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Aiming to minimize service delay, we propose a new random caching scheme in device-to-device (D2D) assisted heterogeneous network. To support diversified viewing qualities of multimedia video services, each video file is encoded into a base layer (BL) and multiple enhancement layers (ELs) by scalable video coding (SVC). A super layer, including the BL and several ELs, is transmitted to every user. We define and quantify the service delay of multiquality videos by deriving successful transmission probabilities when a user is served by a D2D helper, a small-cell base station (SBS), and a macrocell base station. We formulate a delay minimization problem subject to the limited cache sizes of D2D helpers and SBSs. The structure of the optimal solutions to the problem is revealed, and then an improved standard gradient projection method is designed to effectively obtain the solutions. Both theoretical analysis and Monte Carlo simulations validate the successful transmission probabilities. Compared with three benchmark caching policies, the proposed SVC-based random caching scheme is superior in terms of reducing the service delay.

Analysis and Optimization of Service Delay for Multiquality Videos in Multitier Heterogeneous Network With Random Caching