The Massive MIMO (multiple-input multiple-output) technology has great
potential to manage the rapid growth of wireless data traffic. Massive MIMO
achieves tremendous spectral efficiency by spatial multiplexing of many tens of
user equipments (UEs). These gains are only achieved in practice if many more
UEs can connect efficiently to the network than today. As the number of UEs
increases, while each UE intermittently accesses the network, the random access
functionality becomes essential to share the limited number of pilots among the
UEs. In this paper, we revisit the random access problem in the Massive MIMO
context and develop a reengineered protocol, termed strongest-user collision
resolution (SUCRe). An accessing UE asks for a dedicated pilot by sending an
uncoordinated random access pilot, with a risk that other UEs send the same
pilot. The favorable propagation of Massive MIMO channels is utilized to enable
distributed collision detection at each UE, thereby determining the strength of
the contenders' signals and deciding to repeat the pilot if the UE judges that
its signal at the receiver is the strongest. The SUCRe protocol resolves the
vast majority of all pilot collisions in crowded urban scenarios and continues
to admit UEs efficiently in overloaded networks.Comment: To appear in IEEE Transactions on Wireless Communications, 16 pages,
10 figures. This is reproducible research with simulation code available at
https://github.com/emilbjornson/sucre-protoco