# NOMA Clustering for Improved Multicast IoT Schemes
## 1 Introduction  
The ever-increasing number of connected devices and the continuous demand of higher data rates are the main drivers for the emergence of new wireless communication technologies from the first to the fourth generation. Since 2019, the fifth generation (5G) of wireless communication has been communicated and deployed in several countries. 5G represents a technological breakthrough with respect to the previous communication networks. It uses revolutionary technologies (e.g., higher frequencies, network function virtualization (NFV), software defined networking (SDN), and network slicing), in addition to evolutionary ones (e.g., massive multiple-input and multiple-output (MIMO)) to make a significant improvement in energy efficiency, enhancing connectivity and reliability, reducing latency, and achieving gigabits per second speeds. Although 5G was designed to support Internet-of-Things applications through ultra-reliable low-latency communications (URLLC) and massive machine-type communications (MTC), it is arguable whether it can cope with the rapid proliferation of ultimately new Internet-of-Everything ( IoE) applications, which are expected to increase by 12% yearly [1]. A forecast study reported by the International Telecommunication Union demonstrates that the volume of mobile data will continue to grow at an exponential rate, reaching up to 5 zettabytes per month in 2030 [2]. These figures urge he introduction of the next generation of wireless networks, 6G, which is envisioned to realize the next generation connectivity, driven by the evolution from connected everything to connected intelligence, thus enabling \"Human-Thing-Intelligence\" interconnectivity [1]. Compared to 5G, in 6G, the energy efficiency is expected to be 10-100 times, the spectrum efficiency 5-10 times, the connection density 10 times and the capacity is expected to be 10,000 times of 5G systems [3]. Besides the revolutionary new services and technologies, efficient next-generation multiple access (NGMA) techniques and D2D communication will be essential for 6G to support the above-mentioned targets. Non-orthogonal multiple access (NOMA), as an NGMA, has proven its performance compared to the traditional orthogonal multiple access (OMA) technologies in terms of improving the bandwidth utilization and efficient allocation of resources [4, 5, 6, 7]. It was presented as key technology in implementing 6G networks [8, 9]. On the other hand, D2D communications technology was widely advocated for increasing the cellular capacity, improving the user throughput and extending the battery lifetime of user equipments. The concept is to exploit the physical proximity of user equipments or devices, and enable them to communicate directly with each other over a D2D link. In the context of 6G, thanks to the THz band, the communication between two nearby UEs will be near real-time [8]. References [10, 11] have envisioned the role of D2D communications and enumerated a number of potential solutions associated with 6G, in terms of mobile edge computing, network slicing, and NOMA cognitive networking, to construct an efficient implementation of intelligent future network systems. In this work, we propose a NOMA-D2D system configuration for IoT applications. We focus, particularly, on multicast services to disseminate data to end devices. Indeed, multicasting is an efficient mechanism for IoT or machine-type applications (e.g., IoT applications in agriculture, control and monitoring in industry, and information broadcasting in multimedia or vehicular applications), and can help in reducing the required bandwidth, increasing resource efficiency, and reducing transmission costs. With the aim to provide an efficient approach to implementing a NOMA-D2D system, we split the users into coalitions. Coalition heads are connected directly to the serving base station in a NOMA fashion, while the other users within a coalition construct a transmission tree over the D2D links to obtain the service.