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This paper investigates the performance of cooperative non-orthogonalmultiple access (C-NOMA) in a cellular downlink system. The system modelconsists of a base station (BS) serving multiple users, where users with goodchannel quality can assist the transmissions between the BS and users with poorchannel quality through either half-duplex (HD) or full-duplex (FD)device-to-device (D2D) communications. We formulate and solve a noveloptimization problem that jointly determines the optimal D2D user pairing andthe optimal power control scheme, where the objective is maximizing theachievable sum rate of the whole system while guaranteeing a certain quality ofservice (QoS) for all users. The formulated problem is a mixed-integernon-linear program (MINLP) which is generally NPhard. To overcome this issue,we reconstruct the original problem into a bi-level optimization problem thatcan be decomposed into two sub-problems to be solved independently. The outerproblem is a linear assignment problem which can be efficiently handled by thewell-known Hungarian method. The inner problem is still a non-convexoptimization problem for which finding the optimal solution is challenging.However, we derive the optimal power control policies for both the HD and theFD schemes in closedform expressions, which makes the computational complexityof the inner problems polynomial for every possible pairing configurations.These findings solve ultimately the original MILNP in a timely manner thatmakes it suitable for real-time and low latency applications. Our simulationresults show that the proposed framework outperforms a variety of proposedschemes in the literature and that it can obtain the optimal pairing and powercontrol policies for a network with 100 users in a negligible computationaltime.

A Low-Complexity Framework for Joint User Pairing and Power Control for Cooperative NOMA in 5G and Beyond Cellular Networks