# On Developing Techniques for Sharing Satellite Spectrum with Indoor Small Cells in 5G +
## 1 Introduction
### Related Work and Problem Statement  
In the existing literature, numerous research contributions can be found on both iSMSs [6, 7] and hSMSs [8, 9, 10, 11, 12], focusing primarily on interference issues. However, instead of being standalone systems as aforementioned, a hybrid space-satellite and terrestrial-mobile system may also consist of an iSMS and an autonomous terrestrial-mobile system (aTMS). For example, in [13], to demonstrate a number of communication modes, the authors proposed and described the overall system features for a hybrid space-satellite and terrestrial-mobile system. CGCs working as repeaters are considered standalone or collocating with the terrestrial cell sites to provide coverage in areas where the satellite signal strength is poor. Whereas the satellite covers areas outside the coverage of CGC stations and terrestrial cell-sites, and terrestrial cell sites cover areas of the coverage of satellite and CGC regions. Furthermore, in [14], it was proposed that traditional land mobile radio (LMR) systems can be deployed with an iSMS to use its resources. LMR systems are provided with the iSMS handset to facilitate advanced features and to provide coverage and capacity in case of an emergency. Likewise, aTMS may also collocate with an iSMS, forming a hybrid iSMS and aTMS system to share the spectrum of iSMS with aTMS. Recall that since most data in TMSs are generated in indoor environments, indoor small cells of an aTMS can help share it with the spectrum of iSMS. In line with this, the Federal Communications Commission (FCC) proposed to make available 100 megahertz of spectrum for sharing between satellites and small cells in the 3.5 GHz band [15]. Additionally, a study item for provisioning non-terrestrial networks including iSMSs and standalone satellite systems for 5G mobile networks has been initiated recently by the Third Generation Partnership Project (3 GPP) standardization bodies [16]. However, research on spectrum sharing between satellites and small cells in indoor environments are not so apparent. A few works (e.g., [17, 18, 19, 20, 21]) in this direction have been carried out. In [17], the authors investigated sharing both indoor and outdoor small cells with an FSS at 3.5 GHz and analyzed the co-channel interference between small cells and the FSS. The authors in [18] studied the performance improvement in spectral and energy efficiencies by offloading satellite UEs whenever a satellite UE was within the coverage of any small cell, so that the satellite spectrum can be shared with 3-dimensional (3D) in-building small cells. In [19], the authors proposed a number of in-building small cell Base Station (BS) architectures by exploiting the number of transceivers as well as operating spectrums including the space-satellite spectrum, to share with small cells deployed within a building subject to satisfy a co-channel interference management technique. Authors in [20] evaluated the interference between the Long-Term Evolution (LTE) Hotspot indoors and the Earth station of the FSS in the 3.4-3.6 GHz band. Likewise, the authors in [21] studied the deployment and efficient sharing of satellite spectrum in the C-band with indoor small cells as well as device-to-device communications. However, other than the traditional hybrid and integrated satellite and terrestrial mobile systems, developing spectrum sharing techniques for a hybrid iSMS and aTMS to reuse the satellite spectrum to small cells in 3D buildings is not obvious in the literature. Recently, in [22], we addressed this issue (i.e., developing spectrum sharing techniques for a hybrid iSMS and aTMS) where we considered that CGC stations and SBSs were collocated in an apartment of a building for the purpose of simplification. However, in practice, CGC stations are deployed under the control of a satellite service provider, whereas SBSs are deployed either by the customer or by the operator of an aTMS [23; 24]. Hence, it is not a necessary condition that CGC stations and SBSs within a building need to be collocated to one another.