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Grant-free random access (RA) with massive MIMO is a promising RA techniquefor massive access with low signaling overhead. In the grant-free RA withmassive MIMO, preamble length has a critical impact on the performance of thesystem. In this paper, the optimal preamble length is investigated to maximizespectral efficiency (SE) of the grant-free RA with massive MIMO, where effectsof the preamble length on the preamble collision and preamble overhead as wellas channel estimation accuracy are taken into account. Simulation results agreewell with our analyses and confirm the existence of optimal preamble length forSE maximization in the grant-free RA with massive MIMO. Moreover, properties ofthe optimal preamble length with respect to system parameters are revealed.Compared to the granted access, it is shown that longer preamble length isrequired for SE maximization in the grant-free RA.

Optimal Preamble Length for Spectral Efficiency in Grant-Free RA with Massive MIMO