Resource Allocation for NOMA-Based D2D Systems Coexisting With Cellular Networks

This paper describes a nonorthogonal multiple access (NOMA)-based device-to-device (D2D) communication system underlaying in a cellular network sharing time and frequency resource. To optimize the total transmit power for all users in the network, we investigate a comprehensive resource allocation solution consisting of joint subchannel assignments, user pairings, and power control. The optimization problem is a mixed integer linear program, which is solved in three low-complexity steps. We first group D2D users to minimize mutual interference from cellular users (CU). Second, we demonstrate a heuristic solution for coupling D2D users in each group. Finally, we minimize the total transmit power of all devices in the network while maintaining the SINR requirements for both CU and D2D users. The optimal solution is derived off-line using the Rayleigh fading channels for any given QoS level of CU and NOMA-D2D users. We recognize that both subchannel assignment and user pairings play the important roles in reducing the total transmit power of the network, in which both cellular and D2D users coexist. However, optimization of the subchannel assignment contributes more toward minimizing the total network transmit power than the proposed model of optimizing user pairings.