Power allocation for NOMA in D2D relay communications

Non-orthogonal multiple access (NOMA) is considered as one of the key technologies for the fifth generation (5G) wireless communications. The integration of NOMA and device-to-device (D2D) communications has recently attracted wide attention. In this paper, a relaying D2D communications assisted with cooperative relaying systems using NOMA (DRC-NOMA) is considered. We analyze the ergodic sum-rate for the proposed system and then derive the closed-form expressions. In addition, an optimal power allocation strategy maximizing the ergodic sum-rate is proposed based on these analysis results. Numerical results show the good agreement between the results of analysis and Monte Carlo method. The proposed DRC-NOMA has a great improvement of the ergodic sum-rate in the small regime of average channel gain of D2D pair.