A Hybrid-Grant Random Access Scheme in Massive MIMO Systems for IoT

The grant-free random access (RA) can minimize the access delay but also brings severe data transmission interferences. To overcome this defect, we propose a new RA scheme that inserts a base station (BS) broadcasting message after user equipments (UEs) transmitting pilots. In this scheme, UEs can determine whether they have colliders by resolving the broadcasting message, and only non-colliding UEs can transmit data in the following step while colliding UEs keep silent. By doing this, the data interferences from colliding UEs are eliminated without costing much extra time. Since this BS broadcasting message is also used in the legacy grant-based RA, we call the new RA scheme as a hybrid-grant RA. We investigate the hybrid-grant RA in massive multiple-input multiple-output (MIMO) systems and obtain a tight closed-form approximation of the spectral efficiency with maximum-ratio-combing (MRC) and zero-forcing (ZF) receivers, respectively. Via simulation, we find that our proposed hybrid-grant RA can obtain a significant gain on the spectral efficiency compared with grant-free RA, especially for ZF receivers. In particular, this gain grows rapidly as the UE number goes up, which means the hybrid-grant RA is more suitable for the system with large amount of UEs and it is a typical scenario in future communications networks. Moreover, we also analyze the optimal pilot length and UE activation probability that maximize the spectral efficiency, which can be used as references for the practical application of the proposed hybrid-grant RA.