CN114374742A - Dynamic cache updating and cooperative transmission method for low-earth-orbit satellite network - Google Patents

Dynamic cache updating and cooperative transmission method for low-earth-orbit satellite network Download PDF

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CN114374742A
CN114374742A CN202111348239.4A CN202111348239A CN114374742A CN 114374742 A CN114374742 A CN 114374742A CN 202111348239 A CN202111348239 A CN 202111348239A CN 114374742 A CN114374742 A CN 114374742A
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王程
童建飞
刘允
宋瑞良
宋志群
卢宁宁
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Beijing University of Posts and Telecommunications
CETC 54 Research Institute
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Abstract

The invention discloses a dynamic cache updating and cooperative transmission method for a low-orbit satellite network, belonging to the field of wireless communication; firstly, building a communication scene of a low-orbit satellite network; aiming at a satellite a which just enters a space covered by a gateway station b, establishing a dynamic cache updating model based on transmission perception, and solving by a continuous convex approximation method to obtain the content of the satellite a which needs to be cached from the gateway station b; the caches of the other satellites remain unchanged. Then, for a specific request file in the transmission process, all satellites in the coverage area of the gateway station b form a cooperative transmission set, each satellite checks whether the cache space of the satellite contains the request file, and a dynamic cooperative satellite set is formed based on the principle of who participates in the request file. And finally, solving the cooperative beam forming weight matrix W under the constraint of satisfying the power, thereby achieving the goal of shortest content request time delay. The invention improves the transmission capability and the cache efficiency under the low-orbit satellite network.

Description

Dynamic cache updating and cooperative transmission method for low-earth-orbit satellite network
Technical Field
The invention belongs to the field of wireless communication, and particularly relates to a dynamic cache updating and cooperative transmission method for a low-earth-orbit satellite network.
Background
Mobile communication systems are updated about every decade, and higher network capacity, lower latency and stronger access capability are important directions for network evolution. Currently, a 5G network is actively being constructed, and at the same time, network requirements and key enabling technology of a sixth generation mobile communication system (6G) are receiving wide attention from global research institutes and personnel.
The air-space-ground integrated network is used as one of key scenes of a 6G network, is used for supplementing a ground network, and realizes global wide-area seamless broadband coverage. As an important component of space-based networks, low-earth satellite networks (such as SpaceX and OneWeb) play a key role in space-ground integrated networks.
In order to cope with the data traffic exponentially growing in the future, the network load is relieved by caching in the network as an important technical means. By caching the files in which the user is interested at the network edge, repeated requests for data from the core network are avoided. In the low earth orbit satellite network, the popular files are cached on the satellite, so that repeated requests from a gateway station side can be avoided, and the time delay of requesting contents by a user is reduced.
In addition, multicast transmission is also an effective way to improve transmission capability, and the spectrum utilization efficiency is improved by transmitting content to users with the same service request at the same time under the same time-frequency resource.
However, there are the following disadvantages in low-earth satellite networks: firstly, an effective cache updating mechanism is lacked under the dynamic topological characteristic of a low-orbit satellite constellation; second, multicast transmission capacity in a single-star scenario is limited by the capacity of bottleneck users within the multicast group.
Disclosure of Invention
Aiming at the technical scheme that in the existing low-earth-orbit satellite on-satellite cache, in order to reduce the time delay requirement of a user for requesting specific content, the dynamic characteristics and physical layer transmission of the low-earth-orbit satellite are considered in a combined manner, the invention provides a dynamic cache updating and cooperative transmission method for a low-earth-orbit satellite network, and the transmission capacity and the cache efficiency under the low-earth-orbit satellite network are improved.
The dynamic cache updating and cooperative transmission method for the low earth orbit satellite network comprises the following specific steps:
step one, building a communication scene of a low-orbit satellite network;
the scene comprises a plurality of satellites, a plurality of ground terminals and gateway stations.
The user link between the satellite and the ground terminal is used for communication, and the satellite and the gateway station are used for communication through a feed link; in the communication scene, when each satellite initially enters a gateway station coverage airspace, file caching is carried out, the cache is not updated after the satellite enters the coverage airspace, and the file is updated until the satellite enters the next gateway station coverage airspace. And multiple satellites in the current coverage space carry out downlink transmission in a cooperative mode and distribute contents to the ground terminal.
Secondly, establishing a dynamic cache updating model based on transmission perception for the satellite a when the satellite a just enters a gateway station b coverage airspace;
the dynamic cache updating model is established based on the channel statistical information and the content request distribution of the user link; the objective function of the model is to minimize the average content request delay for all terrestrial users, as shown in the following formula:
Figure BDA0003355015280000021
w represents a beamforming weight matrix, and each element in the matrix W is represented as omegas,gWherein S is a satellite index value, S is 1,2, …, S; and S represents the total number of satellites in the airspace. G denotes the index of the multicast group, G1, 2, …, G; g is the total number of multicast groups.
C represents a cache indication matrix, and each element of the matrix C is a binary number Cs,fF is the index value of the popular file, F is 1,2, …, F is the total number of the popular files, cs,fFor indicating whether or not a satellite s contains a popular file f, cs,f1 means that the s-th satellite contains the f-th content file, and vice versa cs,f＝0。
T represents the number of training time slots; u represents the number of terrestrial users, U is the user index in each multicast group, and U is 1,2, …, Ng；NgIndicating the number of users in each multicast group. Tau isg,uRepresenting the propagation delay of user u in the g multicast group;
Figure BDA0003355015280000022
indicating a specific request file fgSize of (a) fgA specific request file indicating the g-th multicast group;
Figure BDA0003355015280000023
denotes the capacity of the g-th multicast group, where B denotes the transmission bandwidth, γg,uIndicating the signal to interference plus noise ratio of user u in the g-th multicast group.
The constraint conditions corresponding to the objective function are as follows:
Figure BDA0003355015280000024
Figure BDA0003355015280000025
Figure BDA0003355015280000026
formula (1a) is an expression of the signal to interference plus noise ratio, where fs,gIs a binary cache indicator if the s-th satellite contains a particular request file fgThen f iss,gIf 1, otherwise fs,g＝0。hg,u,sIndicating the channel between user u and the s-th satellite within the g-th multicast group,
Figure BDA0003355015280000027
representing a noise component;
formula (1b) indicates that the total power of all beams under a single satellite must not exceed the maximum satellite power; psRepresents the maximum satellite power;
equation (1c) indicates that the total size of all the contents of the files cached on the s-th satellite must not exceed the maximum cache limit L of the satellites。MfIndicating the size of the f-th popular file.
Solving the dynamic cache updating model by a continuous convex approximation method to obtain the content of the satellite a needing to be cached from the gateway station b side, and updating the cache record of the satellite a;
the cache of other satellites within the coverage area of the gateway station remains unchanged.
Step four, for the specific request file f in the transmission processgAll satellites in the space covered by the gateway station b form a cooperative transmission set CsEach satellite checks whether the request file f is contained in the cache space of the satellitegBased on who is involvedPrinciple of forming dynamic cooperative satellite set
Figure BDA0003355015280000031
Step five, based on dynamic cooperative satellite set
Figure BDA0003355015280000032
Under the constraint of satisfying power, the cooperative beam forming weight matrix W is solved, and the goal of shortest content request time delay is achieved.
The objective function and constraint conditions are as follows:
Figure BDA0003355015280000033
Figure BDA0003355015280000034
Figure BDA0003355015280000035
and solving by using a convex optimization tool box to obtain a beam forming weight matrix W.
The invention has the advantages that:
the invention relates to a dynamic cache updating and cooperative transmission method for a low-earth orbit satellite network, which models a cache problem as a non-convex optimization problem of transmission perception, obtains a cache result by solving, and further reduces the average content request delay of users in the low-earth orbit satellite network by a multi-satellite cooperative transmission mode, thereby improving the service capability of the network.
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FIG. 1 is a flow chart of a dynamic cache updating and cooperative transmission method for a low earth orbit satellite network according to the present invention;
FIG. 2 is a schematic view of a communication scenario for building a low earth orbit satellite network according to the present invention;
FIG. 3 is a comparison graph of the performance simulation of the present invention and three prior algorithms.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
The invention relates to a dynamic cache updating and cooperative transmission method for a low-orbit satellite network, which considers that the low-orbit satellite network adopts a 'celestial satellite-earth network' architecture, namely, a gateway station covering the whole world is arranged on the ground to be responsible for data uploading, unloading and controlling of the satellite network, each gateway station is determined according to the minimum visible elevation angle constrained by an antenna system, and the gateway station can manage the coverage airspace of a satellite. Further, the invention is realized by two steps: caching dynamic update and cooperating multicast transmission.
As shown in fig. 1, the specific steps are as follows:
step one, building a communication scene of a low-orbit satellite network;
as shown in fig. 2, the scene includes a number of satellites, a number of ground terminals and gateway stations.
The satellite and the ground terminal communicate with each other through a user link of the low-orbit satellite communication system, and the satellite and the gateway station communicate with each other through a feeder link of the low-orbit satellite communication system. In the communication scene, if the files requested by the ground terminal are not cached on the satellite, the satellite needs to obtain the files from the gateway station, the files are cached based on a dynamic cache strategy when each satellite initially enters a gateway station coverage airspace, the cache is not updated after the satellite enters the coverage airspace, and the files are updated until the next gateway station coverage airspace is entered. Multiple satellites in the current coverage area can perform downlink transmission in a cooperative mode and distribute contents to the ground terminal.
Step two, when each satellite just enters a gateway station coverage airspace, establishing a dynamic cache updating model based on transmission perception for the satellite;
the dynamic cache updating model is established based on the channel statistical information and the content request distribution of the user link in the current airspace;
the objective function of the model is to minimize the average content request delay for all terrestrial users, as shown in the following formula:
Figure BDA0003355015280000041
equation 1 is the optimization goal of the transmission-aware-based dynamic cache update model, i.e. the average content request latency of all terrestrial users is minimized.
W represents a beamforming weight matrix, and each element in the matrix W is represented as omegas,gWherein S is a satellite index value, S is 1,2, …, S; and S represents the total number of satellites in the airspace. G denotes the index of the multicast group, G1, 2, …, G; g is the total number of multicast groups.
C represents a cache indication matrix, and each element of the matrix C is a binary number Cs,fF is the index value of the popular file, F is 1,2, …, F is the total number of the popular files, cs,fFor indicating whether or not a satellite s contains a popular file f, cs,f1 means that the s-th satellite contains the f-th content file, and vice versa cs,f＝0。
T represents the number of training time slots, and the long-term time delay performance is approximated by the average time delay performance of a plurality of training time slots by a multi-training time slot sampling point averaging method.
U represents the number of terrestrial users, U is the user index in each multicast group, and U is 1,2, …, Ng；NgIndicating the number of users in each multicast group. Tau isg,uRepresenting the propagation delay of the user u in the g-th multicast group, wherein the delay is equal to the satellite-ground propagation delay (distance divided by the speed of light) between the satellite farthest away from the user in the cooperative satellite set and the user;
Figure BDA0003355015280000042
indicating a specific request file fgSize of (a) fgA specific request file indicating the g-th multicast group;
Figure BDA0003355015280000043
denotes the capacity of the g-th multicast group, where B denotes the transmission bandwidth, γg,uIndicating the signal to interference plus noise ratio of user u in the g-th multicast group.
The constraint conditions corresponding to the objective function are as follows:
Figure BDA0003355015280000044
Figure BDA0003355015280000051
Figure BDA0003355015280000052
formula (1a) is an expression of the signal to interference plus noise ratio, where fs,gIs a binary cache indicator if the s-th satellite contains a particular request file fgThen f iss,gIf 1, otherwise fs,g＝0。hg,u,sRepresenting the channel between user u and the s-th satellite within the g-th multicast group, the upper right hand symbol H represents the conjugate transpose,
Figure BDA0003355015280000053
representing a noise component;
formula (1b) indicates that the total power of all beams under a single satellite must not exceed the maximum satellite power; psRepresents the maximum satellite power;
equation (1c) indicates that the total size of all the contents of the files cached on the s-th satellite must not exceed the maximum cache limit L of the satellites。MfIndicating the size of the f-th popular file.
And step three, solving the dynamic cache updating model through a continuous convex approximation method to obtain the content of the satellite which initially enters the coverage airspace of the current gateway station and needs to be cached from the gateway station side, and updating the cache of the satellite.
The caches of other satellites in the coverage space domain remain unchanged.
Step four, after the dynamic cache updating is finished, the specific request files f of all the surface user groups in the transmission process are subjected togIn the space covered by the gateway stationAll satellites form a cooperative transmission set CsEach satellite in the set checks whether the request file f is contained in the cache space of the satellitegDynamic cooperative satellite set is formed based on the principle of who participates
Figure BDA0003355015280000054
Step five, based on the dynamic cooperative satellite set
Figure BDA0003355015280000055
Under the constraint of satisfying power, solving the weight matrix W shaped by wave beam, and achieving the goal of shortest content request time delay.
The objective function and constraint conditions are as follows:
Figure BDA0003355015280000056
Figure BDA0003355015280000057
Figure BDA0003355015280000058
the non-convex problem model is relaxed into a convex problem step by step through a continuous convex approximation method, a convex optimization tool box can be further used for solving, and a beam forming weight matrix W is obtained, so that higher transmission rate is realized, and lower average content request time delay is further obtained.
The invention provides a dynamic cache updating strategy aiming at the dynamic topological characteristics of a low-orbit satellite, wherein the strategy is used for updating the cache of each satellite only when each satellite just enters a gateway station coverage airspace, and the cache of the existing satellite in the airspace is kept unchanged. Therefore, when the cache of the satellite is updated, the cache states of other satellites, channel statistical information and a content request distribution model of a user link in a current airspace are considered, a dynamic cache updating model based on transmission perception is further established, and after the satellite initially enters the airspace, what contents need to be cached from a gateway station side is solved through a continuous convex approximation method, so that the cache hit rate aiming at the user content request in the area can be improved, and the average content request delay of a ground user is further reduced.
In order to solve the problem that the multicast capacity is limited by users with poorer links in single-satellite multicast transmission, a multi-satellite cooperative transmission strategy is adopted. Content request f for a specific user groupgAll satellites in the coverage space of the gateway station are defined as a set C of cooperable transmissionss. Further, s ∈ C for all LEO satellites ssChecking whether the requested content f is contained in the buffer space of the devicegDynamic collaboration sets are formed based on the principle of who participates
Figure BDA0003355015280000061
Since the cooperative transmission set is determined by the caching result and the caching calculation is based on the transmission perception, the dynamic caching updating model based on the transmission perception can be modeled as a problem of minimizing the average time delay of the user, and the problem is solved by a convex optimization tool box and other tools. And obtaining an expected cache indication matrix C, and caching the content of the satellite newly entering the airspace based on the indication of the matrix C.
As shown in fig. 3, it is the performance simulation result of the invention under three algorithms of random cache, most popular cache and probabilistic cache; uncooperative multicasting refers to multiple multicast user groups served by a single satellite that is closest in distance. The random cache is to select content files for caching in a Gaussian random mode; the most popular cache is to cache the most popular files based on content popularity; the probability cache is a request probability random cache file based on user content, and simulation results show that the scheme provided by the invention has better time delay performance.

Claims (4)

1. A dynamic cache updating and cooperative transmission method facing a low earth orbit satellite network is characterized by comprising the following steps:
firstly, building a communication scene of a low-orbit satellite network comprising a plurality of satellites, a plurality of ground terminals and a gateway station;
aiming at a satellite a which just enters a coverage airspace of a gateway station b, establishing a dynamic cache updating model based on transmission perception for the satellite a, and aiming at minimizing the average content request time delay of all ground users; solving the model by a continuous convex approximation method to obtain the content of the satellite a needing to be cached from the gateway station b side, wherein the caches of other satellites in the coverage space of the gateway station are kept unchanged;
then, for a specific request file f in the transmission processgAll satellites in the space covered by the gateway station b form a cooperative transmission set CsEach satellite checks whether the request file f is contained in the cache space of the satellitegDynamic cooperative satellite set is formed based on the principle of who participates
Figure FDA0003355015270000011
Under the constraint of satisfying power, the cooperative beam forming weight matrix W is solved, the goal of shortest content request time delay is achieved, and cooperative transmission is realized.
2. The dynamic cache updating and cooperative transmission method for the low earth orbit satellite network as claimed in claim 1, wherein in the communication scenario, the user link between the satellite and the ground terminal performs communication, and the satellite and the gateway station perform communication through the feeder link; when each satellite initially enters a gateway station coverage airspace, file caching is carried out, the cache is not updated after the satellite enters the coverage airspace, and the file is updated until the satellite enters the next gateway station coverage airspace; and multiple satellites in the current coverage space carry out downlink transmission in a cooperative mode and distribute contents to the ground terminal.
3. The method according to claim 1, wherein the dynamic cache update model is as follows:
Figure FDA0003355015270000012
w represents a beamforming weight matrix, and each element in the matrix W is represented as omegas,gWherein S is a satellite index value, S is 1,2, …, S; s represents the total number of satellites in the airspace; g denotes the index of the multicast group, G1, 2, …, G; g is the total number of the multicast groups;
c represents a cache indication matrix, and each element of the matrix C is a binary number Cs,fF is the index value of the popular file, F is 1,2, …, F is the total number of the popular files, cs,fFor indicating whether or not a satellite s contains a popular file f, cs,f1 means that the s-th satellite contains the f-th content file, and vice versa cs,f＝0；
T represents the number of training time slots; u represents the number of terrestrial users, U is the user index in each multicast group, and U is 1,2, …, Ng；NgIndicating the number of users in each multicast group; tau isg,uRepresenting the propagation delay of user u in the g multicast group;
Figure FDA0003355015270000013
indicating a specific request file fgSize of (a) fgA specific request file indicating the g-th multicast group;
Figure FDA0003355015270000014
denotes the capacity of the g-th multicast group, where B denotes the transmission bandwidth, γg,uRepresenting the signal-to-interference-and-noise ratio of the user u in the g-th multicast group;
the constraint conditions corresponding to the objective function are as follows:
Figure FDA0003355015270000021
Figure FDA0003355015270000022
Figure FDA0003355015270000023
formula (1a) is an expression of the signal to interference plus noise ratio, where fs,gIs a binary cache indicator if the s-th satellite contains a particular request file fgThen f iss,gIf 1, otherwise fs,g＝0；hg,u,sIndicating the channel between user u and the s-th satellite within the g-th multicast group,
Figure FDA0003355015270000024
representing a noise component;
formula (1b) indicates that the total power of all beams under a single satellite must not exceed the maximum satellite power; psRepresents the maximum satellite power;
equation (1c) indicates that the total size of all the contents of the files cached on the s-th satellite must not exceed the maximum cache limit L of the satellites；MfIndicating the size of the f-th popular file.
4. The dynamic cache updating and cooperative transmission method for the low earth orbit satellite network according to claim 1, wherein the objective function and the constraint condition for solving the cooperative beamforming weight matrix W under the constraint of satisfying the power are as follows:
Figure FDA0003355015270000025
Figure FDA0003355015270000026
Figure FDA0003355015270000027
and solving by using a convex optimization tool box to obtain a beam forming weight matrix W.
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