# On Developing Techniques for Sharing Satellite Spectrum with Indoor Small Cells in 5G +
## 2 System Model  
### System Architecture  
In Figure 1, the system architecture of the proposed hybrid integrated satellite-mobile system and an autonomous terrestrial-mobile system (iSMS/aTMS) was shown, which consists of an iSMS and aTMS. The iSMS consists of a satellite station integrated with CGC stations and serves satellite UEs in both outdoor and indoor environments. However, the aTMS incorporates a set of small cells, namely femtocells, which are deployed within a set of multi-storage buildings. All buildings were assumed to be located within the macrocell coverage of the aTMS. Both SBSs and CGC stations were assumed to deploy randomly within each building and operate at the same satellite spectrum. Note that only one building is shown in Figure 1 for simplicity. For aTMS, we assume that a certain percentage of macro UEs were located within buildings, whereas a group of outdoor macro UEs is offloaded by picocells. Macro UEs can operate only at the spectrum of aTMS by the macrocell. Each building consisted of a set of floors and each floor consisted of a set of regular square-grid apartments. Furthermore, it was assumed that the transmission power of both an SBS and a CGC station was the same so that both had the same area of coverage, which was equal to the area of an apartment. Consider that an agreement between an aTMS operator and an iSMS provider exists so that all small cell UEs of aTMS can operate exclusively at the same satellite spectrum. Moreover, as shown in Figure 1, a global time-domain (TD) resource scheduler (RS) was considered to assign spectrum resources to both CGC stations and SBSs. When moving from the outdoor into a building, a satellite UE informs either the satellite station or an outdoor CGC station, depending on which one of them is serving it using the physical cell identity (ID) of the indoor CGC station that will serve the satellite UE within the building. To avoid co-channel interference, the RS informs the corresponding frequency-domain scheduler of the indoor CGC station to stop scheduling data signals during the ABSs of all collocated SBSs, corresponding to the indoor CGC station within the same building. Since an iSMS uses a common network and resource management system and a common core network, any specific handover procedure [13] can be defined for satellite UEs to minimize or to avoid the impact of speed of satellite on the UEs' handover. Hence, the satellite system can be either a geostationary Earth orbit (GEO) or a low Earth orbit (LEO). Furthermore, the operating frequency of the satellite system is either the L-band or the S-band due to the suitable propagation characteristics in these bands. Modeling Placement of Complementary Ground Component (CGC) Stations and Small Cell Base Stations (SBSs)  
We assumed that the deployments of SBSs and CGC stations in a building followed two independent and random processes. Let \(S\) denote the sample space representing the total number of apartments per building. Let \(S_{\mathbf{SBS}}\) and \(S_{\mathbf{CGC}}\) denote the set of SBSs and CGC stations deployed per building, respectively. Since in an apartment at most one SBS and one CGC station may exist, SBSs and CGC stations can be deployed in four ways within an apartment, assuming that the occurrence of each deployment option is equally likely. In other words, there are four possible events constituting the whole sample space for the deployment of SBSs and CGC stations in an apartment. Each event may  
Figure 1: The system architecture of the proposed hybrid integrated satellite-mobile system and an autonomous terrestrial-mobile system (iSMS/aTMS) [22].