  Massive MIMO is a promising technology to enable a massive number of Internet
of Things nodes to transmit short and sporadic data bursts at low power. In
conventional cellular networks, devices use a grant-based random access scheme
to initiate communications. This scheme relies on a limited set of orthogonal
preambles, which simplify signal processing operations at network access
points. However, it is not well suited for Internet of Things (IoT) devices due
to: (i) the large protocol overhead, and (ii) the high probability of
collision. In contrast to the grant-based scheme, a grant-free approach uses
user-specific preambles and has a small overhead, at the expense of more
complexity at access points. In this work, a grant-free method is proposed,
applicable for both co-located and cell-free deployments. The method has a
closed form solution, which results in a significantly low complexity with
respect to the state-of-the art. The algorithm exploits the static nature of
IoT devices through the use of prior channel state information. With a power
budget of 1 mW, 64 antennas are sufficient to support 1000 nodes with 200
simultaneous access requests with a probability of false alarm and miss
detection below 10-6 and 10-4, respectively.
