This paper considers a massive random access scenario where a small random set of k active users out of a larger number of n total potential users seek to transmit data to a base station. Specifically, we examine an approach in which the base station first determines the set of active users based on an uplink pilot phase, then broadcasts a common feedback message to all the active users for the scheduling of their subsequent data transmissions. Our main question is: What is the minimum amount of common feedback needed to schedule k users in k slots while completely avoiding collisions? Instead of a naive scheme of using k log(n) feedback bits, this paper presents upper and lower bounds to show that the minimum number of required common feedback bits scales linearly in k, plus an additive term that scales only as Θ(log log(n)). The achievability proof is based on a random coding argument. We further connect the problem of constructing a minimal length feedback code to that of finding a minimal set of complete k-partite subgraphs that form an edge covering of a k-uniform complete hypergraph with n vertices. Moreover, the problem is also equivalent to that of finding a minimal perfect hashing family, thus allowing leveraging the explicit perfect hashing code constructions for achieving collision-free massive random access.
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