  Ultra network densification and Massive MIMO are considered major 5G enablers
since they promise huge capacity gains by exploiting proximity, spectral and
spatial reuse benefits. Both approaches rely on increasing the number of access
elements per user, either through deploying more access nodes over an area or
increasing the number of antenna elements per access node. At the
network-level, optimal user-association for a densely and randomly deployed
network of Massive MIMO empowered access nodes must account for both channel
and load conditions. In this paper we formulate this complex problem, report
its computationally intractability and reformulate it to a plausible form,
amenable to acquire a global optimal solution with reasonable complexity. We
apply the proposed optimization model to typical ultra-dense outdoor small-cell
setups and demonstrate: (i) the significant impact of optimal user-association
to the achieved rate levels compared to a baseline strategy, and (ii) the
optimality of alternative network access element deployment strategies.
