Abstract
In this article, we investigate the performance of spectrum sharing inunderlay cognitive radio (CR)-inspired nonorthogonal multiple access (NOMA) systems, at which a primary transmitter with a weak channel shares its spectrum with a secondary transmitter (ST) with a strong channel to promote the network spectrum efficiency. In this article, we derive closed-form expressions of the outage probabilities (OPs), in terms of elementary functions, and the ergodic capacities (ECs) of the two transmissions, in terms of exponential integral function, assuming imperfect successive interference cancellation (SIC) and cochannel interference. Moreover, the feasible range for the power-allocation factor to achieve a NOMA instantaneous data-rate gain for both nodes is investigated. The results reveal the degradation effects of imperfect SIC on the OP and EC of the ST. Additionally, the simulation shows a better sum EC of the proposed NOMA scheme compared with two reference schemes, namely the orthogonal multiple access and the underlay CR with interference temperature constraint scheme. Finally, the accuracy of the analytical results is verified with representative numerical simulations.