# NOMA-Based Rate Optimization for Multi-UAV-Assisted D2D Communication Networks
## 2 System Model
### Network Model  
Consider the NOMA-based UAV-assisted D2D communication model shown in Figure 1, where \(U\) UAVs exist in the air to act as airborne base stations and provide communication services to nearby ground users using Orthogonal Frequency Division Multiple Access (OFDMA) technology. Specifically, the set of airborne base stations is denoted as \(\mathcal{U}=\{1,2,\cdots,u,\cdots,U\}\), and \(\{\mathcal{S}_{1},\mathcal{S}_{2},\cdots,\mathcal{S}_{u},\cdots,\mathcal{S}_{ U}\}\) is the set of available channels for the \(U\) airborne base stations. Meanwhile, we consider the powerful maneuverability of a UAV, assume that the mission period of the UAV's flight in the case of dynamic changes in altitude is \(T_{\max}\), partition \(T_{\max}\) into \(L\) sufficiently small and equal time slots of length \(T=T_{\max}/L\), and denote the set of time slot serial numbers as \(\mathcal{L}=\{1,2,\cdots,t,\cdots,L\}\); therefore, the position of the UAV is nearly constant in each time slot, and the UAVs have a position change in the neighboring time slots. In each UAV base station, cellular users occupy a subchannel resource block alone, and these subchannels are independently orthogonal to each other. In order to enhance the spectrum utilization and improve the network transmission efficiency, the NOMA technique is applied to D2D to form D2D-NOMA clusters. Therefore, a large number of cellular users as well as D2D-NOMA clusters are randomly generated in the ground presence, and the number of cellular users as well as the number of D2D-NOMA clusters are denoted as \(M\) and \(N\), respectively, with the cellular users denoted by the set \(\mathcal{C}=\left\{C_{1},C_{2},\cdots,C_{i},\cdots,C_{M}\right\}\), the set of \(M\) orthogonal channels pre-allocated to the cellular users denoted by the set \(\mathcal{S}=\{S_{1},S_{2},\cdots,S_{i},\cdots,S_{M}\}\), and the D2D-NOMA clusters denoted by the set \(\mathcal{D}=\left\{D_{1},D_{2},\cdots,D_{j},\cdots,D_{N}\right\}\). For convenience, let \(S_{i}\in\mathcal{S}_{u}\) be the channels pre-allocated to users \(C_{i}\in\mathcal{C}_{u}\) and \(C_{i}\in\mathcal{C}_{u}\) be the subset of cellular users associated with UAV base stations \(u\). The D2D-NOMA cluster will reuse the uplink subchannel resources of the cellular users, considering that the user terminal equipment has less processing power and limited battery power consumption compared to the UAV base stations. Meanwhile, in order to keep the signal processing delay as well as the hardware decoding complexity low when transmitting using the D2D-NOMA technique, each D2D cluster \(D_{j}\) contains one transmitter(\(DT_{j}\)) and two receivers (\(DR_{j,1}\) and \(DR_{j,2}\)). More importantly, it is assumed that a D2D cluster \(D_{j}\) can multiplex the channel resources of at most one cellular user, while the channel resources of one cellular user can be multiplexed by multiple D2D clusters. Furthermore, since some D2D clusters are located in the overlapping area of multiple UAV services, when the UAV base station is \(u\in\mathcal{U}\), let \(\xi_{u}\) be the maximum number of D2D clusters associated with the UAV base station.