CN114157337B - Low-orbit satellite inter-satellite switching prediction method based on time-varying graph - Google Patents
Low-orbit satellite inter-satellite switching prediction method based on time-varying graph Download PDFInfo
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- H—ELECTRICITY
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
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Abstract
The invention belongs to the technical field of satellite communication, and discloses a time-varying graph-based low-orbit satellite inter-satellite switching prediction method, wherein in a low-orbit satellite system, the coverage of a satellite to a user can be represented by using a time-varying graph; the node set in the time-varying graph represents a satellite covering a user in a period of time; the coverage relation of the satellite to the user is represented by an arc set; the parameters of interest for the handover selection are characterized as a set of weight values. Compared with the traditional real-time switching strategy, the predicted switching path can enable a user to know a switching target satellite in advance, directly select the target satellite during switching, and reduce signaling overhead and switching time delay during switching. The invention provides a method for predicting the number of idle channels in a multi-user scene and a greedy algorithm-based switching path selection method, and the accuracy of the prediction of the number of idle channels of a satellite in a certain period of time in the future is improved. Compared with the switching selection method between the satellites based on the shortest path, the path selection method provided by the invention has better performance in balancing the load.
Description
Technical Field
The invention belongs to the technical field of satellite communication, and particularly relates to a time-varying graph-based low-orbit satellite inter-satellite switching prediction method.
Background
At present, satellite communication has the outstanding characteristics of wide coverage, long communication distance, large transmission capacity, no limitation of geographical environment and the like, is complementary with a ground communication system, and is widely applied to the field in which the ground communication system is not easy to cover or has overhigh construction cost. Compared with medium and high orbit satellites, the low orbit satellite communication system has the advantages of low orbit, low time delay, low transmission loss and the like, and plays an increasingly important role in the fields of national security, aerospace, environmental monitoring, traffic management, industry and agriculture and the like.
Low orbit satellites have high speed mobility and the duration of coverage to a given area is only a few minutes. In order to ensure the continuity of the call, the user needs to switch frequently between the satellites. In addition, the time-varying topology of the satellite, the high speed of movement of the users, and the variation of the channel resources all cause difficulties in the user handover and the channel resource management.
Compared with a real-time inter-satellite switching method, the inter-satellite switching method for predicting the switching path of the user in advance can reduce signaling overhead and switching time delay. At present, methods for predicting user switching paths include a shortest path method based on a static graph and a shortest path method based on a time evolution graph. The core idea of handover prediction using static graphs is as follows: and using nodes and directed arcs in the directed graph to represent the coverage of each satellite to the user, wherein the parameters considered by the switching strategy are represented as weight values of the arc set in the directed graph. And selecting the shortest path from the initial node to the final node in the directed graph as an inter-satellite switching path of the user in a future time period. Different from real-time switching, the method adopts the directed graph to represent the inter-satellite switching, takes the influence of the switching on the future time period into consideration, and effectively realizes the prediction of the switching. However, the method only considers static users, and does not consider the influence of topology time-varying caused by user movement. The switching prediction model based on the time evolution diagram adds periodic updating to the topological model on the basis of the static diagram, and the accuracy of switching time prediction can be improved. However, both methods have the following two problems. One of the problems is that the method only considers the change of parameters such as satellite coverage, elevation angle and the like along with time, does not consider the time-varying property of satellite channel resources, lacks the prediction of idle channel number, and cannot effectively perform the inter-satellite switching based on the idle channel number criterion. Secondly, in the static graph and the time evolution graph model, there is no correlation between topological graphs of users, and the influence of switching selection of users on each other is not considered, so that the load balancing capability is poor. Therefore, a new method for predicting inter-satellite handoff of low-orbit satellites is needed.
Through the above analysis, the problems and defects of the prior art are as follows:
(1) the existing shortest path method based on the static graph only considers static users, and does not consider the influence of topology time-varying caused by user movement.
(2) The existing method for predicting the user switching path only considers the change of parameters such as satellite coverage, elevation angle and the like along with time, does not consider the time-varying property of satellite channel resources, lacks the prediction of the number of idle channels, and cannot effectively perform the inter-satellite switching based on the idle channel number criterion.
(3) In the existing method for predicting the user switching path, in a static graph and a time evolution graph model, the topological graphs of the users have no relevance, the influence of the switching selection of the users on each other is not considered, and the load balancing capability is poor.
The difficulty in solving the above problems and defects is:
1. satellite services may be directed not only to pedestrians moving at low speeds, but also to vehicles moving at high speeds. How to reduce the influence of the movement of the terminal on the whole network topology needs to establish a prediction model according to the historical speed and direction of the terminal.
2. The number of channels for each satellite is affected by the handovers that occur for each user in the future time period. Predicting the change in satellite channel resources requires consideration of new access users, disconnected users, handoff access users, etc. in future time periods. A special modeling is required for channel number prediction.
3. Considering the constraint of satellite channel resources, the switching of each user will affect the access and switching of other users, so how to correlate the switching of a single user with the state of the whole user needs to establish a corresponding model.
The significance of solving the problems and the defects is as follows:
1. the user's location directly determines the set of satellites that cover the user, which can lead to inconsistency between the satellites that cover the user and the set of satellites that are predicted to switch over if the user moves too far within a period of time. The future position of the user is predicted, the influence of the movement of the user on the switching prediction can be reduced, and the prediction accuracy is improved.
2. The inter-satellite switching strategy of load balancing mainly considers satellite channel resources to select a switching satellite, and the accuracy of predicting the satellite channel resources in a future time period can be improved by establishing a satellite channel resource prediction model, so that the accuracy of a satellite switching path is improved.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a time-varying graph-based low-orbit satellite inter-satellite switching prediction method, and particularly relates to a time-varying graph-based low-orbit (LEO) satellite inter-satellite switching prediction method, system, equipment and terminal.
The invention is realized in such a way, and the low-orbit satellite inter-satellite switching prediction method comprises the following steps:
in a low-orbit satellite system, the coverage of a user by a satellite can be characterized by using a time-varying graph; wherein the set of nodes in the time varying graph represents a satellite covering the user for a period of time; the coverage relation of the satellite to the user is represented by an arc set; the parameters of interest for handover selection are characterized by a set of weight values.
Further, the low-orbit satellite inter-satellite switching prediction method comprises the following steps:
step one, when a user is at the coverage edge of a satellite, the geocentric angle between the satellite and the user is a critical angle; calculating a geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite, wherein the method is a precondition for solving the coverage relation of the satellite to the user;
step two, calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future, thereby obtaining the start-stop time of each satellite covering the user in a certain time in the future, and completely determining the coverage relation of the satellite to the user through the start-stop time of each satellite covering the user;
step three, representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
step four, the weight value of the user time-varying graph is associated with the switching path of other user time-varying graphs, the occupation condition of other users on the switching satellite channel at the moment is determined, the number of idle channels of the switching satellite is predicted, the number of idle channels is used as the weight value of a directed graph, and the selection of the final user switching path is determined;
selecting a switching path by adopting a greedy algorithm; and periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology changes caused by the dual movement of the user and the satellite.
Further, in the step one, the determining, by the satellite, the user coverage includes:
by the latitude and longitude position (Lat) of the user user ,Lon user ) And satellite position (Lat) sat ,Lon sat ) Then, the center angle β is calculated as arccos (sin (Lat) user )sin(Lat sat )+cos(Lat user )cos(Lat sat )cos(Lon user -Lon sat ) ); when the user is at the edge of the satellite coverage, the user's geocentric angle with respect to the satellite is β max (ii) a If beta is less than or equal to beta max The satellite covers the user, otherwise the satellite does not cover the user.
Further, in step three, for the user, characterizing each overlay satellite as a node in a time-varying graph; establishing directed connection arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal, wherein the directed connection arcs include:
characterizing each overlay satellite for a userFor the nodes in the time-varying graph, a node set Sa ═ Sa 1 ,Sa 2 ,…,Sa n ,…,Sa M Wherein the ith satellite is denoted as Sa i I belongs to 1,2, …, M; establishing directed connection arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal, representing the inter-satellite switching of the user, wherein an arc set A is { A } (1,1) ,A (1,2) ,…,A (n,m) ,…,A (M,M) A, wherein a directed arc from the ith node to the jth node is defined as A (i,j) I, j ∈ 1,2, …, M, indicating that the user switched from satellite i to satellite j.
Further, in step four, the predicting the number of idle channels includes:
storing the start-stop time of each user occupying the satellite at the initial time in a table; a user who is switched at first obtains a satellite which can be switched by the user according to a time-varying graph of the user; inquiring the starting and stopping time of each user occupying the satellite in a table entry corresponding to the satellite node; if the starting time of other users occupying the satellite is before the switching time and the ending time is after the switching time, the user occupies the channel of the satellite at the switching time; according to the total number of users occupying the satellite channels at the switching moment, the number of idle channels of each target satellite can be obtained; after the user who is switched selects the optimal satellite according to the maximum idle channel number criterion, the user adds the time occupied by the satellite into the list item of the satellite, and simultaneously predicts the idle channel number of the target satellite when other users are switched by adopting the same method according to the switching time occurrence sequence.
Further, in step five, the switching path selection includes:
and when the user switches, the satellite with the largest number of idle channels inquired according to the satellite table entry is selected as the target satellite, so that the function of balancing the load is realized efficiently.
The time-varying graph updating comprises the following steps:
(1) when a user accesses, setting access time and a timer to realize the periodic updating of a switching time-varying graph of the user;
(2) predicting the position of each future time slot again according to the current position and speed of the user, thereby updating the node set and the arc set of the time-varying graph;
(3) according to the channel prediction method, the number of idle channels caused by the fact that a new user accesses and an original user releases resources within a time period from the last time of updating the time-varying graph is considered, and the number of channels of the target satellite in the future user switching process is predicted again.
Another object of the present invention is to provide a low-orbit satellite inter-satellite switching prediction system using the low-orbit satellite inter-satellite switching prediction method, the low-orbit satellite inter-satellite switching prediction system comprising:
the satellite coverage judging module is used for taking a geocentric angle between the satellite and a user as a critical angle when the user is at the coverage edge of the satellite; calculating the geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite;
the satellite coverage time calculation module is used for calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future so as to obtain the start-stop time of each satellite coverage user in a certain time in the future;
the directed connection arc establishing module is used for representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
the idle channel number prediction module is used for determining the occupation condition of other users on a switching satellite channel at the moment according to the correlation between the weight value of the user time-varying graph and the switching path of other user time-varying graphs, and predicting the idle channel number of the switching satellite;
the switching path selection module is used for selecting a switching path by adopting a greedy algorithm; and periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology changes caused by the dual movement of the user and the satellite.
It is a further object of the invention to provide a computer device comprising a memory and a processor, the memory storing a computer program which, when executed by the processor, causes the processor to perform the steps of:
in a low-orbit satellite system, the coverage of a user by a satellite can be characterized by using a time-varying graph; wherein the set of nodes in the time varying graph represents a satellite covering the user for a period of time; the coverage relation of the satellite to the user is represented by an arc set; the parameters of interest for handover selection are characterized by a set of weight values.
It is another object of the present invention to provide a computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of:
in a low-orbit satellite system, the coverage of a user by a satellite can be characterized by using a time-varying graph; wherein the node sets in the time-varying graph represent satellites covering users for a period of time; the coverage relation of the satellite to the user is represented by an arc set; the parameters of interest for the handover selection are characterized as a set of weight values.
Another object of the present invention is to provide an information data processing terminal, which is used for implementing the low-orbit satellite inter-satellite handoff prediction system.
By combining all the technical schemes, the invention has the advantages and positive effects that: the inter-satellite switching prediction method for the low orbit satellite solves the problem of predicting the user switching satellite selection under the satellite and user double-movement scene based on the time-varying graph. The switching prediction of the invention can lead the user to know the switching target in advance, and reduce the signaling overhead of selecting the target satellite when the user switches. The implementation scheme of the invention is as follows: in a low-orbit satellite system, the coverage of a user by a satellite can be characterized by using a time-varying graph; wherein, the node set in the time-varying graph represents a satellite covering the user in a period of time; the coverage relation of the satellite to the user is represented by an arc set; the parameters of interest for handover selection are characterized by a set of weight values. Due to the mobility of the satellite and the user, the coverage relation of the satellite to the user is constantly changed along with time, and the time-varying graph needs to be periodically updated to reduce prediction errors; through the time-varying graph, the problem of predicting user switching is converted into the problem of finding a proper path on the graph. Compared with the existing shortest path switching prediction algorithm, the method for predicting the number of idle channels in the multi-user scene and the switching path selection method based on the greedy algorithm are provided aiming at the inter-satellite switching criterion of the maximum number of idle channels, and the accuracy of switching prediction and the load balancing capability are greatly improved.
The method is different from static graphs and time evolution graphs, the weighted value of the arc set for representing the channel resources in the time-varying graph provided by the method is not an independent parameter of a topological graph constructed by each user, and the channel resources can be effectively predicted according to the joint estimation of the occupation condition of each user on the node satellite in the future. According to the constructed time-varying graph, the method adopts a greedy algorithm to select the switching path, overcomes the defect that the original method cannot be applied to the switching rule based on the maximum idle channel, and greatly improves the load balancing capability.
Compared with the traditional real-time switching strategy, the predicted switching path can enable a user to know a switching target satellite in advance, directly select the target satellite during switching, and reduce signaling overhead and switching time delay during switching. Compared with the shortest path selection method, the path selection method provided by the invention has better performance in load balancing. The invention can improve the accuracy of the number of the idle channels of the satellite.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flowchart of a method for predicting inter-satellite handoff of a low-orbit satellite according to an embodiment of the present invention.
Fig. 2 is a block diagram of a low-orbit satellite inter-satellite handoff prediction system according to an embodiment of the present invention;
in the figure: 1. a satellite coverage judging module; 2. a satellite coverage time calculation module; 3. a directed connection arc establishing module; 4. a number of idle channels prediction module; 5. and a switching path selection module.
Fig. 3 is a schematic view of the earth center and elevation angles of a satellite and a covered user according to an embodiment of the present invention.
Fig. 4 is a time-varying graph of a periodic update structure of a switching sub-graph according to an embodiment of the present invention.
Fig. 5 is a simulation diagram of the handover failure rate under the maximum number of idle channels criterion according to an embodiment of the present invention.
Fig. 6 is a simulation diagram of the forced outage rate under the maximum number of idle channels criteria provided by an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the problems in the prior art, the invention provides a low-orbit satellite inter-satellite switching prediction method based on a time-varying graph, and the invention is described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the method for predicting inter-satellite handoff of a low orbit satellite according to the embodiment of the present invention includes the following steps:
s101, when a user is at the coverage edge of a satellite, the geocentric angle between the satellite and the user is a critical angle; calculating the geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite;
s102, calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future, thereby obtaining the starting and ending time of each satellite covering the user in a certain time in the future;
s103, representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
s104, the weight value of the user time-varying graph is associated with the switching paths of other user time-varying graphs, the occupation condition of other users to the switching satellite channel at the moment is determined, and the number of idle channels of the switching satellite is predicted;
s105, selecting a switching path by a greedy algorithm; and periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology is changed due to the dual movement of the user and the satellite.
As shown in fig. 2, the low-orbit satellite inter-satellite handoff prediction system provided in the embodiment of the present invention includes:
the satellite coverage judging module 1 is used for setting a geocentric angle between a satellite and a user as a critical angle when the user is at a coverage edge of the satellite; calculating the geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite;
the satellite coverage time calculation module 2 is used for calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future, so as to obtain the start-stop time of each satellite coverage user in a certain time in the future;
the directed connection arc establishing module 3 is used for representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
the idle channel number prediction module 4 is used for determining the occupation condition of other users on the switched satellite channel at the moment according to the correlation between the weight value of the user time-varying graph and the switching path of other user time-varying graphs, and predicting the idle channel number of the switched satellite;
a switching path selection module 5, configured to select a switching path by using a greedy algorithm; and periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology changes caused by the dual movement of the user and the satellite.
The technical solution of the present invention is further described below with reference to specific examples.
Example 1
Aiming at the defects of the prior art, the invention provides an inter-satellite switching prediction method based on an idle channel number criterion. Different from a static graph and a time evolution graph, the weight value of the arc set for representing the channel resources in the time-varying graph provided by the method is not an independent parameter of a topological graph constructed by each user, and the prediction of the channel resources can be effectively realized according to the joint estimation of the occupation situation of each user on the node satellite at the future time. According to the constructed time-varying graph, the method adopts a greedy algorithm to select the switching path, overcomes the defect that the original method cannot be applied to the switching rule based on the maximum idle channel, and greatly improves the load balancing capability.
In order to achieve the purpose, the technical scheme of the invention comprises the following steps:
(1) when the user is at the coverage edge of the satellite, the geocentric angle between the satellite and the user is a critical angle; the geocentric angle is calculated according to the positions of the user and the satellite, and whether the user is covered by the satellite or not can be judged;
(2) the geocentric angle of each time slot user and each satellite in the future can be calculated by predicting the position of each time slot user and each satellite in the future, so that the start-stop time of each satellite covering user in a certain time in the future can be obtained;
(3) for the user, each coverage satellite is characterized as a node in a time-varying graph, and a node set Sa is { Sa ═ 1 ,Sa 2 ,…,Sa n ,…,Sa M Wherein the ith satellite is denoted as Sa i I ∈ 1,2, …, M; establishing directed connection arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal, representing the inter-satellite switching of the user, wherein an arc set A is { A } (1,1) ,A (1,2) ,…,A (n,m) ,…,A (M,M) Wherein a directed arc from the ith node to the jth node is defined as A (i,j) I, j ∈ 1,2, …, M, indicating that the user can switch from satellite i to satellite j;
(4) the weighted value of the directed arc in the time-varying graph is related to the number of idle channels at the switching moment, and the more the number of idle channels is, the larger the weighted value is; the invention mainly solves the problems of idle channel number prediction and switching path selection;
(4a) because the number of idle channels changes with the change of the switching path of each user, the weight value of the user time-varying graph is correlated with the switching paths of other user time-varying graphs, and the number of idle channels of the predicted switching satellite needs to know the occupation condition of other users on the switching satellite channel at the moment; the method stores the start-stop time of each user occupying the satellite at the initial moment in a table; the user who is switched firstly can obtain the satellite which can be switched by the user according to the time-varying graph of the user; inquiring the starting and stopping time of each user occupying the satellite in a table entry corresponding to the satellite node; if the starting time of other users occupying the satellite is before the switching time and the ending time is after the switching time, the users occupy the channel of the satellite at the switching time; according to the total number of users occupying the satellite channels at the switching moment, the number of idle channels of each target satellite can be obtained; after a user who is switched selects an optimal satellite according to the maximum idle channel number criterion, adding the time occupied by the user in a table entry where the satellite is located, and simultaneously predicting the idle channel number of a target satellite when other users are switched by adopting the same method according to the switching time occurrence sequence;
(4b) the method adopts a greedy algorithm to select a switching path, namely, a satellite with the largest idle channel number is selected as a target satellite during user switching, so that the function of load balancing can be best realized;
(4c) and updating the affected satellite table entries whenever a new user is accessed or the user fails to switch or the topology changes caused by the dual movement of the user and the satellite, thereby maintaining the accuracy of channel prediction.
Compared with the prior art, the invention has the following advantages:
1) compared with the traditional real-time switching strategy, the predicted switching path can enable a user to know a switching target satellite in advance, directly select the target satellite during switching, and reduce signaling overhead and switching time delay during switching.
2) Compared with the shortest path selection method, the path selection method provided by the invention has better performance in load balancing.
3) The method and the device can improve the accuracy of the number of the idle channels of the satellite.
Example 2
The invention relates to an inter-satellite switching prediction method based on a time-varying graph, which provides the following examples:
the present example illustrates the implementation of the present invention from the Iridium satellite communications network scenario. Suppose that there are 66 satellite nodes in the networkSa 1 ,Sa 2 ,…Sa 66 Located on the same orbit at an inclination of 86.4 deg. and at an altitude of 780km, and the number of available channels per satellite is 500. The minimum elevation angle of the user relative to the satellite is 8.2 °. The user follows the distribution of the actual population density of continents in the world, and the service obeys the Poisson distribution with the arrival rate of 30 calls/second. The simulation is in a discrete time stepping manner.
Judging whether a satellite covers a user according to the position of the user and the geocentric angle between the satellite and the user;
1a) the motion of the user terminal has certain randomness, and the invention predicts the future time slot positions according to the average values of the current and historical speeds and directions of the user; the satellite follows a fixed operation cycle, the track is relatively fixed, and the longitude and latitude of all satellites in each time slot in the future can be obtained by looking up an ephemeris;
1b) the coverage of the satellite to the user is obtained by calculating the geocentric angle of each time slot satellite and the user; when the user is at the satellite coverage boundary, the elevation angle of the user is the minimum elevation angle theta relative to the satellite elevation angle min . The geocentric angle of the satellite and the user is the maximum geocentric angle beta of the satellite covering the user max . According to the trigonometric formula, there are
1b1) If the longitude and latitude of the user in a certain time slot is [ Lat ] user ,Lon user ]The longitude and latitude of the satellite is [ Lat sat ,Lon sat ]Then, the user and satellite earth center angle β ═ arccos (sin (Lat) user )sin(Lat sat )+cos(Lat user )cos(Lat sat )cos(Lon user -Lon sat ) ); if beta is less than or equal to beta max The satellite covers the user, otherwise the satellite does not cover the user.
The schematic view of the earth center angle and the elevation angle of the satellite and the covered user is shown in fig. 3.
Step two, comparing the geocentric angle beta of each time slot user and the satellite with the maximal geocentric angle beta max The start-stop time of the user covered by each satellite can be obtained; the node set of the time-varying graph represents a set of satellites covering a user over a period of time;
setting the start-stop time of coverage of the satellite covering the user to be
And
wherein the starting time of the ith satellite covering the user is recorded as
The end time of the ith satellite covering the user is recorded as
Step three, constructing a time-varying graph; the switching of the user is necessarily performed between the satellites covering the user, so the time-varying graph selects the satellites covering the user terminal as a node set, the set is Sa ═ { Sa ═ Sa 1 ,Sa 2 ,…,Sa n ,…,Sa M H, wherein the ith satellite is denoted as Sa i I ∈ 1,2, …, M; establishing directed connection arcs among nodes according to the coverage duration of each satellite on a user terminal, representing the satellite switching possibility, wherein an arc set A is { A ═ A } (1,1) ,A (1,2) ,…,A (n,m) ,…,A (M,M) Wherein a directed arc from the ith node to the jth node is defined as A (i,j) ,i,j∈1,2,…,M；
3a) According to the step one, the start-stop time of the satellite node i and the node j covering the user can be obtained as
And
3a1) if the relational expression is satisfied
When the user is no longer covered by the representative service satellite node i, the user is covered by the node j, and the user can be switched to the node j by the node i, thereby establishingDirected arc A from node i to node j (i,j) ；
3a2) If the relational expression is satisfied
When the representative service satellite node j no longer covers the user, the node j covers the user, and the user can be switched from the node j to the node i, so that a directed arc A from the node j to the node i is established (j,i) ；
3a3) If covering the time interval
And
there is no overlap, and there is no directional arc between node i and node j, indicating that the user cannot switch between satellite i and satellite j.
The time-varying graph formed by periodically updating the switching subgraph is shown in fig. 4.
Step four, the weighted value of the directed arc in the time-varying graph is related to the number of idle channels at the switching moment, and the more the number of idle channels is, the larger the weighted value is; the prediction of the number of idle channels and the selection of a switching path are as follows;
4a) the maximum number of idle channels requires that when inter-satellite handover occurs, a user always selects a satellite with the maximum number of idle channels in a visible satellite to access, so that a directed arc between satellite nodes is set as the number of idle channels of a handover destination satellite. The number of free channels of a satellite may change over time, mainly 4 cases as follows:
1) among users occupying satellite nodes, users end conversation before predicting an idle channel;
2) among users occupying the satellite node, users are switched to other satellites before the predicted time;
3) among users of other satellite nodes, a user is switched to the satellite before the predicted time;
4) before the number of idle channels is predicted, a new user accesses the satellite node;
the invention reduces the influence of new user access and user call end on the number of idle channels of each satellite by continuously updating the time-varying graph; the channel prediction of the invention mainly considers the influence of the switching of a large number of existing users accessing the satellite in a future time period on the number of idle channels. The specific steps of channel prediction are as follows:
4a1) the target satellite and the time for occupying the target satellite in the switching paths of all the users with the predicted switching paths are recorded in the table, and the condition that each user occupies each satellite channel at the time of sending switching by the user with the switching paths to be predicted can be obtained by inquiring the table, for example, as shown in table 1.
TABLE 1 predicted satellite occupancy time for each user
| User 1 | |
|
|
|
|
|
| Satellite 1 | (0，2) | (11，20) | (9，11) | (0，6) | (0，17) | |
| Satellite 2 | (2，5) | (0，4) | (0，9) | (13，20) | (17，20) | |
| Satellite 3 | (12，20) | (3，11) | (4，11) | (11，17) | (6，10) | |
| Satellite 4 | (5，12) | (0，3) | (11，20) | (17，20) | (10，13) |
This table is only for convenience in explaining the prediction principle, assuming that there are 4 satellites, 6 users have access to and a time-varying graph has been constructed. The start-stop time periods for each user to occupy each satellite are shown in table 1. If a new user is accessed and handed off at time 13 and satellite 3 is one of the target satellites, it is necessary to predict the number of free channels for satellite 3 at time 13. From the rows corresponding to the satellites 3 in the table, it can be found that, among the existing users, the users 2, 3, and 5 end their occupation of the satellite 3 before the time 13, the user 6 does not occupy the satellite 3, and only the users 1 and 4 occupy the satellite 3 at the time 13. From the number of users occupying satellite 3 at time 13, we can predict the number of free channels on satellite 3.
4a2) Setting the directional arc weight values among the satellite nodes as the number of idle channels of the predicted subsequent node satellite by adopting the method in 4a 1); because the maximum idle channel number criterion requires that a user selects a satellite with the maximum idle channel number when switching occurs, so as to realize network traffic balance, a greedy algorithm is adopted for selecting a switching path in a switching graph, and the user always selects a node with the maximum predicted idle channel number from an initial node every time switching, namely the node with the maximum directed arc weight with the current node is taken as the next node of the current node until the last node.
4b) The satellite channel resources and the dual-movement characteristics of the user and the satellite cause that the coverage relation of the satellite to the user is constantly changed along with time, a time-varying graph which needs to be periodically updated is needed, and the idle channel number prediction error caused by the access of a new user and the coverage relation prediction error caused by the movement of the user are reduced, so that the accuracy of switching prediction is ensured;
4b1) as shown in fig. 4, the handover diagram of all the user terminals needs to periodically update the topology relationship between the satellite and the user and the weight values representing the number of idle channels. When the user accesses, the access time t is set a And updating the time delta t if the current time t satisfies t-t a K Δ t, k ∈ N, the user's switching time-varying graph performs the update.
4b2) And predicting the position of each future time slot again according to the current position and speed of the user, thereby updating the node set and the arc set of the time-varying graph.
4b3) According to the channel prediction method in 4a1), the channel number of the target satellite during future user switching is re-predicted by considering the change of the idle channel number caused by the access of a new user and the release of the resource of the original user within the time period delta t from the last time update time change diagram.
The effects of the present invention can be further illustrated by the following simulations:
the effect of load balancing according to the present invention is further described below with reference to simulation experiments.
Firstly, establishing simulation environment
Firstly, based on the switching path prediction method provided by the invention, a program for user access and inter-satellite switching in a global satellite communication system is designed by using MATLAB.
Then, constructing a motion track of a satellite network under the iridium satellite system by using STK software; generating global service distribution according to population density of each region of the world and a certain arrival rate; the user movement model employs a gaussian markov model.
Then, the switching path prediction method and the existing shortest path switching path prediction method are adopted to carry out simulation comparison under the maximum idle channel number criterion. The present invention focuses mainly on the handover failure rate and forced outage rate, since the maximum number of idle channels criterion aims at achieving traffic balancing and reducing the handover failure rate.
Finally, according to the simulation result, compared with the shortest path prediction method, the prediction method of the invention is more accurate to the inter-satellite switching prediction of the maximum idle channel number criterion, and can better realize load balancing.
Second, simulation content and result analysis
Based on the constructed simulation environment, the access strategy uniformly adopts a load balancing strategy. Under the switching criterion of the maximum number of idle channels, the switching failure rate and the forced interruption rate under the iridium satellite system are obtained by using the switching path prediction method and the shortest path prediction method of the invention, and the results are shown in fig. 5 and fig. 6.
As can be seen from the results in fig. 5 and fig. 6, the shortest path algorithm considers that the larger the number of idle channels, the smaller the weight setting of the arc set in the handover graph, and the total weight and the minimum path from the start node to the end node are taken as the predicted handover path. The weight sum of the path is mainly influenced by the switching times, and the situation that a user selects a visible satellite with the maximum idle channel number as a target satellite in each switching process cannot be guaranteed, so that resource shortage caused by unbalanced flow distribution is easy to occur, and the switching failure rate is high. In contrast, in the time-varying graph, the node with the maximum number of idle channels is selected as the switching satellite node every time, the criterion that the user selects the maximum number of idle channels is better met, and the method is obviously superior to the shortest path switching path selection method in the switching failure rate and the forced interruption rate.
In the above embodiments, the implementation may be wholly or partially realized by software, hardware, firmware, or any combination thereof. When used in whole or in part, can be implemented in a computer program product that includes one or more computer instructions. When the computer program instructions are loaded or executed on a computer, the procedures or functions according to the embodiments of the present invention are wholly or partially generated. The computer may be a general purpose computer, a special purpose computer, a network of computers, or other programmable device. The computer instructions may be stored in a computer readable storage medium or transmitted from one computer readable storage medium to another computer readable storage medium, for example, the computer instructions may be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wire (e.g., coaxial cable, fiber optic, Digital Subscriber Line (DSL), or wireless (e.g., infrared, wireless, microwave, etc.)). The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device, such as a server, a data center, etc., that includes one or more of the available media. The usable medium may be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid state disk (ssd)), among others.
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.
Claims (7)
1. A low orbit satellite inter-satellite switching prediction method is characterized in that in a low orbit satellite system, the coverage of a satellite to a user can be represented by a time-varying graph; wherein the set of nodes in the time varying graph represents a satellite covering the user for a period of time; the coverage relation of the satellite to the user is represented by an arc set; switching and selecting concerned parameters to be characterized as a weight value set;
the method for predicting the inter-satellite switching of the low-orbit satellites comprises the following steps:
step one, when a user is at the coverage edge of a satellite, the geocentric angle between the satellite and the user is a critical angle; calculating a geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite;
step two, calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future, thereby obtaining the start-stop time of each satellite covering the user in a certain time in the future;
step three, representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
step four, the weight value of the user time-varying graph is associated with the switching path of other user time-varying graphs, the occupation condition of other users to the switching satellite channel at the moment is determined, and the number of idle channels of the switching satellite is predicted;
selecting a switching path by adopting a greedy algorithm; periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology changes due to the double movement of the user and the satellite;
in step four, the predicting the number of idle channels includes:
storing the start-stop time of each user occupying the satellite at the initial time in a table; a user who is switched at first obtains a satellite which can be switched by the user according to a time-varying graph of the user; inquiring the starting and stopping time of each user occupying the satellite in a table entry corresponding to the satellite node; if the starting time of other users occupying the satellite is before the switching time and the ending time is after the switching time, the user occupies the channel of the satellite at the switching time; according to the total number of users occupying the satellite channels at the switching moment, the number of idle channels of each target satellite can be obtained; after the user who is switched selects the optimal satellite according to the maximum idle channel number criterion, the user adds the time occupied by the satellite into the list item of the satellite, and simultaneously predicts the idle channel number of the target satellite when other users are switched by adopting the same method according to the switching time occurrence sequence.
2. The method according to claim 1, wherein the step one, the satellite determining the user coverage comprises:
by the latitude and longitude position (Lat) of the user user ,Lon user ) And satellite position (Lat) sat ,Lon sat ) Then, the center angle β is calculated as arccos (sin (Lat) user )sin(Lat sat )+cos(Lat user )cos(Lat sat )cos(Lon user -Lon sat ) ); when the user is at the edge of the satellite coverage, the user's geocentric angle with respect to the satellite is β max (ii) a If beta is less than or equal to beta max The satellite covers the user, otherwise the satellite does not cover the user.
3. The method of claim 1, wherein in step three, each overlay satellite is characterized as a node in a time-varying graph for the user; establishing directed connection arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal, wherein the directed connection arcs include:
for the user, each coverage satellite is characterized as a node in a time-varying graph, and a node set Sa is { Sa ═ 1 ,Sa 2 ,…,Sa n ,…,Sa M Wherein the ith satellite is denoted as Sa i I ∈ 1,2, …, M; establishing directed connection arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal, representing the inter-satellite switching of the user, wherein an arc set A is { A } (1,1) ,A (1,2) ,…,A (n,m) ,…,A (M,M) Wherein a directed arc from the ith node to the jth node is defined as A (i,j) I, j ∈ 1,2, …, M, representing the user's slaveSatellite i switches to satellite j.
4. The method for predicting the inter-satellite handoff of the low earth orbit satellite according to claim 1, wherein in the step five, the selecting of the handoff path comprises:
when a user switches, the satellite with the largest number of idle channels inquired according to the satellite table entry is selected as a target satellite, and the function of balancing load is efficiently realized;
the time-varying graph updating comprises the following steps:
(1) when a user accesses, setting access time and a timer to realize the periodic updating of a switching time-varying graph of the user;
(2) predicting the position of each future time slot again according to the current position and speed of the user, thereby updating the node set and the arc set of the time-varying graph;
(3) according to the channel prediction method, the number of idle channels caused by the fact that a new user is accessed and the original user releases resources within the time period of updating the time-varying graph last time is considered, and the number of channels of the target satellite when the future user is switched is predicted again.
5. An inter-low-orbit satellite handoff prediction system implementing the inter-low-orbit satellite handoff prediction method according to any one of claims 1 to 4, the inter-low-orbit satellite handoff prediction system comprising:
the satellite coverage judging module is used for setting a geocentric angle between the satellite and the user as a critical angle when the user is at the coverage edge of the satellite; calculating the geocentric angle according to the positions of the user and the satellite, and judging whether the user is covered by the satellite;
the satellite coverage time calculation module is used for calculating the geocentric angle of each time slot user and each satellite in the future by predicting the position of each time slot of the user and the satellite in the future so as to obtain the start-stop time of each satellite coverage user in a certain time in the future;
the directed connection arc establishing module is used for representing each coverage satellite as a node in a time-varying graph aiming at a user; establishing directed connecting arcs among nodes in a time-varying graph according to the coverage time relation of each satellite to a user terminal;
the idle channel number prediction module is used for determining the occupation condition of other users on a switching satellite channel at the current moment according to the correlation between the weight value of the user time-varying graph and the switching path of other user time-varying graphs, and predicting the idle channel number of the switching satellite;
the switching path selection module is used for selecting a switching path by adopting a greedy algorithm; periodically updating the time-varying graph when a new user is accessed or the user fails to switch or the topology changes due to the double movement of the user and the satellite;
the prediction of the number of idle channels comprises:
storing the start-stop time of each user occupying the satellite at the initial time in a table; a user who is switched at first obtains a satellite which can be switched by the user according to a time-varying graph of the user; inquiring the starting and stopping time of each user occupying the satellite in a table entry corresponding to the satellite node; if the starting time of other users occupying the satellite is before the switching time and the ending time is after the switching time, the users occupy the channel of the satellite at the switching time; according to the total number of users occupying the satellite channels at the switching moment, the number of idle channels of each target satellite can be obtained; after the user who is switched selects the optimal satellite according to the maximum idle channel number criterion, the user adds the time occupied by the satellite into the list item of the satellite, and simultaneously predicts the idle channel number of the target satellite when other users are switched by adopting the same method according to the switching time occurrence sequence.
6. A computer device for low-orbit satellite inter-satellite handoff prediction, the computer device comprising a memory and a processor, the memory storing a computer program that, when executed by the processor, causes the processor to perform the low-orbit satellite inter-satellite handoff prediction method of claim 1.
7. A computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the low-orbit satellite inter-satellite handoff prediction method of claim 1.
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