Abstract-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.  From analog phone calls through to all Internet Protocol services, including voice and messaging, each transition has been encouraged by the need to meet the requirements of the new generation of mobile technology. Subsequently, mobile communications technology is presently facing a new challenge, giving birth to a hyper-connected society through the emergence of fifth-generation (5G) services. With enormous potential for both consumers and industry, 5G is expected to roll out by 2020. From the next-generation radio access technology viewpoint, a step change in data speed and a significant reduction in end-to-end latency is a major concern for 5G, since the rapid development of the mobile Internet and the Internet of Things (IoT) exponentially accelerates the demand for high data-rate applications. In particular, many of the industry initiatives that have progressed with work on 5G declare that the network-level data rate in 5G should be 10-20 Gbps (that is, 10-20 times the peak data rate in 4G), and the user-experienced data rate should be 1 Gbps (100 times the user-experienced data rate in 4G). They also set the latency (end-to-end round-trip delay) at 1 millisecond (one-fifth of the latency in 4G). The underlying physical connection in a cellular network is called radio access technology, which is implemented by a radio access network (RAN). A RAN basically utilizes a channel access technique to provide the mobile terminals with a connection to the core network. The design of a suitable multiple access technique is one of the most important aspects in improving the system capacity. Multiple access techniques can broadly be categorized into two different approaches [1], namely, orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA). An orthogonal scheme allows a perfect receiver to entirely separate unwanted signals from the desired signal using different basis functions. In other words, signals from different users are orthogonal to each other in orthogonal schemes. Time division multiple access (TDMA), and orthogonal frequency-division multiple access (OFDMA) are a couple of examples of OMA schemes. In TDMA, several users share the same frequency channel on a time-sharing basis. The users communicate in rapid succession, one after the other, each using their assigned time slots. OFDMA allows multi-user communications through an orthogonal frequency-division multiplexing (OFDM) technique in which subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other. In contrast to OMA, NOMA allows allocating one frequency channel to multiple users at the same time within the same cell and offers a number of advantages, including improved spectral efficiency (SE), higher cell-edge throughput, relaxed channel feedback (only the received signal strength, not exact channel state information (CSI), is required), and low transmission latency (no scheduling request from users to base station is required). The available NOMA techniques can broadly be divided into two categories, namely, power-domain and code-domain NOMA. This paper focuses on the power-domain NOMA that superposes multiple users in power domain and exploits the channel gain difference between multiplexed users. At the transmitter side, signals from various users are superposed and the resulting signal is then transmitted over the same channels (i.e., the same time-frequency resources). At the receiver sides, multiuser detection (MUD) algorithms, such as successive interference cancellation (SIC) are utilized to detect the desired signals. Although this paper primarily surveys the power-domain superposition coding (SC)-based NOMA, a brief discussion of other classes of NOMA is given in the next section. OMA is a realistic choice for achieving good performance in terms of system-level throughput. However, due to the aforementioned upcoming wave, 5G networks require further enhancement in system efficacy. In this regard, researchers all over the globe have started investigating NOMA as a promising multiple access scheme for future radio access. NOMA achieves superior spectral efficiencies by SC at the transmitter with SIC at the receiver [2, 3] . On the top of that, the evolution of wireless networks to 5G poses new challenges for energy efficiency (EE), since the entire network will be ultra-dense. With an extreme increase in the number of infrastructure nodes, total energy consumption may simply surpass an acceptable level. Although substantial energy is basically consumed by the hardware, NOMA has an inherent ability to adapt the transmission strategy according to the traffic and users' CSIs. Thus, it can achieve a good operating point, where both spectrum efficiency and EE become optimum. Over the past few years, NOMA has attracted a great deal of attention from researchers trying to meet 5G requirements. As a consequence, many research efforts in this field already exist. Research trends in NOMA include diverse topics, for example, various performance analysis methods, fairness analysis, EE, and user pairing. Many researchers are attempting to further enhance the performance of other existing wireless technologies, such as cooperative communications, multiple-input multiple-output (MIMO), light communications, and relay networks Fig. 1 . The organization and structure of this paper.
In the power domain NOMA, the non-orthogonality is introduced by allocating different power levels to the users REF .
Power-Domain Non-Orthogonal Multiple Access (NOMA) in 5G Systems: Potentials and Challenges
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