# NOMA Clustering for Improved Multicast IoT Schemes
## 2 Related Works  
Many recent research studies have considered combining NOMA and D2D communications techniques with the objective of exploiting their potential benefits in improving system throughput, energy and spectrum efficiencies. A taxonomical classification and comparison of the different approaches to the implementation of NOMA-based D2D schemes was proposed in [12]. Other works, such as [13], used a game theoretic approach to improve the rate of weak users in D2D-NOMA contexts or, as in [14], to maximize the system sum rate by optimizing sub-channel and power allocations. In [15], [PERSON] et al. studied the resource allocation problem and the tradeoff between spectral efficiency (SE) and energy efficiency (EE) for downlink traffic in power domain--NOMA and D2D based heterogeneous cloud radio access networks (H-CRANs) for 5G. [PERSON] et al. in [16] considered an optimization-based model for multicast device-to-device (MD2D) communications, where the cellular users (CUs) communicate in an orthogonal fashion with the base station, while the D2D users use NOMA in the transmission and can share the same channels with other CUs or D2D transmitters. They also investigated the rate and energy efficiency of the network through a dynamic power control algorithm. With the objective of maximizing the network throughput by investigating a joint resource allocation problem of user clustering, power control, and D2D mode selection, the authors in [17] considered a NOMA-D2D system where D2D users can reuse the resources in cellular networks or share the unlicensed band with WiFi users. In [18], the authors investigated the advantage of employing NOMA in D2D communications, where D2D pairs are clustered with content demands.  
Each D2D transmitter communicates with its corresponding receiver, while simultaneously, communicates with a demander via NOMA technique. D2D pairs communicate in overlay. The authors in this work were interested in improving the physical layer security of D2D users by employing this NOMA D2D scheme. [PERSON] and [PERSON] and [PERSON] (2020) approached the traffic offloading problem by employing NOMA-enabled D2D and unlicensed access technologies. The objective here is to maximize the total achievement rate of the D2D network, while ensuring the capacity of cellular and WiFi systems.  
Compared to other works in the literature, our proposed model presents the following contributions:  
* In our proposed design, the base station (BS) resources are split among limited selected users, which communicate in a NOMA fashion with the serving BS, in order to improve the system spectrum efficiency and increase the rate of the selected users users (and hence, the system sum rate).
* The rest of the users join different D2D coalitions where the coalitions' heads are the previously selected users from the BS. Within the coalitions, the D2D users get the service over their best D2D link using short range communications for higher individual and system sum rate. This can be further upgraded, especially in 6G context, where the THz and mmWave bands should be used.
* Due to reusing the same resources, the underlying D2D communications may cause high interference to cellular users, and thus reduce the network throughput. To alleviate this issue, we consider connecting D2D users in out-band mode using available licensed or unlicensed spectrum. We consider that the signaling functions for D2D communications are insured centrally at the BS or via an other defined equipment in the network.
* The proposed system uses low complexity algorithms which can provide a dynamic network configuration adapted to IoT new services based on different users' profiles and applications.
* Based on Monte-Carlo simulations, we demonstrate that the proposed system model outperforms the traditional OMA and NOMA models, in terms of spectrum efficiency and energy efficiency, especially in low mobility users' profiles.