# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 1 Introduction
The performance evaluation in terms of outage probability and ergodic capacity showed the advantage of C-NOMA in the satellite-terrestrial network against time-division multiple access (TDMA)-based OMA. As NOMA relies on user pairs, it is easy to observe that the cooperation between them, in cases where the weak user fails to decode its own message, can lead to robust communication and guaranteed fairness. In various studies, the advantages of device-to-device (D2D) communication have been investigated [12, 13]. Thus, the adoption of the D2D communication paradigm opens up tremendous opportunities to further strengthen the performance of integrated satellite-terrestrial NOMA networks, as long as the channel conditions and rate requirements of each pair are jointly considered with the user pairing process of NOMA. Integrated satellite and terrestrial networks represent an important research topic for 5G and beyond networks and have received several contributions in recent years. Through integrated satellite-terrestrial wireless architectures, multiple wireless systems and access techniques collaborate towards improving the quality-of-service (QoS), coverage and load balancing [14]. At the same time, low Earth orbit (LEO) satellites have been proposed as a viable means for providing additional degrees of freedom for connectivity and data offloading due to reduced latency, compared to other satellite-based solutions with larger distance to the Earth's surface and increased coverage, compared to standalone terrestrial small cells [3, 15]. Other works have shown diversity gains by facilitating the cooperation among satellite and terrestrial network segments where users directly transmit and receive to and from the satellite [16]. Moreover, it has been observed that user cooperation in integrated satellite-terrestrial networks, following the D2D paradigm, improves the diversity of the transmission and the performance of spectral-efficient communication techniques, such as NOMA [11]. Finally, from an industrial perspective, the high potential of integrated satellite-terrestrial networks is outlined by the forthcoming plans to deploy dense LEO constellations by OneWeb [17] and SpaceX [18] and cooperate with traditional cellular operators, providing ubiquitous and high QoS communication. Furthermore, the integration of satellites to complement terrestrial communication provides an interesting trade-off of additional complexity in the architecture, production costs and round-trip delay versus the benefits of improved link quality and coverage. However, it has been shown that the negative aspects of adding a satellite segment are significantly mitigated in the case of LEO satellites. More specifically, the low orbit altitude of LEO satellites, compared to the medium Earth orbit (MEO) and geosynchronous Earth orbit (GEO) satellites, results in one-directional trip delay of less than 14 ms, and a round-trip delay of less than 50 ms being comparable to those of terrestrial links [3]. Furthermore, owing to the the small size of satellites and pipeline production, manufacturing costs of LEO satellites are significantly minimized [3, 15]. At the same time, the interests of industrial stakeholders in deploying dense LEO constellations highlights the importance of adding more communication segments in the architecture of 5G and beyond networks [17, 18]. It is important to note that satellite segments can act in collaboration with other novel propositions, such as spectrum sharing and cognitive radio in order to boost their performance [14]. In this work, we aim at improving the performance of integrated satellite-terrestrial NOMA networks comprising multiple ground users by enabling cooperative D2D communication among them. For this purpose, a pairing scheme for ground users is presented, considering each user's satellite channel, and the terrestrial channels between the users.