The coded caching scheme originally proposed by Maddah-Ali and Niesen (MAN) transmits coded multicast messages from a server to users equipped with caches via a capacitated shared-link and was shown to be information theoretically optimal within a constant multiplicative factor. This work extends the MAN scheme to a class of two-hop wired-wireless networks including one server connected via fronthaul links to a layer of H helper nodes (access points/base stations), which in turn communicate via a wireless access network to K users, each equipped with its own cache. Two variants are considered, which differ in the modeling of the access segment. Both models should be regarded as abstractions at the “network layer” for physical scenarios such as local area networks and cellular networks, spatially distributed over a certain coverage area. The key of our approach consists of routing MAN-type multicast messages through the network and formulating the optimal routing scheme as an optimization problem that can be solved exactly or for which we give powerful heuristic algorithms. Our approach addresses at once many of the open practical problems identified as stumbling blocks for the application of coded caching in practical scenarios, namely: asynchronous streaming sessions, finite file size, scalability of the scheme to large and spatially distributed networks, user mobility and random activity (users joining and leaving the system at arbitrary times), decentralized prefetching of the cache contents, end-to-end encryption of HTTPS requests, which renders the helper nodes oblivious of the users' demands.

Coded caching is an information theoretic scheme to reduce high peak hours traffic by partially prefetching files in the users local storage during low peak hours. This paper considers heterogeneous decentralized caching systems where users' caches and content library files may have distinct sizes. The server communicates with the users through a Gaussian broadcast channel. The main contribution of this paper is a novel joint coded caching and modulation strategy to map the multicast messages generated in the coded caching delivery phase to the symbols of a signal constellation, such that users can leverage their cached content to demodulate the desired symbols with higher reliability and for the sake of simplicity, in this paper we focus only on “uncoded” modulation and symbol-by-symbol error probability. However, our scheme in conjunction with multilevel coded modulation can be extended to channel coding over a larger block lengths.

The coded caching scheme proposed by Maddah-Ali and Niesen (MAN) critically hinges on the ability of the system to deliver a common coded multicast message from a server to all users in the system at a fixed rate, independent of the number of users. In order to apply this paradigm to a spatially distributed wireless network, it is important to make sure that such a common multicast rate does not vanish, as the number of users in the network and/or the network area increase. This paper starts from a variant of the MAN scheme successively proposed for the so-called combination network, where the multicast message is further encoded by a maximum distance separable (MDS) code, and the MDS-coded blocks are sent to multiple spatially distributed single-antenna edge nodes (ENs), transmitting at a fixed rate with no channel state information. The users have multiple antennas. They obtain receiver channel state information from the standard downlink pilots and can select to decode a desired number of EN transmissions while either nulling or treating as noise the others. The system is reminiscent of the so-called evolved Multimedia Broadcast Multicast Service, since the fundamental underlying transmission mechanism is multipoint multicasting, where each user can independently (in a user-centric manner) decide which EN to decode, without any explicit association of users with ENs. We study the performance of the proposed system when users and ENs are distributed according to homogeneous Poisson point processes in the plane, and the propagation is affected by Rayleigh fading and distance-dependent pathloss. Our analysis allows the optimization of the PHY parameters (PHY coding rate at the ENs and MDS coding rate) for given MAN scheme parameters. The proposed scheme achieves full spatial scalability in the following sense: for an extended network with arbitrary constant ratio of users per EN and area A = O(NE), where NE denotes the number of ENs, the system achieves a per-user delivery rate that does not vanish as NE→ ∞.