Placement Delivery Array Design for Coded Caching Scheme in D2D Networks

Ji et al. (IEEE TRANSACTIONS ON INFORMATION THEORY, 62(2): 849-869, 2016) first studied coded caching in device-to-device (D2D) networks, and proposed a D2D coded caching scheme, which is referred to as the JCM scheme. In practice, we prefer to design a scheme with its two important targets, i.e., the rate (the maximal total amount of transmission) and packet number F, as small as possible. In this paper, we first propose a simple array called D2D placement delivery array (DPDA) to characterize the placement phase and the delivery phase in D2D networks. Consequently, some D2D coded caching schemes can be realized by an appropriate DPDA. Second, a lower bound on the rate of a DPDA is derived. And, we show that the JCM scheme achieves our lower bound. However, it is well known that its packet number F increases exponentially with the number of users K. So, we propose two classes of new schemes by constructing DPDAs. One reduces the packet number exponentially with K compared with the JCM scheme while keeping the rate near to our lower bound. The other further reduces F to increasing sub-exponentially with K.