# Machine Learning-Based Methods for Enhancement of UAV-NOMA and D2D Cooperative Networks
## 2 System Model  
From the system point of view, we consider a cooperative UAV and D2D-aided wireless communication system, where the UFBS is mainly responsible for communication. The D2D scheme is employed between the ground mobile terminals (GMTs) to achieve higher data rates and spectral efficiency without the involvement of any additional terrestrial or flying base station. The wireless network architecture is depicted in Figure 1, where a two-tier heterogeneous network is formed, operating in two different and non-overlapping spectrum bands. From now on, these two ways of communication will be referred to as UFBS NOMA transmission when the GMTs receive the data directly from the UFBS through the NOMA scheme and the D2D cooperative transmission when the GMTs cooperate to improve the overall communication quality. Concerning the UFBS NOMA transmission, all GMTs are served by the UFBS via the air-to-ground (A2G) link, utilizing the NOMA technique according to an optimal power allocation and user pairing strategy [27; 28]. More specifically, the total available UFBS' bandwidth \(B_{u}\) is divided into \(K\) slots, equally distributed to the GMT pairs, as depicted in Figure 1. Each GMT pair \(k\) (\(1\leq k\leq K\)) consists of a strong GMT\({}_{i}\) and a weak GMT\({}_{i}\) ground terminal, with \(i\
eq j\), which are sharing the same sub-channel in the frequency/time domain. The UFBS classifies the GMTs of each pair as either weak or strong based on the A2G channel conditions. Following the NOMA principle, in each pair of users the strong GMT\({}_{i}\) first decodes the signal of the weak GMT\({}_{j}\) from the received superposition-coded signal and then performs successive interference cancellation (SIC) to retrieve its signal. Hence, leveraging this knowledge, the utilization of the D2D cooperative transmission scheme on the ground can further enhance the communication quality of the weak users of the system. Concerning the D2D ground communication procedure, each strong GMT\({}_{i}\) decodes and forwards (DF) the received UFBS's signal to the weak GMT\({}_{j}\) of its pair, thus providing reception diversity through the ground assistance. Consequently, each weak GMT\({}_{j}\) in each pair will receive two different copies of the same signal, one from the UFBS and the other from its pair, i.e., the strong GMT\({}_{i}\), which acts as a relay. From a technical standpoint, the communication system consists of \(N=2K\) GMTs, where \(K\) is the number of GMT pairs and a UFBS located in an \(R\)-radius circle region of interest \(A\). Each GMT\({}_{l}\) (\(1\leq l\leq N\)) is randomly placed in the region of interest, and its location is expressed as \(u_{l}=(x_{l}^{u},y_{l}^{u},z_{l}^{u})\in A\). The 3D location of the UFBS is denoted as \(p_{1}=(x_{1}^{p},y_{1}^{p},z_{1}^{p})\in A\). The UFBS is equipped with an antenna with transmit gain \(G_{r}^{u}\) and total available transmit power \(P_{u}\). Also, the downlink operating frequency of the UFBS is \(F_{u}\). Furthermore, the operating frequency, the total available bandwidth, and the transmit power for the D2D transmission are denoted as \(F_{d}\), \(B_{d}\), and \(P_{d}\), respectively. Moreover, the GMTs are equipped with two antennas, one for the reception of the UFBS's signals with reception gain \(G_{r}^{u}\), and the other for D2D communication, i.e., for transmission and reception, with transmit and receive gain \(G_{r}^{d}\)=\(G_{r}^{d}\). We consider that the common antenna for transmission and reception regarding D2D communication is implemented through a radio frequency (RF) switch. Hence, each GMT can only transmit or receive during a D2D frequency/time slot. Finally, the seamless communication between the UFBS and the GMTs requires a reliable and efficient backhaul network. In this regard, we propose the use of zero-touch commissioning (ZTC) cloud radio access network (C-RAN) for the UAV backhaul, as it can provide efficient and automated network management [25; 29].