Research On Caching Strategy In Heterogeneous Celluar Networks

With the development of intelligence services and the rapid growth of mobile data,the capacity of mobile communication network can not meet the increasing demand of data,which brings great challenges to mobile cellular network.The caching space of edge devices can be used to store popular content,which can reduce the requirement of capacity.Therefore,edge caching becomes the key technology of next generation mobile communication.In cache-enabled heterogeneous cellular networks,how to cache content will influence the performance of network directly,so the caching strategy of content has become a hot research topic.This thesis focuses on the edge caching strategy in heterogeneous networks(Het Net).The two-layer heterogeneous network hybrid caching strategy is devised,and the energy consumption of serving users is analyzed.The cache hitting probability of the three-layer heterogeneous network based on the random caching strategy is derived,and a joint optimization algorithm of the three-layer caching based on difference of convex algorithm(DCA)is proposed.The caching placement strategy based on MDS code is proposed for the three-layer heterogeneous network consisting of macro base station,small base stations and user equipments,and the average proportion of coded packets downloaded from macro base station is deduced.The details are as follows:(1)Combining hybrid caching with cooperation between base stations,we devise hybrid cooperative caching in the two-layer heterogeneous cellular network composed of macro base stations and small base stations.The closed-access policy is adopted and the distribution of base stations is modelled as independent Poisson point process(PPP).Based on stochastic geometry theory,we derive the outage probability of users’ wireless transmission under co-channel and orthogonal channel deployment scenarios.Based on hybrid cooperative caching,we derive the cache hitting probability.Furthermore,based on the above,with content retransmission,the energy consumption of serving users is derived.Numerical results show that the analysis results are consistent with the simulation results,which validates the correctness of our derivation.The hybrid cooperative caching policy proposed in this thesis can improve the cache hitting probability of users’ requests,thus saving the energy consumption of the system.(2)We derive the expression of the cache hitting probability with random caching policy and propose the joint optimization algorithm based on difference of convex algorithm(DCA)in the three-tier caching heterogeneous cellular network assisted by macro base stations,helpers and users.Under the constraint of the caching capacity of caching devices,we establish the optimization problem to maximize the cache hitting probability of the network.To solve this problem,a convex function is introduced to convert the nonconvex problem to a difference of convex(DC)problem and then we utilize DCA to obtain the optimal caching probability of macro base stations,helpers and users for each content respectively.Simulation results show that when the density of caching devices is relatively low,popular contents are supposed to be cached to achieve a good performance.However,when the density of caching devices is relatively high,each content ought to be cached evenly.The algorithm proposed in this thesis can achieve the higher cache hitting probability with the same density.(3)A caching placement strategy based on MDS code is proposed for the three-layer heterogeneous network consisting of macro base station,small base stations and user equipments.The content is encoded by MDS code to generate the corresponding coded packets.Each caching device caches a certain number of coded packets for each content.Under this caching mechanism,the expression of the average proportion of file coded packets downloaded from macro base station by the system is derived.Under the condition of limited caching capacity of edge devices,the optimization problem of minimizing the average proportion of coded packets downloaded from macro base station is constructed,and the convex optimization problem is proved by using the property that the maximum point function of two convex functions is still convex.Simulation results show that the proposed encoded caching strategy can significantly reduce the average proportion of file encoded packets downloaded from the macro base station,thus reducing the power consumption and delay of the system.