# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 5 Conclusions  
The integration of satellite segments to terrestrial wireless networks has been shown to facilitate the provision of massive connectivity to coexisting users and devices. At the same time, incorporating efficient non-orthogonal multiple access, in terms of wireless resource utilization can guarantee fairness and system capacity. In this context, SANOCO-D2D, a satellite-aided NOMA cooperative D2D scheme was presented for integrated terrestrial satellite networks. Through SANOCO-D2D, the sum rate of the system was improved for different channel conditions, and it was shown that under a properly selected value for the terrestrial bandwidth, enhanced spectral efficiency, compared to standalone NOMA and OMA can be achieved, especially for low and medium transmit SNR. SANOCO-D2D offers increased QoS homogeneity through proper power allocation depending on the channel asymmetry between the strong and weak users and the possibility for simultaneous out-band D2D cooperation, using the strong users as relays. As a result, the fairness in the system is improved and at the same time, leveraging the increased sum rate, more packets are served, leading to a reduction of the total delay.  
Integrated satellite-terrestrial networks provide a fertile research field and there are several future directions that can expand this work. Given the various benefits of SANOCO-D2D, a distributed weighted matching algorithm could be adopted in order to further decrease the computational complexity and achieve workload sharing between the nodes of the network [29]. Recently, the adoption of artificial intelligence and machine learning has been a driving force towards fully autonomous zero-touch wireless networks [30; 31]. SANOCO-D2D can adopt different learning techniques to reduce the complexity of user pairing and channel state information acquisition and processing. Another important technique that should be studied is full-duplex communication, either in-band or out-band where dynamic bandwidth allocation should be optimized in the device-to-device link. There have been several full-duplex cooperative transmission schemes that can be integrated in SANOCO-D2D, incorporating successive transmissions by the source and relay nodes [32; 33] or full-duplex reception and transmission from a single node [34]. Finally, the issue of outdated CSI represents another important future direction for this work, since in practical systems, the acquired CSI of a link might be different from the actual one, due to delays generated by the feedback mechanism [35; 36].  
**Author Contributions:** Conceptualization, M.K., N.N. and D.V.; methodology, M.K. and N.N.; software, M.K.; validation, M.K., N.N. and D.V.; formal analysis, M.K.; investigation, M.K. and N.N.; writing--original draft preparation, M.K. and N.N.; writing--review and editing, N.N. and D.V.; supervision, D.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Acknowledgments:** The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the HFRI PhD Fellowship grant (Fellowship Number: 1507).  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **SANOCO-D2D** & **NOMA Scheme [6]** \\ \hline
**Complexity** & Medium & Low \\ \hline
**D2D Cooperation** & Yes & No \\ \hline
**Relaying Mode** & Half Duplex & N/A \\ \hline
**CSI Overhead** & Medium & Low \\ \hline
**Power Allocation** & Optimal & Optimal \\ \hline
**Dynamic D2D Channel Bandwidth** & Yes & N/A \\ \hline
**Reliability** & High & Medium \\ \hline
**Spectral Efficiency** & High & Medium \\ \hline
**Sum Rate** & High & Medium \\ \hline
**Fairness** & High & Low \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparative table regarding the key characteristics of the SANOCO-D2D and the scheme of [6].