On the Performance of NOMA in the Two-User SWIPT System

Abstract

In this paper, we investigate nonorthogonal multiple access (NOMA) for a simultaneous wireless information and power transfer (SWIPT) system consisting of a battery powered access point and two energy harvesting users. In SWIPT, there exists an information-energy tradeoff. Since the adoption of NOMA may cause more energy consumption than the orthogonal multiple access (OMA), it is not known whether NOMA can always enhance the spectral efficiency in SWIPT systems compared to OMA. We prove that NOMA performs better than OMA when the decoding energy consumption is negligible. However, for the nonnegligible decoding energy consumption, we show that NOMA is not always superior to OMA. Interestingly enough, for the nonnegligible decoding energy consumption, OMA can outperform NOMA when the channel power gains of the two users are not sufficiently different. Moreover, we study the performance of cognitive radio inspired NOMA (CR-NOMA), in which the power is allocated to the user with poor channel condition such that its quality of service (QoS) requirement is met. Different from the battery powered CR-NOMA, the analytical results show that CR-NOMA can outperform OMA in SWIPT systems only if the channel power gains of users are sufficiently different and the QoS of the weak user is above a threshold.