A Low-Complexity Approach for Sum-Rate Maximization in Cooperative NOMA Enhanced Cellular Networks

This paper investigates the performance of cooperative non-orthogonal multiple access (C-NOMA) in a cellular downlink system. The system model consists of a base station (BS) serving multiple users, where users that have the capability of full-duplex (FD) communications can assist the transmissions between the BS and users with poor channel quality through device-to-device (D2D) communications. To maximize the achievable sum rate of the whole system while guaranteeing a certain quality of service (QoS) for all users, we formulate and solve a novel optimization problem that jointly determines the optimal D2D user pairing and the optimal power control scheme. The formulated problem is a mixed-integer non-linear program (MINLP), which has extremely high complexity. To overcome this issue, a two-step policy is proposed to solve the problem in polynomial time. First, we derive a closed-form expression of the optimal power control scheme that maximizes the sum rate of a given pair of users with a required QoS. Then, using the derived closed-form in the first step, we employ the Hungarian algorithm as the pairing policy in multi-user settings. Our simulation results show that the proposed scheme prevails some previously proposed heuristic approach for the given problem.