Grant-free random access in pilot-reused multiceli massive MIMO systems with backoff mechanisms

Grant-free random access (RA) is attractive for future network due to the minimized access delay. In this paper, we investigate the grantfree RA in multicell massive multiple-input multiple-output (MIMO) systems with pilot reuse. With backoff mechanism, user equipments (UEs) in each cell are randomly activated, and active UEs randomly select orthogonal pilots from a predefined pilot pool, which results in a random pilot contamination among cells. With the help of indicators that capture the uncertainties of UE activation and pilot selection, we derive a closed-form approximation of the spectral efficiency per cell after averaging over the channel fading as well as UEs' random behaviors. Based on the analysis, the optimal backoff parameter and pilot length that maximize the spectral efficiency can be obtained. We find that the backoff mechanism is necessary for the system with large number of UEs, as it can bring significant gains on the spectral efficiency. Moreover, as UE number grows, more backoff time is needed.