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In device-to-device (D2D) coded caching problems, it is possible that not allusers will make file requests in the delivery phase. Hence, we propose a newD2D centralized coded caching problem, named the 3-user D2D coded caching withtwo random requesters and one sender (2RR1S), where in the delivery phase, anytwo of the three users will make file requests, and the user that does not makeany file request is the designated sender. We find the optimal caching anddelivery scheme, denoted as the 2RRIS scheme, for any number of files N byproving matching converse and achievability results. It is shown that codedcache placement is needed to achieve the optimal performance. Furthermore, theoptimal rate-memory tradeoff has a uniform expression for N>=4 and differentexpressions for N=2 and 3.  To examine the usefulness of the proposed model and scheme, we adapt the2RR1S scheme to three scenarios. The first one is the 3-user D2D coded cachingmodel proposed by Ji et al. By characterizing the optimal rate-memory tradeofffor the 3-user D2D coded caching when N=2, which was previously unknown, weshow that the adapted 2RR1S scheme is in fact optimal for the 3-user D2D codedcaching problem when N=2 and the cache size is medium. The benefit comes fromcoded cache placement which is missing from existing D2D coded caching schemes.The second scenario is where in the delivery phase, each user makes a filerequest randomly and independently with the same probability p. We call thismodel the request-random D2D coded caching problem. Adapting the 2RR1S schemeto this scenario, we show the superiority of our adapted scheme over otherexisting D2D coded caching schemes for medium to large cache size. The thirdscenario is the K-user D2D coded caching with K-s random requesters and ssenders problem, for which an achievability result is obtained by generalizingthe 2RR1S scheme.

Three-user D2D Coded Caching with Two Random Requesters and One Sender