  Non-orthogonal multiple access (NOMA) is one of the promising radio access
techniques for performance enhancement in next-generation cellular
communications. Compared to orthogonal frequency division multiple access
(OFDMA), which is a well-known high-capacity orthogonal multiple access (OMA)
technique, NOMA offers a set of desirable benefits, including greater spectrum
efficiency. There are different types of NOMA techniques, including
power-domain and code-domain. This paper primarily focuses on power-domain NOMA
that utilizes superposition coding (SC) at the transmitter and successive
interference cancellation (SIC) at the receiver. Various researchers have
demonstrated that NOMA can be used effectively to meet both network-level and
user-experienced data rate requirements of fifth-generation (5G) technologies.
From that perspective, this paper comprehensively surveys the recent progress
of NOMA in 5G systems, reviewing the state-of-the-art capacity analysis, power
allocation strategies, user fairness, and user-pairing schemes in NOMA. In
addition, this paper discusses how NOMA performs when it is integrated with
various proven wireless communications techniques, such as cooperative
communications, multiple input multiple output (MIMO), beamforming, space time
coding, and network coding, among others. Furthermore, this paper discusses
several important issues on NOMA implementation and provides some avenues for
future research.
