## 1 Introduction
### Related Work  
Numerous spectrum sharing techniques both for the licensed and unlicensed bands have already been proposed to operate small cells at the same spectrum by avoiding the co-channel interference using the elCIC technique, namely dedicated access, co-primary shared access (CoPSA), and licensed shared access (LSA) for the licensed access as well as licensed assisted access (LAA) operating in the 60 GHz unlicensed band for the unlicensed access as shown in Figure 1.  
For the spectrum sharing, MNOs that adhere with the strict QoS requirements, licensed access horizontal spectrum sharing licensing policies, particularly, the CoPSA policy has been proposed where any MNO can explore the shared spectrum allocated for the 5G mobile systems [5]. In the existing literature, numerous researches have studied CoPSA [5; 6; 7; 8; 9]. The CoPSS includes mainly two access techniques, namely spectrum pooling and mutual renting [6]. In spectrum pooling, licensed spectrum bands are shared by the national regulatory agency (NRA) among different MNOs, whereas  
Figure 1: Scopes of the spectrum licensing policies realized using elCIC technique in multiband in-building small cells.  
in mutual renting, the spectrum allocated to one MNO can be rented by another to address mainly the temporal capacity shortage [6].  
In [10], LSA has been proposed to exploit for the inter-cell interference coordination (ICIC) by intelligently allocating the LSA spectrum to center and cell-edge of cells in an MNO to reduce interference considerably. In [11], the use of the dedicated spectrum and the shared spectrum to in-building small cells have been discussed. In addition, the challenges for operating indoor wireless systems with the shared spectrum as compared to the dedicated spectrum and the importance of interference coordination due to the massive deployment of small cells in indoor wireless systems have been emphasized. In [12], co-channel interference in indoor systems due to sharing the satellite spectrum with indoor small cells have been analyzed. A study item for 5G mobile systems to support Non-Terrestrial Networks such as satellite system has been recently initiated by the third generation partnership project (3 GPP) [13; 14; 15]. Moreover, the Federal Communications Commission (FCC) has also proposed to share spectrum between small cells of mobile systems and satellite systems at 3.5 GHz [16].  
Likewise, LAA has been proposed for the LTE to operate at unlicensed bands with an incumbent system such as WiFi with or without employing the Listen-Before-Talk (LBT) function to avoid the collision with the WiFi system. In line with this, to address interference between the WiFi system and the LAA based LTE system without LBT, in [17] the authors have proposed a modified ABS based scheme such that the LTE system is absolutely inactive in a certain number of subframes per ABS pattern period (APP). The authors in [18] have discussed the potentials and challenges of coexisting WiFi and small cells of LTE systems sharing the same unlicensed spectrum. In addition, authors have presented the ABS mechanism and an interference avoidance scheme to address interference between LTE and WiFi systems. Besides, numerous studies such as [6; 19] have also addressed, mainly qualitatively, the overview of different spectrum sharing techniques. A number of studies have also studied the performance comparison among a number of spectrum sharing techniques such as [6; 9; 20].