Energy Harvesting in Overlay Cognitive NOMA Systems With Hardware Impairments

Combining energy harvesting with cognitive radio and nonorthogonal multiple access (NOMA) techniques will be a notable candidate for improving both energy and spectral efficiency of wireless systems. In this article, we appraise the performance of an overlay cognitive NOMA (OCNOMA) system by employing an energy-harvesting-based spectrum sharing cooperation (SSC) scheme in the presence of hardware impairments (HIs) at the transceiver nodes. Herein, an energy-constrained secondary transmitter node harvests energy from primary transmitter’s radio-frequency signal and utilizes this energy to relay primary information signal as well as to transmit its own information signal using NOMA technique. For this, we investigate two SSC schemes using amplify-and-forward (AF) and decode-and-forward (DF) relaying strategies, named as SSC-AF and SSC-DF schemes. Importantly, we consider the impact of the imperfect successive interference cancellation in NOMA and distortion noises in signal processing due to HIs, which are inevitable in practical systems. Adopting Nakagami- m fading environments, we derive the expressions of outage probability for primary and secondary networks under both SSC-AF and SSC-DF schemes and, thereby, disclose some pertinent ceiling effects in the system. Furthermore, we evaluate the system throughput and energy efficiency for the considered OCNOMA system. Our results manifest the advantages of the proposed SSC schemes over the benchmark direct primary transmission and orthogonal multiple access schemes.