Energy efficiency maximization for cooperative NOMA with hardware impairments

The massive connectivity and limited energy pose significant challenges to deploy the enormous devices in energy-efficient and environmentally friendly in the Internet of Things (IoT). Motivated by these challenges, this paper investigates the energy efficiency (EE) maximization problem for downlink cooperative non-orthogonal multiple access (C-NOMA) systems with hardware impairments (HIs). The base station (BS) communicates with several users via a half-duplex (HD) amplified-and-forward (AF) relay. First, we formulate the EE maximization problem of the system under HIs by jointly optimizing transmit power and power allocated coefficient (PAC) at BS, and transmit power at the relay. The original EE maximization problem is a non-convex problem, which is challenging to give the optimal solution directly. First, we use fractional programming to convert the EE maximization problem as a series of subtraction form subproblems. Then, variable substitution and block coordinate descent (BCD) method are used to handle the sub-problems. Next, a resource allocation algorithm is proposed to maximize the EE of the systems. Finally, simulation results show that the proposed algorithm outperforms the downlink cooperative orthogonal multiple access (C-OMA) scheme.