Finite length analysis of caching-aided coded multicasting

In this work we study a noiseless broadcast link serving K users whose requests arise from a library of N files. Every user is equipped with a cache of size M files each. It has been shown that by splitting all the files into packets and placing individual packets in a random independent manner across all the caches, it requires at most N/M file transmissions for any set of demands from the library. The achievable delivery scheme involves linearly combining packets of different files following a greedy clique cover solution to the underlying index coding problem. This remarkable multiplicative gain of decentralized random caching has been established in the asymptotic regime when the number of packets per file F, scales to infinity. In this work, we initiate the finite-length analysis of random caching schemes when the number of packets F is a function of the system parameters M, N, K. Specifically, we show that existing random placement and greedy clique cover delivery schemes, while providing large gains in the asymptotic regime, can have at most a multiplicative gain of 2 if the number of packets is sub-exponential. Further, for any clique cover based coded delivery and a large class of random caching schemes, that includes the existing ones, we show that the number of packets required to get a multiplicative gain of 2g is at least O((N/M)g). We exhibit a caching and an efficient clique cover based coded delivery scheme that approximately achieves this lower bound. We also provide tight concentration results that show that the average (over the random caching involved) number of transmissions concentrates very well with only polynomial number of packets in the rest of the parameters.