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Massive MIMO systems, where base stations are equipped with hundreds ofantennas, are an attractive way to handle the rapid growth of data traffic. Asthe number of user equipments (UEs) increases, the initial access and handoverin contemporary networks will be flooded by user collisions. In this paper, arandom access protocol is proposed that resolves collisions and performs timingestimation by simply utilizing the large number of antennas envisioned inMassive MIMO networks. UEs entering the network perform spreading in both timeand frequency domains, and their timing offsets are estimated at the basestation in closed-form using a subspace decomposition approach. Thisinformation is used to compute channel estimates that are subsequently employedby the base station to communicate with the detected UEs. The favorablepropagation conditions of Massive MIMO suppress interference among UEs whereasthe inherent timing misalignments improve the detection capabilities of theprotocol. Numerical results are used to validate the performance of theproposed procedure in cellular networks under uncorrelated and correlatedfading channels. With $2.5\times10^3$ UEs that may simultaneously become activewith probability 1\% and a total of $16$ frequency-time codes (in a givenrandom access block), it turns out that, with $100$ antennas, the proposedprocedure successfully detects a given UE with probability 75\% while providingreliable timing estimates.

Random Access in Massive MIMO by Exploiting Timing Offsets and Excess Antennas