Over the last few years, interference has been a major hurdle for successfully implementing various end-user applications in the fifth-generation (5G) of wireless networks. During this era, several communication protocols and standards have been developed and used by the community. However, interference persists, keeping given quality of service (QoS) provision to end-users for different 5G applications. To mitigate the issues mentioned above, in this paper, we present an in-depth survey of state-of-the-art non-orthogonal multiple access (NOMA) variants having power and code domains as the backbone for interference mitigation, resource allocations, and QoS management in the 5G environment. These are future smart communication and supported by device-to-device (D2D), cooperative communication (CC), multiple-input and multiple-output (MIMO), and heterogeneous networks (HetNets). From the existing literature, it has been observed that NOMA can resolve most of the issues in the existing proposals to provide contention-based grant-free transmissions between different devices. The key differences between the orthogonal multiple access (OMA) and NOMA in 5G are also discussed in detail. Moreover, several open issues and research challenges of NOMA-based applications are analyzed. Finally, a comparative analysis of different existing proposals is also discussed to provide deep insights to the readers.

中文翻译：

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对 5G 及以后的 NOMA 变体的系统审查

在过去几年中，干扰一直是在第五代 (5G) 无线网络中成功实施各种最终用户应用程序的主要障碍。在这个时代，社区开发并使用了多种通信协议和标准。然而，干扰仍然存在，为不同 5G 应用的最终用户提供给定的服务质量 (QoS)。为了缓解上述问题，在本文中，我们对最先进的非正交多址 (NOMA) 变体进行了深入调查，这些变体以功率和代码域作为干扰缓解、资源分配、和 5G 环境中的 QoS 管理。这些是未来的智能通信，由设备到设备 (D2D)、协作通信 (CC)、多输入多输出 (MIMO)、和异构网络（HetNets）。从现有文献中可以看出，NOMA 可以解决现有提案中的大部分问题，以在不同设备之间提供基于竞争的无授权传输。还详细讨论了 5G 中正交多址 (OMA) 和 NOMA 之间的主要区别。此外，还分析了基于 NOMA 的应用程序的几个未解决的问题和研究挑战。最后，还讨论了对不同现有提案的比较分析，以便为读者提供深刻的见解。还详细讨论了 5G 中正交多址 (OMA) 和 NOMA 之间的主要区别。此外，还分析了基于 NOMA 的应用程序的几个未解决的问题和研究挑战。最后，还讨论了对不同现有提案的比较分析，以便为读者提供深刻的见解。还详细讨论了 5G 中正交多址 (OMA) 和 NOMA 之间的主要区别。此外，还分析了基于 NOMA 的应用程序的几个未解决的问题和研究挑战。最后，还讨论了对不同现有提案的比较分析，以便为读者提供深刻的见解。