On the Exact Rate-Memory Trade-off for Multi-access Coded Caching with Uncoded Placement

Abstract

We study a cache-aided content delivery network consisting of a central server which hosts a catalog of N files, and K caches each with limited memory M which store content related to the files. There are K users, each of which requests a file from the catalog, and has access to the data stored in L ≥ 1 neighboring caches (with a cyclic wrap-around). The server transmits a common message to all the users, so that each of them can recover their requested file. This setup was recently studied in [1], where a coloring-based placement and coded-delivery policy was proposed and the required server transmission size was shown to be order-optimal with respect to information-theoretic bounds. We propose an alternate index coding-based placement and delivery scheme for this setup, which performs better than the previously proposed strategy. Furthermore, for multiple special cases including the (N, K ≤ 4, L) and (N, K, L = K - 1)setups, we show that the scheme is exactly optimal under the restriction of uncoded placement. This extends other recent work [2], [3] which studies exact optimality for the single cache-access case (L = 1) to the multi cache-access case (L > 1).