Research On Content Caching In Large-Scale Wireless Networks

The rapid proliferation of smart mobile devices has triggered an unprecedented growth of global mobile data traffic.Meanwhile,the demand for wireless communication services has been shifting from traditional connection-oriented services such as voice telephony and messaging to content-oriented services such as video streaming applications.Motivated by the fact that a large portion of mobile data traffic is generated by many duplicate downloads of a few popular files,wireless caching has been proposed as a promising approach for massive content delivery.The idea of caching is to store the frequently requested files at BSs so as to reduce the distance between contents and requesters and alleviate the burden of backhaul links.In this thesis,we focus on caching designs in large-scale wireless networks,revealing the impacts of system parameters on caching performance,designing low-complexity optimal or near-optimal caching strategies,and providing first-order insights into the design of practical cache-enabled wireless networks.The main contributions are summarized as follows.Firstly,to facilitate massive video dissemination,we study joint caching and multicasting for multi-quality videos encoded using two video encoding techniques,namely scalable video coding(SVC),and HEVC or H.264 as in dynamic adaptive streaming over HTTP(DASH)respectively.We propose a random caching and multicasting scheme,carefully reflecting the relationship between layers of an SVC-based video or descriptions of a DASH-based video.For each type of videos,we derive tractable expressions for the successful transmission probability(STP)in the general and high user density regions,respectively.We consider the maximization of the STP in the high user density region,which is a convex problem with an exceedingly large number of optimization variables.We propose a two-stage optimization method to obtain a low-complexity near optimal solution by solving a relaxed convex problem and a related packing problem.Numerical results demonstrate the respective operating regions of the proposed random caching and multicasting designs for SVC-based and DASH-based videos.Secondly,to make better use of file diversity provided by random caching,we study the impact of antenna resource at users on improving the performance of random caching,and consider the maximal ratio combining(MRC)receiver and partial zero forcing(PZF)receiver at users.For each receiver,we derive an integral expression and a closed-form upper bound for the STP.Consider the maximization of the STP.In the case of the MRC receiver,we maximize the STP by optimizing the caching probabilities,which is a non-convex problem.We obtain a stationary point by solving an equivalent difference of convex(DC)programming problem using concaveconvex procedure(CCCP).In the case of the PZF receiver,we maximize the STP by optimizing the caching probabilities and the degrees of freedom(Do F)allocation,which is a mixed discrete-continuous problem.Based on structural properties,we obtain a low-complexity near optimal solution by using an alternating optimization approach.The analysis and optimization results reveal the impact of antenna resource at users on random caching.Thirdly,to further improve file diversity,we investigate two partitionbased caching designs,i.e.,a coded caching design based on Random Linear Network Coding and an uncoded caching design in a successive interference cancellation(SIC)-enabled wireless network.Under each caching design,we derive tractable or closed-form expressions for the STPs in the general,small and large file size regimes,respectively.Then,we consider the STP maximization in the general file size regime,which is an NP-hard problem.By exploring structural properties,we transform the orignal problem into a multi-choice knapsack problem and obtain a near optimal solution with 1/2approximation guarantee and polynomial complexity using a greedy method.We also obtain closed-form asymptotically optimal solutions.The analysis and optimization results show the advantage of the coded caching design over the uncoded caching design,and reveal the impact of caching and SIC capabilities.