# Spectrum Sharing Opportunities for 6G Terrestrial and Non-Terrestrial Networks +
## 2 Case Study: Spectrum Sharing in the 28 GHz Band  
Space-based connectivity at 28 GHz (i.e., 27.5-29.5 GHz) is increasingly important in Europe with an ever-growing number of Earth satellite gateways. In the ITU Radio Regulation [10], the 27.5-29.5 GHz band is allocated on a co-primary basis to the fixed service (FS), fixed satellite service Earth-to-space (FSS E-S) and to the mobile service (MS). In Europe, the regulatory framework for the use of the 28 GHz band is provided in [11]. In Italy, the fixed service operates through WLL (Wireless Local Loop) technology with P-P (Point-to-Point) and P-MP (Point-to-Multipoint) connections. To facilitate coordination at the national level, the study of the technical conditions that guarantee the protection of the FS system becomes necessary. At the international level, the protection and coordination of the satellite component of the FSS system is managed within the ITU. In [9], the coexistence scenarios between the FSS and FS in the 27.5-29.5 GHz band have been identified and the coexistence conditions between the FSS E-S system and the FS WLL P-P system have been evaluated. In this paper, the methodology already developed for the coexistence study between FSS E-S (GSO) systems and FS WLL P-P systems has been extended and adapted to the coexistence scenario between the FSS E-S (GSO) and the FS WLL P-MP systems. WLL P-MP systems allow for the connection of a central station (CS) to several terminal stations (TSs), thus being able to provide wireless broadband services. Figure 1 shows the coexistence scenario that will be analyzed in this work. In the coexistence analysis between the FSS Earth station and FS WLL P-MP link, the protection of both the CS and TS must be guaranteed. The final coexistence conditions take into account the joint results of the two analyses, considering that the protection of the TS must be evaluated on all locations (pixels) of the FS coverage area where it is assumed that a P-MP receiver could be present. The general methodology for carrying out coexistence studies has been described in [9]. The methodology applied in this work is reported, for convenience, below. The methodology adopted to evaluate the interference on FS WLL P-MP links due to FSS Earth stations is based on the Minimum Coupling Loss (MCL) approach [11; 12; 13] taking into account the long-term FS protection criterion based on the I/N ratio (interference-to-noise ratio). The I/N ratio in dB is evaluated with the following formula:  
\[\text{I/N}\left(\Delta\text{f},\,\text{d},\,\text{\text{\text@b{\text@b{1}}}}, \,\text{\text@b{\text@b{2}}}\right)=\text{Pt}+\text{Att}(\Delta\text{f})+ \text{Att}(\text{\text@b{\text@b{1}}})+\text{Gr}(\text{\text@b{\text@b{0}}} )-\text{PL}(\text{\text@b{\text@b{0}}})-\text{N}\]  
Figure 1: Coexistence scenario: FSS E-S interfering FS WLL P-MP. In the above formula, Pt is the transmitted power (dBm) of the interferer that in the analyzed scenario is represented by the FSS E-S Earth station; \(\Delta\)f is the frequency separation (in MHz) between the carrier frequencies of the interferer and the victim systems; Att(\(\Delta\)f) is the Net Filter Discrimination (NFD) that must be computed according to the considered band and service; Gt(\(\bar{\text{0}}\)1) and Gr(\(\bar{\text{0}}\)2) are, respectively, the gains (dBi) of the FSS interferer antenna at angle \(\bar{\text{0}}\)1 and of the FS WLL CS/TS victim antenna at angle \(\bar{\text{0}}\)2 between the interfering system site and victim system site; PL(d) is the path loss attenuation (dB) due to the propagation along distance d (km); and N is the noise level (dBm) of the victim receiver. In this paper, the identification of the technical conditions to ensure coexistence between the FSS E-S (GSO) and FS WLL P-MP systems has been studied, analyzing the co-channel (i.e., Att(\(\Delta\)f) = 0 dB) coexistence scenario. The antenna pattern of the FSS transmitter follows Recommendation ITU-R S.465-6 [14] as used in the coexistence analysis in [9].