# Challenges of Using the L-Band and S-Band for Direct-to-Cellular Satellite 5G-6G NTN Systems
## 1 Introduction  
The concept of integrating satellite technology with cellular mobile networks has been around for a long time. In the mid-1990s, the International Telecommunications Union (ITU) introduced the IMT-2000 (International Mobile Telecommunications 2000) initiative with the goal of achieving global wireless access in the 21 st century. This initiative led to the establishment of the Third-Generation Partnership Project (3 GPP). Several standards were developed, such as CDMA-2000 (Code Division Multiple Access 2000), UMTS (Universal Mobile Telecommunications System), and TD-CDMA (Time Division-Code Division Multiple Access), collectively known as 3G. IMT-2000 was intended to include both terrestrial and satellite components, and the ITU-R (ITU Radiocommunication Sector) developed various reports on the satellite aspect of IMT-2000 [1]. However, due to the high costs involved and a lack of sufficient user demand, the idea of implementing a satellite-based 3G network was not pursued. Today, an increasing number of LTE and 5G users have begun to prioritize ubiquitous connectivity rather than increased data rates [2,3]. As many individuals enjoy traveling to remote areas with no existing connectivity, satellite technology has become an increasingly convenient solution to provide such services. Consequently, the development of direct-to-cellular 5G satellite NTN (non-terrestrial networks) has become a prominent and essential objective. Furthermore, it is anticipated that satellites will become integral part of future 6G technology, making it imperative to establish suitable conditions for the development of 5G-6G satellite networks. One of the most challenging issues in achieving this goal is securing a sufficient amount of spectrum for such networks. Currently, there are two approaches to spectrum assessment for satellite 5G-6G networks [4]. The first approach involves the convergence of the satellite and terrestrial components of 5G-6G within the frequency bands already designated for terrestrial GSM, 3G,LTE and 5G [5]. This approach has been adopted by companies like Lynk and AST Space-Mobile, which utilize the 700 MHz and 800 MHz frequency bands. Additionally, SpaceX, in partnership with T-Mobile, announced plans to use the 1910-1915/1990-1995 MHz frequency bands for their next generation of Starlink satellites, which will provide direct-to-cellular services. The FCC even released a regulatory framework document describing how to regulate the work of direct-to-cellular satellite services within terrestrial frequency bands [6]. The second approach is to utilize the spectrum allocated for mobile satellite services defined by 3 GPP explicitly for non-terrestrial network (NTN) technologies. Currently, 3 GPP is considering a 5G satellite (NTN) and has specified non-terrestrial frequency ranges, the n255 band and n256 band. Table 1 provides frequency number details for these two bands. These frequency ranges offer advantages such as lower propagation losses and compatibility with legacy communications, which means existing components can be utilized. However, a notable drawback is the well-known spectrum crunch as these bands are already heavily occupied, restricting the usable bandwidth. The maximum overall bandwidth anticipated is up to 40 MHz. To maintain compatibility with 5G/6G, the NTN will adopt the bandwidth part (BWP) methodology. Each user equipment (UE) can be configured with one or multiple BWPs, and the UE need not be concerned about the SAN (satellite access node) channel bandwidth. Flexibility is possible via the segmentation of the overall SAN channel bandwidth using different numerologies.