Hierarchical video caching in wireless cloud: Approaches and algorithms

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We had introduced video caching techniques in the Radio Access Network (RAN) in [1] as a way to reduce the need to bring requested videos from Internet CDNs, thus reducing overall backhaul traffic, improving video quality of experience and increasing network capacity to support more simultaneous video requests. In this paper, we investigate supplementing the resulting wireless cloud with a hierarchical caching scheme, where the gateways in the Core Network (CN) also have video caches. The hierarchical caching approach further improves network capacity by enabling multiple cell sites to share caches at higher levels of the hierarchy, thereby improving overall cache hit ratio, without increasing the total cache size used. In addition, we exploit hierarchical caching to better accommodate mobility, so that when a user with an active video session moves from one cell to a neighboring cell, it is likely that the video currently being downloaded is already in a cache within the RAN or CN network associated with the new cell. To achieve the goal of improving capacity and supporting mobility, we extend our User Preference Profile (UPP) based caching policies [1] to accommodate the hierarchical caching structure introduced in this paper. For all the videos that miss the cache in any layer of hierarchy, we propose a scheduling approach to allocate RAN and CN backhaul resources judiciously so as to maximize the capacity of the wireless network. We extend our discrete event statistical simulation framework developed in [1] to study the performance of the proposed hierarchical caching approach. Our simulation results show that using hierarchical caching can enhance cache hit ratio by 24% and network capacity by up to 45% compared to caching only in the RAN. Significant capacity gains are also observed when additionally considering user mobility.