阅读说明：本技术 基于联盟形成博弈的无人设备内容协同实现方法 (Unmanned equipment content cooperation realization method based on alliance formed game ) 是由 孙彦赞 钟心琳 张舜卿 陈小静 徐树公 于 2021-07-30 设计创作，主要内容包括：一种基于联盟形成博弈的无人设备内容协同实现方法,根据用户间相似度,利用谱聚类算法对用户进行分簇,并基于用户偏好和目前内容流行度分布,在无人设备上进行内容缓存,再根据无人设备系统效用函数最大化原则,将无人设备的协作内容传输问题建模为联盟博弈模型,进行无人设备结盟博弈,最终收敛形成稳定的联盟结构。本发明基于联盟博弈算法构建无人设备联盟,分析无人设备传输能耗和用户满意度以及系统可靠性,建立多无人设备协同通信策略,以达到最大化系统效用目标。(A cooperative implementation method for contents of unmanned equipment based on a league-forming game is characterized by clustering users by utilizing a spectral clustering algorithm according to similarity among the users, caching contents on the unmanned equipment based on user preference and current content popularity distribution, modeling a cooperative content transmission problem of the unmanned equipment into a league game model according to a maximum principle of an unmanned equipment system utility function, carrying out league game on the unmanned equipment, and finally converging to form a stable league structure. The invention constructs the unmanned equipment alliance based on the alliance game algorithm, analyzes the transmission energy consumption and the user satisfaction degree of the unmanned equipment and the system reliability, and establishes a multi-unmanned equipment cooperative communication strategy so as to achieve the aim of maximizing the system utility.)

1. A cooperative implementation method for contents of unmanned equipment based on league formation game is characterized in that users are clustered by using a spectral clustering algorithm according to similarity among the users, content caching is performed on the unmanned equipment based on user preference and current content popularity distribution, then a cooperative content transmission problem of the unmanned equipment is modeled into a league game model according to a maximum principle of an unmanned equipment system utility function, league game of the unmanned equipment is performed according to league formation criteria, and finally a stable league structure is formed in a converging mode.

2. The cooperative implementation method for content of unmanned aerial vehicle based on alliance formation game as claimed in claim 1, wherein the clustering is performed on users based on content preference similarity and distance similarity among users and adding different weights to form clustering standard, and specifically comprises:wherein: similarity of user preferences to content Similarity of physical distancesp_uIs the distribution of user u's preference for all content, p_vIs the distribution of user v's preferences, dist, over all content_u，vIs the distance between users u and v, maxdist represents the maximum distance between all users, α₁And alpha₂Respectively, a weight parameter, a user-associated variable y_n，u∈{0，1}，y_n，u1 denotes that user u is associated to drone n.

3. The cooperative implementation method for content of unmanned aerial vehicle based on league-forming game as claimed in claim 2, wherein the clustering standard takes into account load of the unmanned aerial vehicle to control the number of the unmanned aerial vehicle access users, so as to avoid link congestion caused by an excessive number of the single unmanned aerial vehicle access users, and specifically comprises: clustering users by using Spectral Clustering (SC) algorithm, and making similarity matrix sim between users as a large-sized pagesim_n，uN belongs to N, U belongs to U and is used as input, and finally the division condition of each cluster, namely the service user distribution condition C (C) of each unmanned aerial vehicle is obtained₁，c₂，...，c_N) The position of the drone is determined as the cluster center for each cluster.

4. The unmanned aerial vehicle content collaborative implementation method based on alliance formation game as claimed in claim 1, wherein the content caching, calculating a content caching strategy based on user preference and popularity, sorting the content according to the caching strategy, selecting the first Q contents for content caching in the unmanned aerial vehicle, specifically: content caching policy c_u，f＝γ₁p_u，f+γ₂q_fWherein: q. q.s_fIndicating content popularity, i.e. the probability of a user u requesting content f within any time periodp_ufIndicates user preference, γ₁And gamma₂The weight parameter is used for expressing the proportion of the content popularity and the user preference in the content caching process.

5. The cooperative implementation method for content of unmanned aerial vehicle based on alliance formation game as claimed in claim 1, wherein the principle of maximum system utility function is: establishing a system utility function through transmission energy consumption, user satisfaction and system reliability, and modeling the unmanned aerial vehicle cooperative content transmission problem into a alliance game model;

the system utility function u_n＝εMOS_n-δE_n+ηR_nWherein: epsilon, delta and eta are respectively a scale factor for dividing the influence of three indexes on the utility function of the unmanned aerial vehicle, and MOS_nTo serve user satisfaction of drone n, E_nFor communication energy consumption, R_nA reliability function for the unmanned aerial vehicle system;

the unmanned aerial vehicle cooperation content transmission problem is that: when the user does not request the required content in the service unmanned aerial vehicle, the unmanned aerial vehicle can communicate with other unmanned aerial vehicles through the cooperative link, and when the content is cached by other unmanned aerial vehicles, the unmanned aerial vehicle can transmit the content to the user, so that the access process of the macro base station is reduced, and further the time delay is reduced.

6. The cooperative implementation method for unmanned aerial vehicle content based on league formation game as claimed in claim 5, wherein the user satisfaction utilizes M0S model to evaluate QoE of user, so that satisfaction of service unmanned aerial vehicle n can be served Wherein: c₁，C₂Is a constant number, C₁＞0， Delay for user request, x_n，fCaching variables for content, x_n，fThat 1 indicates that drone n has cached content f, x_n，f0 denotes drone n uncached content f, x_n′，fCaching variables for the content of drone n' (other drones than drone n);

the communication energy consumptionWherein: the energy consumption generated by the unmanned aerial vehicle transmitting the content to the user isThe energy consumption generated by the communication between the unmanned aerial vehicle and other unmanned aerial vehicles through the cooperative link isThe energy consumption generated when the content is not cached under other unmanned planes and is requested to the macro base station through the backhaul link isP₀And P_nRepresenting a macro base station transmit power and an unmanned aerial vehicle transmit power;

the reliability function of the unmanned aerial vehicle systemWherein: r_n＝In(Re_n)，Re_nReliability for communication of a single drone, D_n，u，fTime delay r for requesting content f for user u under unmanned aerial vehicle n service_n，u，f∈{0, 1} is a binary variable of the user request content, r_x，u，f1 indicates that the user u under drone n has requested content f.

7. The cooperative implementation method for content of unmanned devices based on league formation game as claimed in claim 1, wherein the league game model specifically comprises:

s.t.x_n，f∈{0，1}，r_n，u，f∈{0，1}

wherein: u shape_sysIs the total utility of the system, S_kIs a union, pi ═ S₁，S₂，...，S_KIs a federation group structure, S_kFederation utilities of u_nFor unmanned aerial vehicle utility, x_n，fBuffer variables for content, r_n，u，fAnd requesting variables for the user, wherein the sum of the content caching quantity expressed by the inequality does not exceed the unmanned aerial vehicle caching space.

8. A league formation game-based unmanned device content collaborative implementation method according to claim 1, wherein the league formation criterion is: when the total utility of the alliance after the unmanned aerial vehicle n is added into the alliance is higher than that of the alliance before the unmanned aerial vehicle n is added into the alliance and the utility of the unmanned aerial vehicle is improved, the unmanned aerial vehicle is added into the new alliance, and definition is performedFor a complete transitive relationship over the set of all possible federations that drone n may form, whenRepresenting drone n compared to federation S_jPreferring to join federation S_iThe method specifically comprises the following steps: when the unmanned aerial vehicle chooses to join the alliance S₁Instead of S₂When the system is used, the utility of the system is increased, and the total utility of the system is increased, namely the federation formation criterion is as follows: wherein:is that unmanned aerial vehicle n joins alliance S_iLater new alliance S_iAnd original alliance S_jThe utility of the federation of (a),is that unmanned aerial vehicle n joins alliance S_iFront connectionAlly S_iAnd original alliance S_jFederation utility of.

9. The cooperative implementation method for content of unmanned aerial vehicle based on league-forming game as claimed in claim 1, wherein the convergence is: through unmanned aerial vehicle according to alliance formation criterion and setting up candidate set and historical selection set, constantly playing and finally converging form stable alliance structure, specifically include:

1) and (3) initializing a union: each unmanned aerial vehicle is an independent alliance, namely the initial alliance is pi { {1}, {2}, } and { K } }, and an unmanned aerial vehicle history selection set H is set_nAnd candidate set Ca_n；

2) Calculating self utility U of unmanned aerial vehicle n under existing alliance_nIndividual federation utilitiesAnd total system utility U_sys；

3) Selecting one alliance from the existing alliances by the unmanned aerial vehicle n, calculating the own utility, the alliance utility and the system utility after the unmanned aerial vehicle n is added, and judging whether the unmanned aerial vehicle n meets the following transfer conditions:

a) unmanned n from the current alliance S_iTransfer to federation S_jWhen the effect is not less than the effect before adding;

b) unmanned n from the current alliance S_iTransfer to federation S_jWhen a new alliance structure is formed, the system utility is greater than that under the original alliance structure before the new alliance structure is added;

c) selected federation S_jIs not present in the history set;

when the transfer condition is satisfied, the alliance S is connected_jAdding the candidate set into a candidate set; when the transfer condition is not met, reselecting one alliance to join;

4) when the candidate set is not empty, selecting the alliance S which enables the system to have the maximum effect from the candidate alliances of the unmanned aerial vehicle n_optJoin the drone and join the federation S_optAdding the data into a history selection set of the unmanned aerial vehicle n, and updating a alliance structure;otherwise, the history selection set is not changed;

5) and when the history sets of all the unmanned aerial vehicles are not changed any more, the game is ended, and a stable alliance structure is obtained.

10. The cooperative implementation method for content of unmanned aerial vehicles based on alliance formation game as claimed in any preceding claim, which comprises:

step A, clustering users based on a spectral clustering algorithm: in order to improve the utilization efficiency of cache resources of the unmanned aerial vehicle, the similarity between users is consideredWherein: inter-user content preference similarity sim1_u，vSimilarity sim2 with physical distance between users_u，v；

Step A-1, content preference similarity, setting the interest of a user u in a content f as p_u，fThe interest of the user v in the content f is p_v，fI.e. the probability that a user requests a certain content, the similarity of content preference among usersWherein: p is a radical of_u＝[p_u，1，p_u，2，...，p_u，F]，p_v＝[p_v，1，p_v，2，…，p_v，F]The interest vectors of the users u and v are shown, and F is the content quantity;

step A-2, similarity of physical distancesWherein: dist_u，vIs the distance between users u and v, max dist represents the maximum distance between all users; in order to avoid link congestion caused by overlarge number of access users of a single UAV, the similarity of content preference and the similarity of physical distance are respectively expressed by alpha₁And alpha₂Weighting is performed, so that the similarity between users isThe similarity between any two users forms a similarity matrix S between the users;

step A-3, taking the matrix S as input, and dividing the user clusters by using a spectral clustering algorithm, wherein the method specifically comprises the following steps:

step A-31, preparing data, constructing a similarity matrix S, and constructing an adjacency matrix W as S and a degree matrix D according to S, wherein: d diagonal element of

Step A-32, calculating Laplace matrix L ═ D-W and normalized Laplace matrix D^-1/2LD^-1/2；

Step A-33, calculation of D^-1/2LD^-1/2The minimum N eigenvalues and corresponding eigenvectors are normalized to obtain an eigenvector matrix F;

step A-34, taking each line in the F as a sample, clustering by using a clustering method to obtain cluster division C (C)₁，c₂，...，c_N) Wherein: n is the number of unmanned aerial vehicles, namely the clustering dimension;

step B, according to the user clustering division result C (C) obtained in step A₁，c₂，...，c_N) And caching the content according to the content popularity, specifically: building popularity distribution of content f using a Zipf model based on user determinations for each drone serviceWherein: f is the content number, beta represents the distribution deviation, and the caching probability of the content F in the unmanned plane n is Wherein: p is a radical of_u，fFor user interest, γ₁And gamma₂Representing the influence degree of the content popularity and the user preference on the caching strategy;

step C, according to the user clustering division result and the unmanned aerial vehicle position information obtained in the step A and the content caching strategy C obtained in the step B_n，fThe method adopts cooperative game-based unmanned equipment alliance construction to realize system utility maximization as an objective to construct alliance, and specifically comprises the following steps:

step C-1, initializing alliances, enabling each unmanned device to serve as an independent alliance, and defining a history selection set H_nAnd candidate set Ca_n；

C-2, calculating the self utility u of the unmanned aerial vehicle n_nFederation utilitiesAnd total utility of the systemWherein: s_kIs a alliance, pi is an alliance structure;

c-3, adding the unmanned aerial vehicle n into one of the existing alliances, calculating the self utility, the alliance utility and the system utility after the unmanned aerial vehicle n is added, and judging whether the unmanned aerial vehicle n meets the following three transfer conditions:

step C-31, unmanned plane n from current alliance S_iJoining to federation S_jWhen the effect is not less than the effect before adding;

step C-32, unmanned plane n from current alliance S_iJoining to federation S_jThen, the system utility is greater than that under the federation structure before joining;

step C-33, selected federation S_jIs not present in H_nPerforming the following steps; if the first two transfer conditions are met, the alliance S_jAdding into Ca_nIf the transfer condition is not met, reselecting one alliance to join;

step C-4, if Ca_nAdding the unmanned aerial vehicle into Ca when the unmanned aerial vehicle is not empty_nFederation S of medium to maximum system utility_optAnd then S is_optAdd to History selection set H_nMore, moreA new alliance structure; otherwise, the history set is unchanged;

and C-5, all the unmanned aerial vehicle historical selection sets are not changed any more, the game is ended, and the optimal alliance is obtained.

Technical Field

The invention relates to a technology in the field of distributed information control, in particular to a cooperative implementation method of unmanned equipment content based on Coalition Formation Game (CFG).

Background

Caching is a potential technique to alleviate link congestion. For a user-dense scene, most of the existing caching work at present is generally performed in a static network without mobility, and the cached content is stored in a ground static base station. However, in areas with ultra-dense users and high-rise buildings, the static terrestrial base station with cache may not meet the high capacity needs of the users. Due to the advantages of high speed and high efficiency, the LoS is linked to the ground node, the movement can be controlled, and the like, the unmanned aerial vehicle has a very high caching prospect in future wireless communication, and can be deployed as an aerial base station to assist a traditional cellular network.

The existing unmanned aerial vehicle group cooperation technology comprises the steps of calculating an acceleration reference value through an artificial potential field and an artificial vortex field constructed by a field method, or taking a Charpy value as a solution of alliance income distribution; however, most of the technologies do not consider the problems of system reliability, transmission delay, load pressure of a single unmanned aerial vehicle and the like in the cluster cooperation, and the joining of the unmanned aerial vehicles through repeated calculation and low convergence speed occur in the algorithm process of building the alliance, so that the actual engineering requirements are difficult to meet.

Disclosure of Invention

The invention provides a cooperative implementation method of unmanned equipment content based on alliance-formed game aiming at the problem of poor user service quality caused by excessive base station load in the prior art.

The invention is realized by the following technical scheme:

the invention relates to a cooperative implementation method of unmanned equipment content based on alliance formation game, which is characterized by clustering users by utilizing a spectral clustering algorithm according to similarity among the users, performing content caching on the unmanned equipment based on user preference and current content popularity distribution, modeling the cooperative content transmission problem of the unmanned equipment into an alliance game model according to the maximization principle of an unmanned equipment system utility function, performing alliance game on the unmanned equipment, and finally converging to form a stable alliance structure.

The method specifically comprises the following steps:

step A, clustering users based on a spectral clustering algorithm: in order to improve the utilization efficiency of cache resources of the unmanned aerial vehicle, the similarity between users is consideredWherein: inter-user content preference similarity sim1_u,vSimilarity sim2 with physical distance between users_u,v。

Step A-1, content preference similarity, setting the interest of a user u in a content f as p_u，fThe interest of the user v in the content f is p_v，fI.e. the probability that a user requests a certain content, the similarity of content preference among usersWherein:

p_u＝[p_u，1，p_u，2，...，p_u，F]，p_v＝[p_v，1，p_v，2，...，p_v，F]the interest vector of the user u, v, and F is the content number.

Step A-2, similarity of physical distancesWherein: dist_u，vIs the distance between users u and v, and maxdist represents the maximum distance between all users. In order to avoid link congestion caused by overlarge number of access users of a single UAV, the similarity of content preference and the similarity of physical distance are respectively expressed by alpha₁And alpha₂Weighting is performed, so that the similarity between users isThe similarity between any two users constitutes an inter-user similarity matrix S.

Step A-3, taking the matrix S as input, and dividing the user clusters by using a spectral clustering algorithm, wherein the method specifically comprises the following steps:

step A-31, preparing data, constructing a similarity matrix S, and constructing an adjacency matrix W as S and a degree matrix D according to S, wherein: d diagonal element of

Step A-32, calculating Laplace matrix L ═ D-W and normalized Laplace matrix D^-1/2LD^-1/2；

Step A-33, calculation of D^-1/2LD^-1/2The minimum N eigenvalues and corresponding eigenvectors are normalized to obtain an eigenvector matrix F;

step A-34, taking each line in the F as a sample, clustering by using a clustering method to obtain cluster division C (C)₁，c₂，...，c_N) Wherein: n is the unmanned aerial vehicle quantity, clustering dimension promptly.

Step B, according to the user clustering division result C (C) obtained in step A₁，c₂，...，c_N) And caching the content according to the content popularity, specifically: building popularity distribution of content f using a Zipf model based on user determinations for each drone serviceWherein: f is the content number, beta represents the distribution deviation, and the caching probability of the content F in the unmanned plane n is Wherein: p is a radical of_u，fFor user interest, γ₁And gamma₂Indicating how much content popularity and user preferences affect the caching policy.

Step C, according to the user clustering division result and the unmanned aerial vehicle position information obtained in the step A and the content caching strategy C obtained in the step B_n，fBy using cooperative-based gamesThe establishing of the unmanned equipment alliance aims at realizing the maximization of system utility, and specifically comprises the following steps:

step C-1, initializing alliances, enabling each unmanned device to serve as an independent alliance, and defining a history selection set H_nAnd candidate set Ca_n。

C-2, calculating the self utility u of the unmanned aerial vehicle n_nFederation utilitiesAnd total utility of the systemWherein: s_kIs a alliance, pi is an alliance structure.

C-3, adding the unmanned aerial vehicle n into one of the existing alliances, calculating the self utility, the alliance utility and the system utility after the unmanned aerial vehicle n is added, and judging whether the unmanned aerial vehicle n meets the following three transfer conditions:

step C-31, unmanned plane n from current alliance S_iJoining to federation S_jWhen the effect is not less than the effect before adding;

step C-32, unmanned plane n from current alliance S_iJoining to federation S_jThen, the system utility is greater than that under the federation structure before joining;

step C-33, selected federation S_jIs not present in H_nIn (1). If the first two transfer conditions are met, the alliance S_jAdding into Ca_nAnd if the transfer condition is not met, reselecting one alliance to join.

Step C-4, if Ca_nAdding the unmanned aerial vehicle into Ca when the unmanned aerial vehicle is not empty_nFederation S of medium to maximum system utility_optAnd then S is_optAdd to History selection set H_hAnd updating the federation structure. Otherwise, the history set is unchanged.

And C-5, all the unmanned aerial vehicle historical selection sets are not changed any more, the game is ended, and the optimal alliance is obtained.

Technical effects

The invention integrally solves the problem that the quality of the user content access service is reduced in the dense place of the user in the prior art. The method comprises the steps that the ground base station is assisted by the unmanned equipment, similarity among users and communication capacity among the unmanned equipment are fully considered, an unmanned equipment alliance is constructed to cooperatively transmit content, a content transmission system is optimized, and system effectiveness is improved; under the condition of intensive user distribution, the system performance obtained by the invention is obviously superior to that of an unmanned device non-cooperative mode. The invention combines the cooperative game theory and the content caching and transmission problem together, aims at maximizing the overall effectiveness of the system, and researches the unmanned equipment cooperative content caching and transmission problem based on game formation alliance. In order to obtain the maximum system utility, the constant game among the unmanned devices forms alliances to better serve users, finally, a stable alliance structure which maximizes the system is obtained, and the effect is more excellent compared with the traditional alliance forming criteria, which shows that the invention has better utility.

Drawings

FIG. 1 is a schematic diagram of an embodiment;

fig. 2 is a schematic diagram of user distribution and unmanned aerial vehicle distribution positions after clustering;

FIG. 3 is a diagram of simulation results for system convergence;

FIG. 4 is a graph of the results of the present invention compared to a non-cooperative approach;

FIG. 5 is a diagram of network utility versus content caching;

FIG. 6 is a graph of network utility versus number of drones;

FIG. 7 is a flowchart illustrating exemplary steps of the present invention.

Detailed Description

The experimental environment is a Windows 1064 bit operating system, the CPU is Intel i5-10210U, the GPU is NVIDIA GeForce MX 250, the memory is 16GB, and the experimental development language is MATLAB.

When all communications use orthogonal channels, interference from other devices is reduced. And when the unmanned aerial vehicle and the user do not move in the data transmission process, the SNR of the signal to noise ratio between the unmanned aerial vehicle and the user (UAV-UE link), between the macro base station and the unmanned aerial vehicle (MBS-UAV link), and between the unmanned aerial vehicle and the unmanned aerial vehicle (UAV-UAV link) are respectively:

1) UAV-UE link: the propagation channel from the unmanned aerial vehicle to the user adopts a standard logarithmic normal shadow model. The standard logarithmic normal shadow model can model the line-of-sight LoS and the non-line-of-sight NLos link by selecting specific channel parameters, and specifically comprises the following steps:

wherein: n is unmanned plane, u is user, f is carrier frequency 5GHz, d_n，uRepresenting the distance, η, between drone n and user u_LoS1.6dBm and eta_NLoS23dBm represents the extra path loss component of line-of-sight Los and non-line-of-sight NLos links, respectively.

The probability of a LoS link depends on the environment (density and height of the building), the positions of the drone and the user, the elevation angle between the drone and the user, in particular:wherein: x-11.9 and Y-0.13 are constants, depending on environmental factors (urban, suburban, dense, etc.),is the pitch angle between unmanned aerial vehicle and the user, and H equals 100m and is unmanned aerial vehicle height of hovering.

The path loss between the unmanned aerial vehicle and the user is as follows:

wherein: a ═ η_LoS-η_NLoS，B＝20log(4πf/c)+η_NLoSThen, the SNR of the drone and the user link is:P_nand 30dBm represents the unmanned plane transmitting power.

2) MBS-UAV Link: los link and NLoS link models are also adopted between the macro base station and the unmanned aerial vehicle. Los and NLos path loss from macro base station to drone are respectivelyWherein: α is 2 is the road loss index,is an additional path loss coefficient of the NLoS link. Then the probability of Los and NLoS links is: wherein:calculating the path loss in the same way Obtaining SNR of the link between the macro base station and the unmanned aerial vehicle as follows:wherein P is₀43dBm represents the macro base station transmit power.

3) UAV-UAV link: Wi-Fi communication is adopted for communication among the unmanned aerial vehicles, and the path loss of Wi-Fi signals in free space transmission, namely the path loss of a cooperative link among the unmanned aerial vehicles is PL_n，n′＝32.44+20lg f_w+20lg d_n，n′Wherein: f. of_w2.4GHz is the working frequency of the unmanned aerial vehicle, d_n，n′Is the distance between drones. Then the SNR of the drone and drone link is:

through the calculation, the signal-to-noise ratio of the UAV-UE, MBS-UAV and UAV-UAV links is obtained, and for data transmission, the transmission rate and the transmission time delay are calculated.

For a wireless access link between the UAV and the user, a backhaul link between the MBS and the UAV and a cooperative link between the UAV and the UAV, when the bandwidth B of the wireless access link is 20MHz, the bandwidth B of the backhaul link is_b10MHz, cooperative link bandwidth B_c10MHz, content size is unified as S_f10Mbits, the data rate of which is Wherein: n is the number of unmanned aerial vehicles, UⁿNumber of users under service for drone n. Therefore, the transmission delays are respectively: d_n，u＝S_f/R_n，u，D_0，u＝S_f/R_0，u，D_n，n′＝S_f/R_n，n′。

As shown in fig. 7, the present embodiment relates to a cooperative implementation method for content of an unmanned aerial vehicle based on league-formed game, which includes the specific steps of:

step one, forming a clustering standard based on similarity of content preferences and distance between users and addition of different weights, clustering the users, and enabling each unmanned aerial vehicle to determine service users thereof so as to improve utilization efficiency of limited unmanned aerial vehicle cache resources, specifically:wherein: similarity of user preferences to contentSimilarity of physical distancesp_uIs the distribution of user u's preference for all content, p_vIs the distribution of user v's preferences, dist, over all content_u，vIs the distance between users u and v, maxdist represents the maximum distance between all users, α₁And alpha₂Respectively, a weight parameter, a user-associated variable y_n，u∈{0，1}，y_n，u1 denotes that user u is associated to drone n.

The clustering standard, consider unmanned aerial vehicle's load condition simultaneously, control unmanned aerial vehicle and insert the quantity of user, avoid single unmanned aerial vehicle to insert that the too big link that causes of user quantity is congested, specifically do: clustering users by using Spectral Clustering (SC) algorithm, and setting a similarity matrix sim between users as { sim }_n，uN belongs to N, U belongs to U and is used as input, and finally the division condition of each cluster, namely the service user distribution condition C (C) of each unmanned aerial vehicle is obtained₁，c₂，...，c_N) As shown on the left of fig. 2; the position of the drone is determined as the cluster center of each cluster, so the drone and the user position are distributed as shown on the right of fig. 2.

Step two, calculating a content caching strategy based on user preference and popularity, sequencing the contents according to the caching strategy, and selecting the first Q contents to cache the contents in the unmanned aerial vehicle, wherein the method specifically comprises the following steps: content caching policy c_u，f＝γ₁p_u，f+γ₂q_fWherein: q. q.s_fIndicating content popularity, i.e. the probability of a user u requesting content f within any time periodp_u，fIndicates user preference, γ₁And gamma₂The weight parameter is used for expressing the proportion of the content popularity and the user preference in the content caching process.

The content caching strategy is determined according to users under each unmanned aerial vehicle service, current content popularity distribution is considered, and preference of each unmanned aerial vehicle service user and limitation of an unmanned aerial vehicle caching space Q are considered.

And step three, establishing a system utility function through transmission energy consumption, user satisfaction and system reliability, and modeling the unmanned aerial vehicle cooperative content transmission problem into a alliance game model.

The system utility function u_n＝εMOS_n-δE_n+ηR_nWherein: epsilon, delta and eta are respectively a scale factor for dividing the influence of three indexes on the utility function of the unmanned aerial vehicle, and MOS_nTo serve user satisfaction of drone n, E_nFor communication energy consumption, R_nIs a reliability function of the drone system.

The user satisfaction utilizes an MOS (mean opinion score) model to evaluate the QoE of the user, so that the satisfaction of the service unmanned plane n can be obtainedWherein: c₁，C₂Is a constant number, C₁＞0，Delay for user request, x_n，fCaching variables for content, x_n，fThat 1 indicates that drone n has cached content f, x_n，f0 denotes drone n uncached content f, x_n′，fVariables are cached for the content of drone n' (other drones than drone n).

The communication energy consumptionWherein: the energy consumption generated by the unmanned aerial vehicle transmitting the content to the user isThe energy consumption generated by the communication between the unmanned aerial vehicle and other unmanned aerial vehicles through the cooperative link isThe energy consumption generated when the content is not cached under other unmanned planes and is requested to the macro base station through the backhaul link isP₀And P_nRepresenting the macro base station transmit power and the drone transmit power.

The reliability function of the unmanned aerial vehicle systemWherein: r_n＝In(Re_n)，Re_nReliability for communication of a single drone, D_n，u，fTime delay r for requesting content f for user u under unmanned aerial vehicle n service_n，u，fE {0, 1} is a binary variable of the content requested by the user, r_n，u，f1 indicates that the user u under drone n has requested content f.

The unmanned aerial vehicle cooperation content transmission problem is that: when the user does not request the required content in the service unmanned aerial vehicle, the unmanned aerial vehicle can communicate with other unmanned aerial vehicles through the cooperative link, and when the content is cached by other unmanned aerial vehicles, the unmanned aerial vehicle can transmit the content to the user, so that the access process of the macro base station is reduced, and further the time delay is reduced. Therefore, the unmanned aerial vehicle cooperation mode can reduce the user request time delay, improve the user satisfaction degree and further improve the system utility.

The alliance game model specifically comprises the following steps:

s.t.x_n，f∈{0，1}，r_n，u，f∈{0，1}

wherein: u shape_sysIs the total utility of the system, S_kIs a union, pi ═ S₁，S₂，...，S_KIs a federation group structure, S_kFederation utilities ofu_nFor unmanned aerial vehicle utility, x_n，fBuffer variables for content, r_n，u，fAnd requesting variables for the user, wherein the sum of the content caching quantity expressed by the inequality does not exceed the unmanned aerial vehicle caching space.

And step four, establishing alliances based on cooperative game, namely unmanned aerial vehicles of the same alliance perform cooperative transmission, performing game between the unmanned aerial vehicles by taking the system effectiveness maximization as a target, and finally achieving stable convergence of the alliance structure.

The league structure is that in N, the league group pi of all players is { S }₁，S₂，…，S_K}, wherein: k is the total number of associations in partition Π. For example, N ═ {1, 2, 3, 4, 5, 6, 7, 8}, then the federation S₁＝{1，2，3}，S₂＝{3，5，6}，S₃7, 8 is a partition of N, when there is no federation S_kThe inner player members will join other alliances by joining To change the current partition or split into smaller non-adjacent leagues, the league structure, pi ═ S₁，S₂，...，S_KIs a stable partition.

The cooperative game based method comprises the following steps: for any unmanned aerial vehicle, the unmanned aerial vehicle can join any alliance and collaboratively transmit content with the unmanned aerial vehicle in the alliance. For drone n, define >_nA complete transitive relationship over the set of all possible federations that drone n may form. When S is_i＞_nS_jDenotes drone n vs. alliance S_jPreferring to join federation S_iThis preference can affect the formation of the final federation structure. The unmanned aerial vehicles play with each other to form alliance and rootAccording to the preference relationship, namely the alliance forming criterion, the new alliance is added or the original alliance is remained, and finally all alliances are stable. In the league forming game, the preference sequence can ensure the existence of league stability. In addition to the order of preference, there are many criteria for federation formation, with different criteria leading to different federation results.

The alliance forming criterion is that: when the total utility of the alliance after the unmanned aerial vehicle n joins the alliance is higher than that before joining the alliance and the utility of the unmanned aerial vehicle is improved, the unmanned aerial vehicle n will join the new alliance, and the method specifically comprises the following steps: when the unmanned aerial vehicle chooses to join the alliance S₁Instead of S₂When it comes, its own utility increases, and the overall utility of the system increases. Thus, the federation formation criteria of the present invention are: wherein:is that unmanned aerial vehicle n joins alliance S_iLater new alliance S_iAnd original alliance S_jThe utility of the federation of (a),is that unmanned aerial vehicle n joins alliance S_iFormer alliance S_iAnd original alliance S_jFederation utility of. Since the addition of the drone only affects the new and old alliances and does not affect other alliances, it is feasible to consider the alliance utility of the new and old alliances and then affect the system utility.

The alliance forming criterion not only needs to meet the requirement that when the unmanned aerial vehicle joins the alliance, the utility of the unmanned aerial vehicle is increased, the total utility of the system is increased, but also needs to meet the requirement that the unmanned aerial vehicle can be transferred to a new alliance only when the unmanned aerial vehicle does not appear in a history selection set, and meanwhile, the alliance is added into a candidate set Ca_nIn (1). Selecting the optimal alliance which enables the system to have the maximum effect from the candidate set as the alliance which the unmanned aerial vehicle n finally joins, and adding the optimal alliance into the candidate setHistorical selection set H of incoming unmanned aerial vehicle n_nIn (1).

The convergence stability refers to: in the alliance game process, in order to ensure that the criterion, namely unmanned aerial vehicle transfer condition can be converged, accelerate the convergence speed and avoid the unmanned aerial vehicle from repeatedly joining the same alliance, the invention defines a history selection set H_nAnd candidate set Ca_nThe method comprises the following steps: for each drone n, its historical selection set H_nAll federations that were added are included.

The stable convergence of the alliance structure is as follows: through unmanned aerial vehicle according to alliance formation criterion and setting up candidate set and historical selection set, constantly playing and finally converging form stable alliance structure, specifically include:

4.1) initializing the alliance: each unmanned aerial vehicle is an independent alliance, namely the initial alliance is pi { {1}, {2}, } and { K } }, and an unmanned aerial vehicle history selection set H is set_nAnd candidate set Ca_n。

4.2) calculating the own utility U of unmanned plane n under the existing alliance_nIndividual federation utilitiesAnd total system utility U_sys。

4.3) selecting one alliance from the existing alliances by the unmanned aerial vehicle n, calculating the self utility, the alliance utility and the system utility after the unmanned aerial vehicle n is added, and judging whether the unmanned aerial vehicle n meets the following transfer conditions:

a) unmanned n from the current alliance S_iTransfer to federation S_jWhen the effect is not less than the effect before adding;

b) unmanned n from the current alliance S_iTransfer to federation S_jWhen a new alliance structure is formed, the system utility is greater than that under the original alliance structure before the new alliance structure is added;

c) selected federation S_jNot present in the history set.

When the transfer condition is satisfied, the alliance S is connected_jAdding the candidate set into a candidate set; when the branch condition is not satisfied, one is reselectedAnd joining the alliance.

4.4) when the candidate set is not empty, selecting the alliance S which enables the system to have the maximum effect among the candidate alliances of the unmanned plane n_optJoin the drone and join the federation S_optAnd adding the data into the history selection set of the unmanned plane n, and updating the alliance structure. Otherwise, the history selection set is unchanged.

4.5) when the history sets of all the unmanned aerial vehicles are not changed any more, the game is ended, and a stable alliance structure is obtained.

As shown in fig. 1, a specific application scenario related to the present embodiment includes: the system comprises a macro base station, N unmanned aerial vehicles and U users distributed at different positions of a two-dimensional plane.

The user models the distribution of the macro base station through a random geometric theory, and models the distribution as Poisson point process distribution with the density of lambda, and the macro base station position is w₀＝(x₀，y₀) User position is w_u＝(x_u，y_u) The position of the unmanned plane on the two-dimensional plane is w_n＝(x_n，y_n) In this embodiment, the drones hover at the same height H.

When the unmanned aerial vehicle assists the ground base station to serve the user, a return link exists between the unmanned aerial vehicle and the ground base station, and a wireless access link exists between the unmanned aerial vehicle and the user. Because the communication interval between the unmanned aerial vehicles is hundreds of meters, according to the radio frequency division and the frequency spectrum application condition, the data communication mode of Wi-Fi is selected for communication between the unmanned aerial vehicles, and the frequency band is selected to be 2.4GHz, the Wi-Fi communication link is adopted as the cooperative link between the unmanned aerial vehicles.

Through specific practical experiments, under the specific environment setting of a 500m × 500m simulation area and the flying height of an unmanned aerial vehicle (H) 100m, the method is operated by using the user density U (200), the content quantity F (100), the number N (7) of unmanned aerial vehicles, the unmanned aerial vehicle cache space Q (30), and the content popularity Zipf parameter beta (0.8), so as to obtain the total utility U of the system_sys16.7405, the final split is 2 associations { S }₁，S₂{ (1, 6), (2, 3, 4, 5, 7) }, with optimal federation utility of [4.0826, 12.6579 }]The utility of each drone is [1.9750, 2.3385, 2.5901, 2.6670 ]，2.4088，2.1077，2.6535]。

As shown in fig. 3, which is a simulation result diagram of the system convergence situation, it can be seen that the method of the present invention can achieve convergence quickly.

As shown in fig. 4, in order to compare the content caching and cooperative transmission method and the non-cooperative method of the unmanned aerial vehicle provided by the present invention under different user numbers, it can be seen that the cooperative method is superior to the non-cooperative method.

As shown in fig. 5, for the comparison between the proposed caching method and the random caching and non-caching methods, it can be seen that the proposed content caching method is superior to the random caching and the non-caching methods.

As shown in fig. 6, which is a graph of the total system utility and the number of drones, it can be seen that the method has great advantages compared with the conventional federation formation algorithm based on the pareto order and the federation formation algorithm based on the selfish order.

In conclusion, the method firstly utilizes the spectral clustering algorithm to cluster the users so as to better manage, secondly considers the user preference and the content popularity to cache the content, and finally models the cooperative content transmission problem of the unmanned aerial vehicle into a alliance game model. Each unmanned aerial vehicle starts from a non-cooperative state, games are played by taking the maximum system total utility as a target according to a alliance forming criterion, and finally, a stable alliance structure is formed through rapid convergence and the system total utility is obviously improved.

The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.