# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 1 Introduction  
Future wireless networks will be characterized by dense topologies and diverse services, ranging from high-throughput multimedia applications to ultra-reliable Internet of Things (IoT) communication. The coexistence of both users and devices results in unprecedented hurdles for the currently deployed terrestrial infrastructure. At the same time, aiming to alleviate the stress on terrestrial wireless networks, the integration of satellite and aerial segments has been proposed [1; 2]. The efficient cooperation among the terrestrial and space segments enables integrated satellite-terrestrial networks to enjoy improved coverage for both urban and remote areas and increased capacity to serve beyond fifth generation (5G) services. At the same time, the addition of satellites acting as base stations (BSs) in space offers several key benefits to the wireless transmission. More specifically, users can experience improved diversity by connecting to either a terrestrial or satellite BS or both, opportunistically choosing the best possible point for transmission and reception [3]. Another dimension that has been investigated in recent years is the development of spectral efficient multiple access techniques to accommodate the massive connectivity requirements for both users and devices. In this field, non-orthogonal multiple access (NOMA) allows multiple users to share the same resources, thereby avoiding the orthogonal use of wireless channels and its inefficiencies. Since NOMA in the power domain relies on appropriate power allocation, both strong and weak users can be served, maintaining both system throughput and user fairness [4]. An important aspect forsuccessful NOMA transmissions lies in determining the users that will be paired and share resources. Generally, if two users with asymmetric channel conditions and/or rate requirements are served simultaneously, the sum rate surpasses that of orthogonal multiple access (OMA) [5]. A distributed matching algorithm for optimal power allocation, considering both channel and rate asymmetries, has been proposed by [PERSON] et al. [6] and it was shown to surpass the performance of OMA without incurring high complexity. Further sum rate enhancement can be achieved by employing hybrid schemes where switching between OMA and NOMA takes place in order to allow at least one user to be served when NOMA transmission fails [7]. In the context of integrated satellite-terrestrial networks, the application of NOMA has been suggested to better utilize the wireless resources among the different segments. An overview of NOMA in such topologies has been given in [8], highlighting the advantages of employing NOMA in scenarios, such as cognitive satellite-terrestrial communication and cooperative networks relying on satellite and terrestrial relays. Then, [PERSON] et al. investigated the downlink communication of ground users with multi-antenna terrestrial BSs and a satellite [9]. More specifically, users were allocated to the satellite according to a channel quality-based scheme, while the remaining users were served by terrestrial BSs through NOMA and paired by solving a max-min problem that maximized their minimum channel correlation. The results revealed a trade-off among fairness and system capacity when the number of users served by the terrestrial BSs increased. The efficient application of NOMA in a two-user network communicating through NOMA with a satellite has been examined by [PERSON] et al. [10]. Towards that end, optimal power allocation was performed for sum rate maximization, while abiding by predetermined target user rates. Additionally, closed-form expressions for the ergodic capacity and the energy efficiency of the system were derived, while performance evaluation showed the superiority of NOMA over OMA. Another work focused on cooperative NOMA (C-NOMA) among two already paired users communicating with a satellite [11]. In greater detail, the strong user forwarded the signal of the weak user when decoding failed at the latter.