Comparison of preamble structures for grant-free random access in massive MIMO systems

Massive MIMO based grant-free random access (mGFRA) is a viable solution for massive access with low signaling overhead. In this letter, the performance of two preamble structures, namely concatenated orthogonal preamble (COP) and single orthogonal preamble (SOP), are compared for mGFRA in terms of the spectral efficiency (SE). To make a trackable comparison, closed-form expressions for the SEs with COP and SOP are first derived. Then, the SE behaviors of the two preamble structures with respect to the number of antennas M in massive MIMO are studied and analyzed. Through analysis, it reveals that there always exists a threshold value of M that decides the superiority of a preamble structure to the other and the superiority gets larger as M grows. Simulation results verify the accuracy of our derivations and analysis.