Analysis and Optimization of Caching and Multicasting for Multi-Quality Videos in Large-Scale Wireless Networks
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Efficient dissemination of videos is an important problem for mobile telecom carriers. In this paper, 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, in a large-scale wireless network. First, for each type of videos, we propose a random caching and multicasting scheme, carefully reflecting the relationships between layers of an SVC-based video or descriptions of a DASH-based video. Then, for each type of videos, we derive tractable expressions for the successful transmission probability in the general and high user density regions respectively, utilizing tools from stochastic geometry. The analytical results reveal that in the high user density region, the marginal increase of the successful transmission probability with respect to the caching probability of a video with certain quality reduces when the caching probability increases. Next, for each type of videos, we consider the maximization of the successful transmission probability 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. The optimization results reveal the impact of the caching gain of a layer for an SVC-based video or a description for a DASH-based video on its caching probability. Finally, we show that the proposed solution for each type of videos achieves a significant performance gain over baseline schemes and SVC is preferable when the popularity distribution over quality levels is flat or the layered encoding overhead is small, by numerical simulations.
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