Packet Re-Management-Based C-NOMA for URLLC: Cooperation in the Second Phase of Relaying.

To realize the efficient transmission under ultra-reliable and low-latency communication (URLLC), cooperative non-orthogonal multiple access (C-NOMA), in which the two-phase cooperative transmission is considered and non-orthogonal multiple access (NOMA) is implemented during the cooperation, has gained much attention in recent years. When the cooperation is implemented in the first phase of relaying, the C-NOMA scheme has superb performance only when the relay (R) is close to the source, since NOMA gain comes from the channel quality difference. To expand the versatility of C-NOMA in the case where R is close to the destination (D), we propose a packet re-management-based C-NOMA transmission scheme, in which the cooperation is implemented in the second phase of relaying. Further, considering the importance of power consumption in 5G, we also propose an efficient algorithm to minimize the power consumption, subject to URLLC requirements. Specifically, to tackle the above non-convex and implicit problem, we first provide a tight upper bound approximation of channel dispersion to make the problem explicit. Then, on the basis of block coordinate descent and successive upper bound approximation frameworks, a recursion method is proposed to solve the joint optimization problem. Simulation results verify the effectiveness of the proposed scheme in the case where R is close to D.