An improved lower bound for device-to-device coded caching.

Device-to-device (D2D) coded caching is considered in which there is a server hosting a library of files and a set of users. Each user is equipped with a cache memory of equal size. The system operates in two phases: first, the placement phase, in which the users’ cache memory is filled with data from the file library, followed by the delivery phase, in which each user demands one file from the library. These demands are then satisfied via user-to-user multicast transmissions only, without any involvement from the central server. For such a system, we obtain a novel lower bound on the optimal D2D sum rate. Moreover, the best known achievable D2D sum rate is shown to be within a constant multiplicative factor of 3.17 of our lower bound, improving upon the previously best known lower bound. Hence, the result in this paper further closes the gap between the best known achievable sum rate and the optimal sum rate for D2D coded caching.