, the one with the least current backlogged connections is dynamically selected to process the current request, so as to improve the utilization efficiency of the backend servers as much as possible and reasonably distribute the load to each machine. The first two methods achieve the balance of the number of requests allocated by service consumers. The least-connection number method observes the load of the system from the perspective of the backend server, rather than the request initiator, and is more suitable for the load balancing of satellite communication systems. The load balancing algorithm in satellite handover is to pre-calculate the load situation after user access after obtaining the load function of each satellite, and obtain the maximum load value of each satellite as the load parameter of the handover algorithm.

[0151] After obtaining the load function of each satellite, pre-calculate the load situation after user access, obtain the maximum load value of each satellite, and use it as the load parameter of the handover algorithm, as follows:

[0152] Set user u i to select the handover path pathj, then user u i 's handover to satellite S j generates a load change amount, which is added to the load change before accessing satellite S j to obtain the following formula:

[0153]

[0154] pathi = {S j | j = u i Satellite numbers on the handover path}

[0155] It is set that at this time, it is necessary to switch from satellite S1 to satellite S2. The k-th handover time of user ui is denoted as hottimei, and its definition is as follows:

[0156]

[0157] L j (t) represents the load change of the satellite not connected to the user terminal u i where t in represents the user terminal access time, and t out represents the time when the user terminal satellite no longer provides services. When the load is greater than L max the load is congested. The maximum load is maxLoad[j].

[0158] Satellite S j average maximum load is defined as follows:

[0159]

[0160] where S j ∈ path, and maxLoadj is the maximum load of satellite S j ;

[0161] Mark the satellites that may have load congestion, that is, during the period when the user accesses, there will be overloaded situations, and eliminate the handover paths containing satellites with load congestion.

[0162] Step 5. Calculate the objective function of all remaining satellite handover paths, and select the satellite handover path with the largest objective function as the final handover path, specifically as follows:

[0163] Step 5.1. Calculate the objective function of the satellite handover path according to the number of hops, average signal quality, and average maximum load of each path, as shown in Table 3:

[0164] Table 3 Satellite Handover Path Parameter Table

[0165]

[0166] Step 5.2. Select the satellite handover path with the largest objective function as the final handover path.

[0167] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for determining a low-orbit satellite switching path based on a directed graph, characterized in that: It includes the following steps: Step 1: The gateway station obtains the task sequence of the user terminal according to the real-time ephemeris information; Step 2: Convert the task sequence into a directed graph; Step 3: Based on the directed graph, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph; Step 4: Calculate three parameters of the hop count, average signal quality, and average maximum load for each path, and eliminate the satellite coverage time with unsatisfactory signal quality and the handover paths containing load-congested satellites; Step 5: Calculate the objective function of all remaining satellite handover paths, and select the satellite handover path with the maximum objective function as the final handover path.

2. The method for determining a low-orbit satellite switching path based on a directed graph according to claim 1, characterized in that: The gateway station described in Step 1 obtains the task sequence of the user terminal according to the real-time ephemeris information, specifically as follows: The gateway obtains the task sequence of the user terminal based on the real-time ephemeris information. The task sequence information includes the satellite numbers of all satellites that the user terminal passes by within the time t that the satellite orbits the earth, the start time and the departure time of a certain satellite. i The task sequence is represented by the following formula: where u i is the user terminal numbered i. The first column represents all satellites in the system. The system has a total of N satellites, where the subscript S represents the satellite number, ranging from 1 to N. The second column represents the start time of the user terminal accessing the satellite corresponding to the row, t sij Represents user terminal u i Access Satellites j The task sequence is sorted in the order of the start time of the user terminal passing the satellite from front to back, that is, the second column of the task sequence has t si1 <t si2 <… <t siN The third column indicates the time when the user terminal is at the edge of the signal coverage of the corresponding satellite in this row, t eij Indicates satellites j After this time, leave the user terminal u i , will not be on satellite s j within the coverage area.

3. The method for determining a low-orbit satellite switching path based on a directed graph according to claim 1, characterized in that: The conversion of the task sequence into a directed graph described in Step 2 is specifically as follows: Compare the satellites in the task sequence pairwise. When the coverage times of two satellites overlap, it indicates that the two satellites can perform a handover. The satellite with an earlier start time hands over to the satellite with a later start time. The former satellite is the arc head of the directed edge in the directed graph, and the latter satellite is the arc tail of the directed edge; after traversing all satellites, a directed graph of satellite handover is obtained.

4. The method for determining a low-orbit satellite switching path based on a directed graph according to claim 1, characterized in that: Based on the directed graph described in Step 3, use the improved depth-first traversal algorithm to obtain all path information between the starting point and the ending point in the directed graph, specifically as follows: Step 3.1: Set the number of paths LNUM to 1, set the current path number i to 1, and set the starting satellite begin as the current node of the current path; Step 3.2: Determine whether the current path is the last path, that is, i = 1 or i < LNUM. If it is the last path, the calculation of all paths ends; if it is not the last path, then go to Step 3.3; Step 3.3: Determine whether the current node is the final node. If it is, then the current path number i is incremented by 1, and the last node of path i is obtained as the current node, and return to Step 3.1 to continue processing the next path; if not, then go to Step 3.4; Step 3.4: Determine whether the branch number N of the current node is greater than 1. If the branch number N of the current node > 1, then add N - 1 paths identical to this path, the number of paths LNUM = LNUM + N - 1, and add the 2nd to Nth adjacent nodes to the last nodes of the corresponding N - 1 paths, and enter Step 3.5; if not, then directly enter Step 3.5; Step 3.5: Set the adjacent node as the current node, and then return to Step 3.3 for the next loop until the calculation of all paths ends.

5. The method for determining a low-orbit satellite switching path based on a directed graph according to claim 1, characterized in that: The calculation of the three parameters of the hop count, average signal quality, and average maximum load for each path described in Step 4, and the elimination of the satellite coverage time with unsatisfactory signal quality and the handover paths containing load-congested satellites are specifically as follows: Step 4.1: Obtain the hop count J information of each path; Step 4.2: In each switching path, each satellite coverage time corresponds to a signal change sequence. According to the distance between the user's location and the center point of the satellite wide beam, the user has certain requirements for signal quality when requesting different services. When the signal quality cannot meet the user's request, switching occurs. In order to reduce switching requests caused by signal quality, the satellite coverage time whose signal quality cannot meet the requirements is first eliminated when obtaining the satellite coverage signal quality. The average signal quality of the remaining path is the sum of the signal quality of each satellite calculated starting from the second satellite. The average signal quality is The definition is as follows: Among them, Q j Satellite S j signal quality; Step 4.3: After obtaining the load function of each satellite, pre-calculate the load after the user accesses and obtain the maximum load value of each satellite as the load parameter of the switching algorithm, as follows: Set user u i Select the switching path pathj, then user u i Switch to satellite S j The load variation generated is related to the access satellite S j Adding the previous load changes gives the following formula: path i ={S j |j=u i Switch the number of satellites on the path} Suppose that it is necessary to switch from satellite S1 to satellite S2. User u i The kth switching time is denoted as hottimei, which is defined as follows: Satellite S j The average maximum load The definition is as follows: Among them, S j ∈path, maxLoadj is the satellite S j Maximum load; The satellites that will be overloaded during the user access period are marked, and the switching paths containing the satellites with load congestion are removed.

6. The method for determining a low-orbit satellite switching path based on a directed graph according to claim 1, characterized in that: The objective functions of all remaining satellite switching paths are calculated as described in step 5, and the satellite switching path with the largest objective function is selected as the final switching path, as follows: Step 5.1, calculating the objective function of the satellite switching path according to the number of hops, average signal quality and average maximum load of each path; Step 5.2: Select the satellite switching path with the largest objective function as the final switching path.

7. A low-orbit satellite switching path determination system based on a directed graph, characterized in that: The system is used to implement the low-orbit satellite switching path determination method based on a directed graph as described in any one of claims 1 to 6. The system includes a first module to a fifth module, and the functions of each module are as follows: In the first module, the gateway obtains the task sequence of the user terminal based on the real-time ephemeris information; The second module converts the task sequence into a directed graph; The third module, based on the directed graph, uses the improved depth-first traversal algorithm to obtain all the path information between the starting point and the end point in the directed graph; The fourth module calculates the three parameters of each path, namely, the number of hops, average signal quality and average maximum load, and eliminates the satellite coverage time whose signal quality cannot meet the requirements and the switching paths containing satellites with load congestion; The fifth module calculates the objective functions of all remaining satellite switching paths and selects the satellite switching path with the largest objective function as the final switching path.

8. A mobile terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the low-orbit satellite switching path determination method based on a directed graph as described in any one of claims 1 to 6 is implemented.

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