# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 2 System Model  
### Topology  
A network consisting of a LEO satellite and \(N=2K\) user equipment (UEs) located in a circle with radius \(R\) and within the LEO satellite coverage area is considered, as depicted in Figure 1. The UEs can communicate with the satellite and with each other via D2D links. The satellite is equipped with an antenna with transmitting gain \(G_{t}^{s}\) and total available transmit power \(P_{s}\). Additionally, the satellite downlink frequency operates at a frequency \(f_{s}\) and the total available bandwidth is denoted as \(B_{s}\). Furthermore, each UE\({}_{i}\) has three antennas, one for the reception of the satellite signal with a reception gain denoted as \(G_{r}^{s}\) and two antennas for the transmission and the reception of the terrestrial signal to and from other devices that are located within the area of interest and with gains \(G_{t}^{d}\) and \(G_{r}^{d}\), respectively. The operating frequency for the terrestrial D2D links is denoted as \(f_{d}\) and the total available bandwidth as \(B_{d}\). Besides, each UE\({}_{i}\) has a maximum available transmit power \(P_{d}\) and the receiver equipment for the satellite and the terrestrial D2D signals have noise temperatures denoted as \(T_{s}\) and \(T_{d}\), respectively.  
UEs are served through a satellite-aided and cooperative D2D network where the satellite is mainly responsible for the communication, while the D2D scheme is employed between the devices to improve the total communication quality. The communication with the UEs is divided into two phases. During the first transmission phase, the satellite employs NOMA based on a user pairing policy in the power domain to transmit the data to pairs of UEs using superposition coding, as illustrated in Figure 1. Practically, the satellite takes into account the channel conditions of each UE\({}_{i}\) regarding the satellite link and the channel conditions of terrestrial D2D links between each UE\({}_{i}\) and UE\({}_{j}\) pair where \(i\
eq j\), and applies user pairing. Each pair of UEs will share the same sub-channel in the frequency/time domain and within this sub-channel, a power allocation scheme will take place to assign different power levels to each UE\({}_{i}\). During the second phase, the UEs with favorable satellite  
Figure 1: System model with \(N=8\) user equipment (UEs).  
channel conditions from each pair decode the received satellite signal and transmit the decoded signal via a D2D communication link to their pair. Thus, the UEs with weak channel conditions towards the satellite will have two copies of their own signal in the power domain and will select the best one in order to achieve the maximum rate. On the other hand, the UEs with strong channel conditions towards the satellite, perform successive interference cancellation (SIC) and get their own signals. Terrestrial communication between D2D pairs takes place over the time/frequency domain using OMA. Hence, each D2D pair communicates in a different sub-channel without interfering with D2D links of neighboring pairs, as illustrated in Figure 1.