# Quality of Service Based NOMA Group D2D Communications
## 1 Introduction
### Related Work and Motivation  
Recently, a FD D2D aided cooperative NOMA is proposed in [14]. The base station (BS) sends a NOMA signal to one NOMA-strong and one NOMA-weak user, where the strong user is equipped with full-duplex ability. By invoking D2D communication between strong and weak NOMA user pair, the authors proposed to improve the outage performance of the weak user with the aid of D2D aided direct and cooperative transmissions. However, they only considered a single-cell scenario where NOMA is conducted at BS while strong and weak users communicate via conventional paired D2D communication. The authors in [15] developed an analytical framework based on stochastic geometry to analyse cellular networks with underlay D2D communications. The D2D users are also equipped with FD transceivers and can operate in FD mode. The authors proposed criteria to select between FD and D2D modes of operation. They derived closed-form expression for outage probability to evaluate the performance of cellular and D2D users. However, they modeled spatial topology of D2D users by a Poisson point process (PPP), which may not be realistic distribution choice for D2D users. The reason is that PPP cannot capture the features of device clustering and spatial separation of D2D communications due to its completely random nature [16]. Furthermore, NOMA protocol is not applied for both cellular and D2D communications. A relay assisted diversity communication is proposed in [17]. The proposed analytical framework analyses the frame error probability performance by considering the effects of node locations, link characteristics, power allocation, diversity methods and distributed coding and constellation signaling. However, they considered an OMA based communication between single source and destination assisted by three relay nodes. The authors in [18] considered a downlink multiuser MIMO NOMA cellular network with underlaid D2D communications. They proposed two beamforming schemes in order to eliminate the inter-beam interference and the one caused to D2D users by BS transmission. In addition, they formulated an optimization problem to jointly study the performance of both cellular and D2D users. A potential limitation to their approach is that the considered system model is limited to single cell and no specific random distribution is utilised to model the spatial topologies of cellular and D2D users. Hence, it is not straightforward to generalize the results for a case of multi-cell network. Furthermore, they considered a paired D2D communication where D2D users do not apply NOMA protocol to communicate with each other. In [19], the authors considered a NOMA-based D2D communications and introduced the concept of D2D group, where a D2D transmitter (DT) is communicating with multiple D2D receivers (DRs) using NOMA protocol. The authors proposed an optimal resource allocation strategy for interference management that enables to realize the NOMA-based D2D group communications. Although the concept of NOMA group D2D was introduced in [19], their system model was comprised of single-cell and lack of interference characterization at DR. Furthermore, it requires interference modeling and performance evaluation at the DR in order to extend the concept of NOMA group D2D to a general scenario, where DTs and DRs are distributed in the entire network. The authors in [20] further proposed cooperative hybrid automatic repeat request (HARQ) assisted NOMA in large-scale D2D networks. They studied the outage and throughput performance of the D2D users under the considered network setting and demonstrated that cooperative HARQ assisted NOMA achieves lower outage probability than non-cooperative case and OMA scheme. However, their NOMA based D2D network model is restricted to the two-user case only i.e., they only considered two-userNOMA transmission from D2D sources. Furthermore, they considered a significant difference between channels of two D2D users by assuming that one user is always closer to D2D source compared to the other user.