This paper considers a two-hop network architecture known as a combination network, where a layer of relay nodes connects a server to a set of end users. In particular, a new model is investigated where the intermediate relays employ caches in addition to the end users. First, a new centralized coded caching scheme is developed that utilizes maximum distance separable (MDS) coding, jointly optimizes cache placement and delivery phase, and enables decomposing the combination network into a set virtual multicast sub-networks. It is shown that if the sum of the memory of an end user and its connected relay nodes is sufficient to store the database, then the server can disengage in the delivery phase and all the end users' requests can be satisfied by the caches in the network. Lower bounds on the normalized delivery load using genie-aided cut-set arguments are presented along with second hop optimality. Next recognizing the information security concerns of coded caching, this new model is studied under three different secrecy settings: 1) secure delivery where we require an external entity must not gain any information about the database files by observing the transmitted signals over the network links, 2) secure caching, where we impose the constraint that end users must not be able to obtain any information about files that they did not request, and 3) both secure delivery and secure caching, simultaneously. We demonstrate how network topology affects the system performance under these secrecy requirements. Finally, we provide numerical results demonstrating the system performance in each of the settings considered.
Index TermsCombination networks with caching relays, coded caching, maximum distance separable (MDS) codes, secure delivery, secure caching.Caching is foreseen as a promising avenue to provide content based delivery services for 5G systems and beyond [1], [2]. Caching enables shifting the network load from peak to off-peak hours leading to a significant improvement in overall network performance. During off-peak hours, in the cache placement phase, the network is likely to have a considerable amount of under-utilized wireless bandwidth which is exploited to place functions of data contents in the cache memories of the network nodes. This phase takes place prior to the end users' content requests, and thus content needs to be placed in the caches without knowing what specific content each user will request. The cached contents help reduce the required transmission load when the end users actually request the contents, during peak traffic time, known as the delivery phase, not only by alleviating the need to download the entire requested data, but also by facilitating multicast transmissions that benefit multiple end users [3]. As long as the storage capabilities increase, the required transmission load during peak traffic can be decreased, leading to the rate-memory trade-off [3], [4]. Various network topologies with caching capabilities have been investigated to date, see for example [5]-[13]. Referenc...
