  We present a novel access protocol for crowd scenarios in massive MIMO
(Multiple-input multiple-output) systems. Crowd scenarios are characterized by
a large number of users with intermittent access behavior, whereby orthogonal
scheduling is infeasible. In such scenarios, random access is a natural choice.
The proposed access protocol relies on two essential properties of a massive
MIMO system, namely asymptotic orthogonality between user channels and
asymptotic invariance of channel powers. Signal processing techniques that take
advantage of these properties allow us to view a set of contaminated pilot
signals as a graph code on which iterative belief propagation can be performed.
This makes it possible to decontaminate pilot signals and increase the
throughput of the system. Numerical evaluations show that the proposed access
protocol increases the throughput with 36%, when having 400 antennas at the
base station, compared to the conventional method of slotted ALOHA. With 1024
antennas, the throughput is increased by 85%.
