Hybrid-Grant Random Access in Massive MIMO Systems

The future wireless network introduces lots of new communication paradigms, such as Internet of Things (IoT) which employs millions of devices to make the everyday objects connected and smart. With limited number of pilots, the access of the great amount of devices becomes a key issue. Considering the short-delay requirement in IoT, the grant-free random access (RA) which minimizes the access delay begins to attracts attentions. Compared with the legacy grant-based RA, the grant-free RA has a timesaving access procedure but also more severe data transmission interference. In this paper, we modify the grantfree RA by inserting a base station (BS) broadcasting message after the user equipment (UE) transmitting pilot. This message contains the identifications of all non-colliding UEs, and UEs can determine whether it has colliders by resolving the message. Only non-colliding UEs can transmit data in the following time while colliding UEs keep silent. This BS broadcasting message is also used in the grant-based RA, so we call the proposed new RA scheme as hybrid-grant RA. In this paper, we investigate the hybrid-RA in massive multiple-input multiple-output (MIMO) systems. After taking into account the extra time consumed by the BS broadcasting, we obtain a tight closed-form approximation of the spectral efficiency using hybrid-grant RA, and the spectral efficiency using grant-free RA is also given for comparison. In particular, we find that our proposed hybrid-grant RA have a significant gain on the spectral efficiency, and this gain grows rapidly as UE number goes up, which means the hybrid-grant RA is suitable for the future network with massive quantities of UEs. Moreover, we also give the optimal pilot length in the hybrid-RA which can be used as references for practical configuration of this scheme.