  Cloud-RAN is a recent architecture for mobile networks where the processing
units are located in distant data centers while, until now, they were attached
to antennas. The main challenge, to fulfill protocol constraints, is to
guarantee low latency for the periodic messages sent from each antenna to its
processing unit and back. The problem we address is to find a periodic sending
scheme of these messages \emph{without contention nor buffering}, when all
messages are of the same size and the period is fixed.
  We study the periodic message assignment problem modeling this situation on a
common topology, where contention arises from a single link shared by all
antennas. The problem is reminiscent of coupled-task scheduling, but the
periodicity introduces a new twist. We study how the problem behaves with
regard to the \emph{load} of the shared link. The main contributions are
polynomial-time algorithms which \emph{always} find a solution for an arbitrary
size of messages and load at most $2/5$ or for messages of size one and load at
most $\phi - 1$, the golden ratio conjugate. We also prove that a randomized
greedy algorithm finds a solution on almost all instances with high
probability, explaining why most greedy algorithms work so well in practice.
