Communication-optimal coding designs for caching networks

Abstract

In this survey paper, we review three recent main results on cache networks, which not only considerably sharpen the approximate characterization of the rate-memory trade off, but also extend those results to more general networks. In these systems, a server with a database of some files (e.g. movies) is connected to multiple users via a communication network. Each user has an isolated memory of limited size that can be used for caching. The system operates in two phases: a placement phase where users each store a portion of the files in their local cache, and a delivery phase, where the users each request a file and the server delivers coded messages to the users, fulfilling their file requests. We start by considering the shared bottleneck network in two flavors of the system, with uncoded prefetching and with coded prefetching. First, for uncoded prefetching, an optimal design is proposed, under both centralized and decentralized settings, for both peak rate and average rate. The exact optimality is proven through a matching converse. Second, for caching with coded prefetching, we present a design that is optimal within a factor of approximately 2, which strictly improves the state of the art. Lastly, we move the focus to more general network topologies, and present an order-wise optimal scheme that is independent of the underlying communication network between the server and the users. This scheme is shown to achieve the minimum delivery delay with a constant factor for all memoryless networks.