# Challenges of Using the L-Band and S-Band for Direct-to-Cellular Satellite 5G-6G NTN Systems
## 3 Materials and Methods
The size of the antenna is related with the frequency and maximum gain by the following expression:  
\[G_{max}=\eta\left(\frac{\pi Df}{c}\right)^{2} \tag{1}\]  
where \(G_{max}\) represents the maximum gain of the antenna, \(\eta\) represents antenna efficiency, \(D\) represents the antenna diameter, \(f\) represents frequency and \(c\) represents the speed of light. As previously mentioned, the uplink for the n255 and n256 bands operates in the frequency ranges of 1626.5-1660.5 MHz and 1980-2010 MHz, respectively. The corresponding downlink operates in the frequency ranges of 1525-1559 MHz and 2170-2200 MHz, respectively. In direct-to-cellular NTN communications, typical smartphones will be utilized. Given that smartphones operate at a maximum power of 200 mW, the interference they cause in the uplink portions of the n255 and n256 bands to the satellite receivers of existing systems is expected to be insignificant. However, in the downlink portions, interference from NTN satellites to the ground and the aerial receivers of the incumbent satellite systems is possible. In particular, interference levels may be significantly higher when the main lobe of the NTN satellite overlaps with the main lobe of the receiving antenna's pattern gain of the incumbent services. The study considers several incumbent satellites networks in the n255 and n256 bands as the interference receptors from the possible future 5G-6G NTN systems. The frequency bands 1525-1559 MHz and 1626.5-1660.5 MHz (n255 band) are split between several operators, according to the GSO/MSL-band multilateral meeting memorandum of understanding. There are several operators that are part of that memorandum, such as Inmarsat, Thuraya, and others [16]. In our study, we considered the Inmarsat system a victim network, which is a GSO satellite system. Inmarsat offers a range of services that can be broadly categorized into maritime, aeronautical, and land applications. Maritime users rely on Inmarsat for the GMDSS satcom service, which plays a crucial role in distress alerts, the transmission and reception of maritime safety information, and vessel tracking [17]. In addition, many airlines utilize Inmarsat's L-band service to support the AMS(R)S service, particularly in oceanic regions where satellite communication is necessary beyond the reach of VHF terrestrial communications. L-band satellite communications are also increasingly adopted for aircraft communication over land to alleviate congestion on VHF  
Figure 3: Antenna pattern of NTN simulated satellite beam: (**a**) one-beam pattern; (**b**) 3D visualization of multiple beams. frequencies. In this study, we analyzed interference in the downlink of the 25E Inmarsat satellite for Aero terminals that are installed in the aircrafts. Typical characteristics were obtained from the ITU-R filing information and are summarized in Table 4.  
For the Inmarsat satellite transmitter, an antenna pattern based on the ITU-R S.672 Recommendation was used since it is the most commonly used pattern for simulating GEO satellites [18]. The simulation of the Inmarsat link with an Aero satellite is illustrated below, in Figure 4. In the figure, 3 dB is shown. In our study, we considered the INM-AERO (H) C-21 terminal, and the aircraft was moving within the 3 dB contour. For the n256 band, two incumbent NGSO satellite systems were considered; the first one was an MEO satellite system, Omnispace. Initially, this system was part of ICO Global Communications, which had intentions to launch 12 satellites that were designed to provide voice communication services. In 2001, the first satellite was successfully launched; later, in 2004, the company filed for bankruptcy, and later, in 2012, Omnispace acquired the ICO-F2 MEO satellite from ICO Global Communications. The Boeing-built satellite was renamed Omnispace-F2 and currently is operating in the S-band, providing mobile communications and data/Internet services and supporting 4500 simultaneous calls.