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Non-orthogonal multiple access (NOMA) systems can serve multiple users incontrast to orthogonal multiple-access (OMA), which makes use of the limitedtime or frequency domain resources. It can help to address the unprecedentedtechnological advancements of the sixth generation (6G) network, which includehigh spectral efficiency, high flexibility, low transmission latency, massiveconnectivity, higher cell-edge throughput, and user fairness. NOMA has gainedwidespread recognition as a viable technology for future wireless networks. Themain characteristic that sets NOMA apart from the conventional orthogonalmultiple access (OMA) techniques is its ability to handle more users thanorthogonal resource slots. NOMA techniques can serve multiple users in the sameresource block by multiplexing users in power or code domain. The purpose ofthis paper is to provide a thorough overview of the promising NOMA systems.Initially, we discuss the state-of-the-art and existing literature on NOMAsystems. This study also examines the practical deployment of NOMAimplementation and key performance indicators. An overview of the most recentNOMA advancements and applications is also given in this survey. We alsobriefly discuss that multiple-input multiple-output (MIMO), visible lightcommunications, cognitive and cooperative communications, intelligentreflecting surfaces (IRS), unmanned aerial vehicles (UAV), HetNets, backscattercommunication, mobile edge computing (MEC), deep learning (DL), and otheremerging and existing wireless technologies can all be flexibly combined withNOMA. This study surveys a thorough analysis of the interactions between NOMAand the aforementioned technologies. Lastly, we will highlight a number ofdifficult open problems and security issues that need to be resolved for NOMA,along with pertinent possibilities and potential future research directions.

A Survey of NOMA: State of the Art, Key Techniques, Open Challenges, Security Issues and Future Trends