# Challenges of Using the L-Band and S-Band for Direct-to-Cellular Satellite 5G-6G NTN Systems
## 6 Conclusions  
Today, many users prioritize greater accessibility over higher data rates. This preference is driven by the increasing number of people who enjoy traveling to remote places with limited connectivity but still require basic features such as text messaging, voice calls, and basic web browsing. Therefore, achieving seamless compatibility with other NGSO systems is vital to ensure the success of NTN technology in these frequency bands. Consequently, the development of 5G NTN and 6G NTN systems becomes crucial as they bridge this gap for numerous individuals. Moreover, the demand for direct-to-cellular NTN systems in smartphones is currently on the rise, allowing users to access these features in one device. To meet users' needs effectively, it is essential to develop NTN systems supported by 3 GPP frequency bands. This eliminates the need for users to purchase additional phones when traveling to remote areas. In line with this goal, 3 GPP has added support for n255 and 256 bands. However, implementing NTN systems in these bands requires compatibility with other NGSO systems. This is due to the complexity of spectrum sharing in satellite systems, given their global nature of operations compared to terrestrial systems. The study examined the impact of a 5G/6G NTN satellite system on incumbent satellite systems in the n255 and n256 bands, revealing a significant challenge in implementing a new NGSO system with contiguous coverage and satisfactory link performance to support 5G and 6G services in these bands. The results unequivocally demonstrate a substantial degradation in link performance and potential adverse effects on the incumbent satellite systems when they are subjected to interference from the 5G/6G NTN satellite system in these frequency bands. The study considered three incumbent satellite networks, and the results indicate that for Inmarsat, the results clearly demonstrate a significant degradation in link performance when subjected to interference. This poses a substantial threat, potentially resulting in a drastic decline in service quality or even complete radio link outages. Similarly, Omnispace experiences a significant degradation in link performance when exposed to interference. This degradation has severe implications, potentially leading to a significant drop in service quality or complete radio link outages. In the case of Lyra, even though it is a more interference-robust system, the study demonstrates a substantial degradation in link performance when external interference is present. Despite the system's robustness, such degradation can still result in radio link outages and significant delays in message delivery. These findings strongly discourage the deployment of the 5G NTN network in the n255 and n256 bands as it would compromise the quality of service, result in radio link outages, and hinder the timely delivery of messages. Additionally, coordination with other operators and administrations within the ITU-R for filing such a system would be practically impossible. This means that providing the required 5G and 6G services in these bands is not feasible in the foreseeable future. While the existing systems in these bands, such as those used by MVNOs for the NB-IoT services provided by Inmarsat, Echostar, Thuraya, and others, may have limited capabilities, since they were not designed to support the full range of services with the lower latency and fast data rate requirements that are the part of the 5G ecosystem and will be a part of the 6G ecosystem. Considering these limitations, it is prudent to explore alternative spectrum bands in which new constellations can be launched to provide the necessary voice and data services based on the 5G and 6G radio interfaces. Future studies should focus on identifying suitable bands that can accommodate these services effectively. In summary, this study highlights the significant challenges and adverse effects associated with deploying a new 5G NTN satellite system in the n255 and n256 bands.