Joint Time and Power Control Scheme for NOMA-enabled D2D Users with Energy Harvesting in IoT Environment

Energy efficiency in underlying wireless commu-nication systems is challenging. There are several device-to-device (D2D) groups in which an individual group contains one transmitter and two receivers laying in a cellular network. The downlink (DL) transmission helps the transmitter of the D2D group (DG) for energy harvesting (EH). Harvested and stored energy is used to transfer the information to the respective D2D receivers $(DRX)$ over the same spectrum, simultaneously. In this paper, the power allocation scheme is utilized to optimize the D2D transmitter $(DT_{X})$ power and to maximise the DG's energy-efficiency (EE). The user of poor channel conditions of a DG is affected by the strong channel conditions due to intra-user interference, which reduces the EE of the respective group. Therefore, a non-orthogonal multiple access (NOMA) technique is employed on the DG's transmitter to reduce intra-user interfer-ence. Numerical results demonstrate that the spectrum efficiency and EE of a network can be enhanced by maximising the EE and optimising the power of transmitters in DGs using successive convex approximation.