NOMA and 5G emerging technologies: A survey on issues and solution techniques

Power Domain Non-Orthogonal Multiple Access (PD-NOMA) is a potential technology for the next generation of cellular networks. Compared to classical orthogonal multiple access (OMA) techniques, PD-NOMA leverages the distinct channel gains of users for multiplexing different signals in a single resource block (time, frequency, code) in power domain. This results in higher spectral efficiency, improved user fairness, better cell-edge throughput, increased reliability and connectivity and low-latency. The flexible combination of PD-NOMA with existing and emerging technologies such as heterogeneous networks (HetNets), multiple-input multiple-output (MIMO), massive MIMO, cooperative communication, cognitive radios (CRs), millimeter wave communication, simultaneous wireless information and power transfer (SWIPT), visible light communication (VLC), mobile edge computing (MEC), intelligent reflecting surfaces (IRS), unmanned aerial vehicles (UAVs), underwater communication etc., is expected to cause further enhancements in performance. Existing survey papers on NOMA mainly focus on its concept, comparison, issues and analysis without any categorization of different techniques to solve the issues related to it. This survey paper highlights the main issues and constraints of resource allocation, signaling, practical implementation and security aspects of NOMA and its integration with 5G and upcoming wireless technologies. Various solutions have been proposed in the literature that involve optimization, analytical, game theory, matching theory, graph theory and machine learning (ML) techniques. We present an in-depth analysis and comparison of these solutions with key insights emphasizing the feasibility and applicability for a qualitative analysis. We finally identify promising future research directions and challenges in the context of PD-NOMA?s application to the existing 5G and next generation wireless networks.