In the expanding 5G era, the integration of Non-Terrestrial Networks (NTN) with terrestrial 5G infrastructure is paramount. Recognizing the need for seamless global connectivity, the TRANTOR project aims to leap forward by transitioning 5G NTN evolution towards 6G. One of the key objectives is to enhance both the satellite User Equipment (UE) and the gNB to facilitate Multi-Connectivity (MC). The project considers a Multi-Satellite configuration with multi-Band (Ku and Ka) capabilities, with the satellites deployed either in the same or different Orbits. This configuration can be referred to as the M-SBO system in this context. As shown in Figure 1, the UE could receive multiple signals from several nodes (Low Earth Orbit (LEO)) at the same time, which is known as the MC case. It is also possible to be connected to a single node and move between nodes sequentially, which is known as Single Connectivity (SC).
Figure 1 Prototype scenario for SC and MC.
As MC enabler technologies, 3GPP identifies various methods on different layers . Among them, the most relevant are coordinated multi-point/multi-transmission points on the physical layer, Carrier Aggregation (CA) on the medium access control layer, and Dual Connectivity (DC) on the packet data convergence protocol layer. While Terrestrial Networks (TN) have seen extensive research on MC , studies on its implications in NTN are still sparse , , .
Notably, utilizing the MC case for the NTN, and more specifically for the M-SBO systems is highly important for the following reasons:
- The integration of TN and NTN involves the employment of MC technologies to manage the combination and provide a smooth transition between the networks.
- The inherent expansive coverage of satellites increases the availability of multi-link connectivity for UE. By utilizing such connectivity, the likelihood of connection failures caused by obstructions or extreme weather could be significantly reduced.
- Satellite transmissions at high frequencies, such as the Ka-band, are susceptible to signal attenuation caused by air absorption, precipitation, and obstructions. Utilizing MC provides broad solutions to these issues, not only enhancing signal reliability by capitalizing on diverse band propagation properties but also incorporating redundancy from multiple nodes.
For the M-SBO system, there are multiple potential usage scenarios for MC. Figure 2 depicts one such scenario in which two Geostationary Earth Orbit (GEO) satellites broadcast in both the Ku and Ka bands to maintain NTN-TN service continuity, which is particularly advantageous for moving vehicles. DC is essential in this context, connecting TN and GEO satellites to enhance transitions between TN and multi-GEO connections and ensure a seamless handover. The UE can establish simultaneous connections with multiple nodes, be it a single GEO with TN, multiple GEOs, or multi-GEO with TN. This configuration increases adaptability and ensures uninterrupted communication. Whenever necessary, the UE can use CA to leverage the multi-band capabilities of the GEO, resulting in enhanced user data rate experiences.
Figure 2 Multi-band, multi-GEO satellites scenario.
 M.-T. Suer, C. Thein, H. Tchouankem, and L. Wolf, “Multi-Connectivity as an Enabler for Reliable Low Latency Communications—An Overview,” IEEE Communications Surveys & Tutorials, vol. 22, no. 1, pp. 156-169, 2020, doi: 10.1109/comst.2019.2949750.
 C. Pupiales, D. Laselva, Q. De Coninck, A. Jain, and I. Demirkol, “Multi-Connectivity in Mobile Networks: Challenges and Benefits,” IEEE Communications Magazine, vol. 59, no. 11, pp. 116-122, 2021, doi: 10.1109/mcom.111.2100049.
 N. Cassiau et al., “5G-ALLSTAR: Beyond 5G Satellite-Terrestrial Multi-Connectivity,” presented at the 2022 Joint European Conference on Networks and Communications & 6G Summit (EuCNC/6G Summit), 2022.
 M. Majamaa, H. Martikainen, L. Sormunen, and J. Puttonen, “Multi-Connectivity in 5G and Beyond Non-Terrestrial Networks,” Journal of Communications Software and Systems, vol. 18, no. 4, pp. 350-358, 2022, doi: 10.24138/jcomss-2022-0155.
 H. Al-Hraishawi, N. Maturo, E. Lagunas, and S. Chatzinotas, “Scheduling Design and Performance Analysis of Carrier Aggregation in Satellite Communication Systems,” IEEE Transactions on Vehicular Technology, vol. 70, no. 8, pp. 7845-7857, 2021, doi: 10.1109/tvt.2021.3093117.