## 7 Conclusions  
The current static and regulated spectrum allocation in SATCOM leads to underutilization of the spectrum. This regulated spectrum allocation cannot meet the emergence of numerous new satellite services and satellite operators that are bound to enter into SATCOM in the next few years. Therefore, to support a dense deployment of new satellite services and accommodate the new operators, it is required to dynamically share and reuse the finite available spectrum. The reuse and sharing of the spectrum can be enabled by employing DSS techniques. Two decentralized DSS techniques have been analyzed in terms of spectrum reuse for opportunistic sharing of satellite and terrestrial systems in downlink Ka-band and uplink Ka-band. The first technique relies on a decentralized CP that connects all entities across different systems and is used for advertising relevant information, while the second technique is based on decentralized spectrum sensing and requires spectrum monitoring capabilities. Achieved results demonstrate that the proposed DSS techniques show remarkable interference mitigation capabilities as compared to the baseline, reducing the downtime of the established communication links by 60-99%. Interference mitigation between GSO and NGSO links is proven. Whereas FWA links are not affected in the downlink use case, protection of FWA links in the uplink use case was achieved with small upgrades to the DSS techniques. It has become evident that with a few more enhancements, a seamless experience can be provided to the application layer users in all cases. DSS techniques enable instantaneous adjustment of the allocated spectrum to match incoming application layer load, making sure no more than the needed spectrum is used at any given time. In conjunction with the use of a smaller basic transmission unit in the form of a slot instead of a channel, this enables increased spectrum efficiency, optimally mapping incoming traffic to the allocated spectrum. In both downlink and uplink scenarios, spectrum utilization was increased from 88% in baseline to 96% with applied DSS techniques. We anticipate seeing higher gains in spectrum utilization in the case of higher density satellite-terrestrial network deployments, where spatial reuse of spectrum plays a bigger role. This work shows preliminary results of DSS techniques for enabling the coexistence of satellite and terrestrial networks. The evaluated scenarios are local and include a small number of satellite and terrestrial entities. In reality, there would be a higher number of satellite systems with their coverage areas including a much higher number of user terminals and terrestrial deployments. Therefore, in future work, large-scale testing is needed to investigate the scalability of proposed solutions and to pinpoint the areas where the design needs to be fine-tuned. Besides, with large-scale testing, we will be able to determine the impact of the proposed techniques in scenarios with large-scale satellite-terrestrial deployments. Another interesting point to discuss in the future is the improved protection of terrestrial links. Improved protection of terrestrial networks may be achieved, for instance, by improving CP alert message for anticipating packet loss or by extending the terrestrial networks with the same DSS techniques as applied in satellite systems. Besides, the anticipation of packet loss by applying advanced techniques for the prediction of interference events could enable proactive resource reallocation before packet loss occurs and further reduce downtime in satellite links. Conceptualization, S.G., A.S., M.R. and I.M. ; methodology, I.J., S.G. and A.S.; software, I.J., F.F. and A.M.; validation, I.J., F.F., S.G. and A.M.; formal analysis, A.S., M.R, and I.M. ; investigation, I.J., F.F., S.G. and A.M.; resources, M.R. and I.M. ; data curation, I.J. and F.F; writing--original draft preparation, I.J. and A.S.; writing--review and editing, I.J., S.G., A.S., A.M., M.R. and I.M. ; visualization, F.F. and A.M.; project administration, M.R. and I.M.