Device-to-device (D2D) communication is a promising approach to optimize theutilization of air interface resources in 5G networks, since it allowsdecentralized opportunistic short-range communication. For D2D to be useful,mobile nodes must possess content that other mobiles want. Thus, intelligentcaching techniques are essential for D2D. In this paper we use results fromstochastic geometry to derive the probability of successful content delivery inthe presence of interference and noise. We employ a general transmissionstrategy where multiple files are cached at the users and different files canbe transmitted simultaneously throughout the network. We then formulate anoptimization problem, and find the caching distribution that maximizes thedensity of successful receptions (DSR) under a simple transmission strategywhere a single file is transmitted at a time throughout the network. We modelfile requests by a Zipf distribution with exponent $\gamma_r$, which results inan optimal caching distribution that is also a Zipf distribution with exponent$\gamma_c$, which is related to $\gamma_r$ through a simple expressioninvolving the path loss exponent. We solve the optimal content placementproblem for more general demand profiles under Rayleigh, Ricean and Nakagamismall-scale fading distributions. Our results suggest that it is required toflatten the request distribution to optimize the caching performance. We alsodevelop strategies to optimize content caching for the more general case withmultiple files, and bound the DSR for that scenario.
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