ACK Feedback based UE-to-CTU Mapping Rule for SCMA Uplink Grant-Free Transmission

Massive machine type communication (mMTC) is one of the three significant scenarios in 5G, which is characterized by massive connection and low energy consumption. To meet these requirements, the contention based grant-free sparse code multiple access (SCMA) transmission which can support a large number of devices and reduce the cost of signaling overhead caused by massive connection is proposed. It allows user equipment (UE) to transmit in the preconfigured radio resources shared by multiple UEs, which is called contention transmission unit (CTU). Since several UEs can be mapped to the same CTU in the transmission, collision may occur. In the conventional set-up, the UE is allocated to a CTU by the fixed mapping rule, which brings the problem of unfairness among the UEs and the probability that UE may collide again in retransmission. In this paper, we propose a new mapping rule, which utilizes the Acknowledgement (ACK) feedback to indicate the radio resource allocation. The theoretical deduction confirms the superiority of the proposed method. The simulation results show that collision probability is reduced by around 20%.