# Mechanism to Estimate Effective Spectrum Availability Inside Smart Buildings
## 3 System Model
### Scenario Description  
Figure 1 shows a dense urban area with highly homogeneous buildings, which maintain a high demand for internal wireless communication services (e.g., an SB) and being part of a megacity (e.g., an SC), shaping a scenario that looks like a Manhattan grid. In the same Figure 1, all the SBs are within the incumbent coverage area, which is radiated by robust systems (e.g., broadcast TV systems); the incumbent operators are the PS.  
Inside every SB, all apartments get allocated one SR-CRWS that can operate like an SS. All SR-CRWS have their own operation frequencies (e.g., ISM bands), but under strong interference conditions, they use the available inner spectrum to maintain the service. Likewise, the deployment of primary users (PUs) is considered, and this is represented by an antenna located on the rooftop of the building. It is important to note that the said antenna feeds an internal distribution system, not shown in the figure, which uses a wired network (i.e., coaxial cable). As a result, the deployment of the PUs occurs when the antenna on the rooftop of the building senses interferences.  
Although it is common for the different SBs in an SC to be heterogeneous, in this study, the internal distribution in the SBs was identical, such that each building had \(N\) apartments of equal dimensions. Therefore, on each floor, the same number of SR-CRWSs was deployed to allow the opportunistic use of the PS spectrum if required.  
To determine the lower amount of spectrum availability inside the building, this study was performed in an SB that received the maximum power level of the PS. This building had one face receiving wave propagation at a normal incidence position; also, it needed a line of sight (LOS) from all its floors to the base station (BS). Finally, the effects of the power level reflected from neighboring  
Figure 1: Primary users and Cognitive Radio (CR)-based short-range wireless system (SR-CRWS) deployment within a smart building (SB).  
buildings were considered. The scenario was conveniently adjusted to achieve the most restrictive spectral availability conditions for obtaining the ESA.  
As shown in Figure 2, a BS of a primary system was located at a distance d opposite to the face of an SB with dimensions \(d_{\mathrm{SB}}\times d_{\mathrm{SB}}\times H_{\mathrm{B}}\). The BS had an antenna with a height \(H_{\mathrm{BS}}\) that radiated the power \(P_{BS}\) using the operating frequency \(f\), while the SB had a height \(H_{\mathrm{B}}=H_{p}p\), where p represents the number of floors of the building and \(H_{p}\) represents the height of each story.  
Additionally, Figure 2 shows a floor of the building where three regions stand out: red apartments, white apartments, and white apartments with red stripes. The white apartments indicate the inner SB area where there is effective availability of a primary band, contrary to the red apartments, which represent apartments without spectrum availability, and the white apartments marked with red stripes represent the zone where an SS producing interference with the PS is located.  
The white apartments represent the zone where the SS can use the spectrum available. This last zone represents the ESA for each floor, the zone where the spectrum can be used for CR-based systems.