# Distance Protection for Coexistence of 5G Base Station and Satellite Earth Station
## 1 Introduction  
The demand for fifth generation (5G) radio frequency (RF) communications continues to grow at an alarming rate; as a new generation of mobile communication technology, 5G has higher data rates, shorter delays, and larger numbers of connections than 4G. The 5G technology utilizes 'high','medium', and 'low' frequencies [1]; therefore, it can exploit the specific characteristics of different frequency bands of the spectrum based on usage scenarios. For instance, low frequencies (below 2 GHz) will continue to be essential for extending the 5G mobile broadband experience to wide areas and deep indoor environments. On the other hand, high frequencies (above 6 GHz) will prove indispensable for providing additional capacity and delivering extremely high data rates required by some of the 5G enhanced mobile broadband (eMBB) applications. Furthermore, medium bands (2-6 GHz) are suitable for managing the trade-offs between extended coverage and high capacity. In 2018, the Republic of Korea completed auctions of the 5G frequency bands, namely the 3.42-3.70 GHz and 26.5-28.9 GHz bands, as shown in Figure 1[1]. In the future, additional frequencies may be allocated from the adjacent frequency bands; however, there are incumbent users in the existing bands, such as fixed-satellite service (FSS) [2, 3], inter-satellite services, Earth-exploration-satellite service, and meteorological-satellite service [4]. UK Ofcom has identified the 3.8-4.2 GHz band to have the potential for increased sharing and is considering the development of proposals to facilitate shared-frequency access in this band. In August 2018, the FCC of the USA adopted a Notice of Proposed Rulemaking to release the 3.7-4.2 GHz band for new wireless services, while accommodating incumbents, and to seek comments on proposals to promote more spectrum-efficient, and intensive,fixed use of the band on a shared basis. Previously, several studies have been conducted on GSM to access the current utilization of the C-band (3.5-4.2 GHz) to define a strategy and roadmap to release this band to support the deployment of 5G services. This research has been, and is being, conducted to ensure independent coexistence with other C-band users [5]. The interference between two communication systems is the main problem of the coexistence of two systems in one frequency band. There are four main types of interference between 5G and FSS systems: interference from 5G base stations (BSs) to FSS Earth stations, 5G users to FSS Earth stations (ESs), FSS devices to 5G BSs, and FSS devices to 5G users. Figure 2 shows the coexistence relationship between the FSS and cellular systems. The signal transmission paths from the space station (SS) to the ESs constitute the downlinks of the FSS system, mainly in the 3.7-4.2 GHz band, and the signal transmission paths from the ESs to a SS constitute the uplinks of the FSS system mainly in the 29.1-29.9 GHz band. In Figure 2, 1 is the signal transmission path from a BS to an ES, and the ES interferes with the 5G system that must be protected [2,6,7]; 2 is the signal transmission path from the user equipment (UE) to the ES, and the downlink from the SS to the ES should also be protected [2,6]. For the interference experienced by the 5G BS/UE from the SS, 3 and 4 are downlinks from the SS to the BS and UE, but it has not been verified whether 3 and 4 can coexist. The uplink from the ES to SS operates in the 29.1-29.9 GHz band, and it can coexist with the links 5 and 6[8]; however, 7 and 8, which are the interference from the ES to BS and UE, should be protected [8,9]. In this study, we focus on minimizing 1, the interference from the BS to the ES. The main problem should be solved, to achieve the coexistence between the FSS and 5G cellular, under the following considerations: Furthermore, compared with 2, the 5G gNBs utilizes massive multi-input multi-output (MIMO) systems, and the FSS ESs use antenna dishes with high directivities and antenna gains. This could lead to the alignment/misalignment of their respective antenna directions [2].