Fundamental Limits of Cache-Aided Interference Channels

We consider a system, comprising a library of files (e.g., movies) and a wireless network with an arbitrary number of transmitters and receivers, where each node is equipped with a local cache memory. The system operates in two phases, the prefetching phase, where each cache is pre-populated from the contents of the library, up to its limited size, and then the delivery phase, where each receiver reveals its request for a file from the library, and the system needs to deliver the requested files. The objective is to design the cache placement and the communication scheme to maximize the rate of delivery for arbitrary set of requested files. We characterize the sum degrees-of-freedom (sum-DoF) of this network to within a factor of 2 for all system parameters, under one-shot linear schemes. In particular, we show that the linear sum-DoF scales linearly with the aggregate cache size in the network (i.e., the cumulative memory available at all nodes). The proposed achievable scheme exploits the redundancy of the content at transmitters' caches to cooperatively zero-force some outgoing interference, and availability of the unintended content at the receivers' caches to cancel (subtract) some of the incoming interference. The outer bound is derived by an optimization argument which bounds the number of communication blocks needed to deliver any requested contents to the receivers. This result demonstrates that in this setting, caches at the transmitters' side are equally valuable as the caches at the receivers' side. In addition, it shows that caching can offer a throughput gain that scales linearly with the size of the network.