I. Introduction
With the rapid development of the Internet of Things, future communication systems are required to support massive connectivity and achieve high spectral efficiency [1]. To meet these requirements, non-orthogonal multiple access (NOMA), as a promising multi-access technology, has gradually attracted widespread attention [1]–[22]. Compared to traditional orthogonal multiple access (OMA), NOMA allows allocating one frequency channel to multiple users at the same time, providing higher spectral efficiency, higher system throughput, and better user fairness [1]–[5].