Caching Policies for D2D-Assisted Content Delivery Systems

We consider a content delivery system consisting of a central server and multiple end-users. The central server stores the entire catalog of contents on offer and can deliver the requested content to the end-users. In addition, the end-users are equipped with limited caching capabilities and have the ability to deliver content to each other via D2D communication. The system also allows a third mode of content delivery where the central server delivers content to some of the end-users who then relay it to the other users. Our goal is to determine which contents to cache at the end-users in order to minimize the cost of service. We characterize the optimal caching policy and evaluate the benefits of allowing the central server to use other end-users as relays to deliver content. The key takeaway from this work is that if end-users have caching capabilities, the benefits of the central server using end-users as relays is negligible. This is in contrast to the case where the end-users cannot cache content where using end-users as relays leads to significant improvement in system performance.