Contents I. Introduction
Non-orthogonal multiple access (NOMA) is a promising technique for the upcoming fifth generation (5G) wireless communications, and it has attracted an increased research interests in recent years. Enhanced latency, spectral efficiency and connectivity are the main factors that stimulated the emergence of NOMA systems, in which multiple users are allowed to share the same time and frequency resources [1]. The key point of NOMA systems is to permit a constrained level of interference from other users that allows the receiver to perform successive interference cancellation (SIC) for the other users' signals before detecting its own signal. NOMA systems rely on exploiting the power domain multiplexing to control interference and maintain user fairness, in a way that grants the far users higher power coefficients and assign low power coefficients to near users [2]. Although NOMA technique enhances users' fairness, in comparison with the conventional systems such as orthogonal multiple access (OMA) systems, quality of service (QoS) of far users is relatively low, which is considered as a performance limiting factor in many scenarios due to error propagation.
