A new paradigm of wireless video delivery between users in the converged D2D network and cellular system is studied in this paper.\nD2D communication enables direct data transmissions of nearby user equipment (UE) by sharing the same time and frequency\nresource, thereby improving spectrum utilization, system throughput, and energy efficiency. Yet in the case that D2D network\ncannot provide the requested video files for the limited storage capacity of UE, users have to access the video files in the traditional\ncellular mode. This paper investigates a D2D-cellular converged network where users are scattered over the area of a single cell\naccording to the spatial Poisson Point Processes and D2D users are able to store part of video files requested by other users according\nto file popularity. First, we derive the energy efficiency of the D2D-cellular converged network. Then, to maximize the energy\nefficiency of the D2D-cellular converged network, the optimal collaborative distance of D2D communication and the number of\ncache files are addressed. Finally, via simulations, we show the effects of these system parameters on energy efficiency and point out\nthe existence of optimal solutions.
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