## 3 Mobile RAN Entity Integrated into 5G and Beyond
### Integration of Satellite Components into the 5G and Beyond Landscape  
The incorporation of satellite components into the 5G system began with its inclusion in the specifications of 3 GPP Release 17 in 2022. As outlined in Figure 2, after this pivotal integration, a series of investigations and initiatives followed, aiming at further enhancement. The initial design objective primarily aimed at establishing the connectivity between traditional mobile phones and a satellite component intricately woven into the architecture of the 5G framework depicted in Figure 3, functioning within the sub-6 GHz frequency band. This integration sought to extend 5G's reach to mobile users situated in geographically challenging areas, such as maritime expanses. The satellite component, illustrated in Figure 3a as a transparent payload, was designed to serve as a repeater, limited to radio frequency (RF) processing tasks, including frequency conversion, amplification, and beam management [36; 37; 38].  
However, since the inception of 5G's design in 3 GPP Release 15 specifications, operating under the presumption of a terrestrial-based radio interface environment [7; 33], numerous challenges emerged due to the unique characteristics associated with satellites. Within the framework of conventional 3 GPP standards-based communication systems, these challenges encompass various critical dimensions [41; 42]. They include significant considerations like the substantial round-trip delays inherent to especially Geostationary Earth Orbit (GEO) satellites. Additionally, complexities have arisen from rapid Doppler Shift and intricate mobility patterns not typically encountered in terrestrial-based RAN equipment, particularly evident within the realm of Low Earth Orbit (LEO) satellites. Moreover, a formidable obstacle has emerged in the form of establishing communication coverage across geographical regions that extend beyond the boundaries of a single country. As delineated in Release 17 [34], the solution adopted has illuminated suboptimal  
Figure 3: Comparison between 5G system architectures with transparent satellite access and with terrestrial NR radio access and typical satellite network architecture.  
Figure 2: Studies and works for satellite access as a 5G New Radio and a backhaul entity in 3 GPP.  
design principles from the perspective of satellite components. These principles have been implemented through the adaptation of key procedures, notably encompassing aspects such as timing advance, hybrid automatic repeat request (HARQ) operations, and tracking area management. Importantly, this adaptation has been executed while concurrently preserving established procedural norms that have conventionally underpinned radio interfaces within the framework of 3 GPP standardization.  
Furthermore, ongoing advancements are currently under way within 5G-Advanced standardization [43; 44; 45]. Aligned with the goals of the IMT-2030 framework including 'ubiquitous connectivity' and 'connecting unconnected', it is anticipated that satellite components will be seamlessly and comprehensively integrated into the foundational design of the 6G communication system within the framework of 3 GPP standardization [36; 46; 47].