Caching in Combination Networks: A Novel Delivery by Leveraging the Network Topology

THIS PAPER IS ELIGIBLE FOR THE STUDENT PAPER AWARD. Maddah-Ali and Niesen (MAN) in 2014 surprisingly showed that it is possible to serve an arbitrarily large number of cache-equipped users with a constant number of transmissions by using coded caching in shared-link broadcast networks. This paper studies the tradeoff between the user’s cache size and the file download time for combination networks, where users with caches communicate with the servers through intermediate relays. Motivated by the so-called separation approach, it is assumed that placement and multicast message generation are done according to the MAN original scheme and regardless of the network topology. The main contribution of this paper is the design of a novel two-phase delivery scheme that, accounting to the network topology, outperforms schemes available in the literature. The key idea is to create additional (compared to MAN) multicasting opportunities: in the first phase coded messages are sent with the goal of increasing the amount of ‘side information’ at the users, which is then leveraged during the second phase. The download time with the novel scheme is shown to be proportional to 1/H (with H being the number or relays) and to be order optimal under the constraint of uncoded placement for some parameter regimes.