Research On Key Technology Of Opportunistic Network Targeting Data Offloading

The agglomeration of various communication networks,the popularization of intelligent communication terminals,as well as people's strong demand for access to data and network services anytime and anywhere,jointly spawned a data-explosion era.In order to solve the problems of traffic overload and network congestion caused by the growing data demands of users upon the cellular system,related technologies for data offloading have been emerging in recent years.Utilizing opportunistic network to achieve cellular data offloading,with no-need for infrastructure,zero traffic costs,network coverage enhancement and other advantages,makes the research and application of opportunistic network face unprecedented opportunities and challenges.Taking data offloading as an entry point,the research on key technologies for opportunistic network will fully demonstrate the complementary advantages of opportunistic network and cellular system,effectively mitigate the impact of future big data traffic upon existing cellular system,and promote the commercialization and generalization of opportunistic network,as well as the process of future heterogeneous network integration.This dissertation focuses on improving the offloading efficiency of opportunistic network.Considering the type of offloading service,the topological changes of network and the offloading cost of data traffic,the research is specified in the following aspects:the design of incentive framework,the analysis and control of network topology,the data dissemination and caching mechanisms.The ultimate goal is to build a theoretical system targeting data offloading through game theory,graph theory,stochastic process and integer optimization theory in order to effectively exploit the terminal resources,topology resources,connection resources,and storage resources.The main contribution of this dissertation is listed as follows:For the problem of low offloading efficiency caused by selfishness of terminal nodes,a behavioral interaction model between cellular network operator and terminal node is built based on Stackelberg Game,and the optimal incentive decisions are determined based on equilibrium analysis under the assumption of unlimited caching space.Meanwhile,under the condition of limited caching space,an auction model is designed for request node and caching node,also the corresponding caching incentive framework is established.The experimental results demonstrate that the proposed caching framework is able to provide profitable reward revenue for terminal nodes,furthermore motivate them to participate in data offloading and effectively improve the offloading efficiency of opportunistic network.In view of the influence of mobility on the offloading efficiency of opportunistic network and for the problem of topology control in data offloading scenario,firstly,the topology features are extracted based on real data sets to verify the periodicity and path redundancy of mobile trajectory.Then,the dynamic evolution of topology in time and space is captured and described based on space-time graph.Finally,topology control algorithms based on least density bunch and fully-covered time window are proposed,and the experimental results based on random graph and realistic traces demonstrate that the proposed algorithm effectively reduces the topological cost of network with respect to cost and edge number while still guarantees the network connectivity.For the problem of large data dissemination using opportunistic network in offloading scenario,combining the advantage of centralized control introduced by base station,a global optimization model including two-tier solution space,objective function and the caching constraint is built in order to improve the delivery ratio of opportunistic transmission.Based on uniform segmentation and stochastic-order-based path selection,the proposed optimization model is simplified and the corresponding data dissemination mechanism is also designed.Compared to four existing dissemination mechanisms,the experimental results based on realistic trace data demonstrate that the proposed mechanism effectively reduces the replication overhead while the opportunistic transmission delay and delivery ratio are still guaranteed.Aiming at the optimal selection of caching nodes in data offloading scenario and based on the principle of "minimum node maximum coverage",a cache protocol framework including cache replacement,multi-path dissemination and node selection is established based on set-cover theory.Considering the time-variance of data popularity and the limitation of terminal nodes on transmission and caching capabilities,a cache management problem that takes system cost as the optimization goal is formulated and the corresponding caching-offloading protocol is designed as well.The experimental results based on realistic and synthetic traces demonstrate that,compared to three existing caching schemes,the proposed caching framework reduces the data accessing delay and improves the offloading efficiency of opportunistic network,while still balances the transmission cost and storage cost.