Fundamental Limits of Device-to-Device Private Caching with a Trusted Server under Uncoded Cache Placement and User Collusion

In the coded caching problem, as originally formulated by Maddah-Ali and Niesen, a server communicates via a noiseless broadcast link to multiple users that have local storage capability. In order for a user to decode the desired file from the coded multicast transmission, the demands of all the users must be globally known, which may violate the privacy of the users. To overcome this privacy problem, Wan and Caire recently proposed several schemes that attain coded multicasting gain while simultaneously guarantee information theoretic privacy of the users' demands. In Device-to-Device (D2D) networks, the demand privacy problem is further exacerbated by the fact that each user is also a transmitter, which should know the files demanded by the remaining users in order to form its coded multicast transmissions. This paper solves this seemingly infeasible problem with the aid of a trusted server. Specifically, during the delivery phase, the trusted server collects the users' demands and sends a query to each user, who then broadcasts coded multicast packets according to this query. The main contribution of this paper is the development of novel achievable schemes and converse bounds for D2D private caching with a trusted server, where users may be colluding (i.e., when some users share cached content and demanded file indices), that are to within a constant factor of one another.