A Random Access Protocol for Pilot Allocation in Crowded Massive MIMO Systems

In this paper, we revisit the random access problem in the Massive MIMO context and develop a re engineered protocol, termed strongest-user collision resolution (SUCRe). The Massive MIMO (multiple-input multiple output) technology has a 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 Equipment's (UE's). These gains are only achieved in practice if many more UE's can connect efficiently to the network than today.

As the number of UE's increases, while each UE intermittently accesses the network, the random access functionality becomes essential to share the limited number of pilots among the UE's.  An accessing UE asks for a dedicated pilot by sending an uncoordinated random access pilot, with a risk that other UE's 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 UE's efficiently in overloaded networks.