The inevitable next era of smart living requires unprecedented advances in enabling technologies. The building blocks of this era are devices such as sensors, actuators, and Internet of Things (IoT) devices. Machine-Type Communication (MTC), also known as Machine-to-Machine (M2M) communication, constitutes the main enabling technology to support communications among such devices. The massive number of these devices and the immense amount of traffic generated by them require a paramount shift in cellular and non-cellular wireless technologies to achieve the required connectivity. Ultra-Dense Network (UDN) is intuitively one of the most convenient approaches to tackle such requirements of massive connectivity and high throughput found in MTC. In UDNs, small cells are deployed in large densities compared to Human-Type Communication Users (HTCUs) like smartphones and tablets. In a different scope, multiple access techniques also require a paradigm shift to cope with the increasing density of required connections while confined by limited resources. Recently, Non-Orthogonal Multiple Access (NOMA) has received a great attention as an efficacious candidate to enhance the network’s performance compared to traditional Orthogonal Multiple Access (OMA) techniques. For this sake, in this paper, we provide a comprehensive survey and illustrative simulation results on the application of NOMA to support MTC in a UDN environment. First, we give a brief discussion on MTC and its different applications and supporting technologies. Then, we focus our effort on investigating the main challenges facing MTC along with the existing opportunities found in both NOMA and UDN, respectively. Finally, we show via simulations the possible gains of both technologies and conclude by discussing the open problems for future research directions.

中文翻译：

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UDN 中的 NOMA 辅助机器类型通信：最新技术和挑战

不可避免的下一个智能生活时代需要使能技术取得前所未有的进步。这个时代的基石是传感器、执行器和物联网 (IoT) 设备等设备。机器类型通信 (MTC)，也称为机器对机器 (M2M) 通信，是支持此类设备之间通信的主要使能技术。大量这些设备及其产生的大量流量需要蜂窝和非蜂窝无线技术的重大转变才能实现所需的连接。超密集网络 (UDN) 直观地是解决 MTC 中的大规模连接和高吞吐量需求的最便捷方法之一。在 UDN 中，与智能手机和平板电脑等人型通信用户 (HTCU) 相比，小型基站的部署密度较大。在不同的范围内，多址技术还需要范式转换，以应对所需连接密度不断增加的问题，同时又受到有限资源的限制。最近，与传统的正交多址 (OMA) 技术相比，非正交多址 (NOMA) 作为增强网络性能的有效候选技术受到了极大的关注。为此，在本文中，我们提供了有关 NOMA 在 UDN 环境中支持 MTC 的应用的全面调查和说明性模拟结果。首先，我们简要讨论 MTC 及其不同的应用和支持技术。然后，我们专注于调查 MTC 面临的主要挑战以及分别在 NOMA 和 UDN 中发现的现有机会。最后，我们通过模拟展示了这两种技术的可能收益，并通过讨论未来研究方向的开放性问题得出结论。