# Mechanism to Estimate Effective Spectrum Availability Inside Smart Buildings
## 1 Introduction
DSA means that a non-licensed system can use an available segment from the spectrum assigned to incumbent operators in any spatial position without causing interference to incumbent users; in this context, the incumbent operator is called the Primary System (PS), while the operator that makes dynamic use of the spectrum is called the Secondary System (SS) [13]. Here, it is crucial to indicate that Cognitive Radio (CR) is a wireless system that uses the DSA to make efficient use of the spectrum [13], so CR-based systems can act like an SS. Wireless systems based on CR have the functionality of intelligently changing their transmission/reception parameters to try to maintain continuity in the service, despite using the time-changing spectrum availability of the spectrum segment of the primary band [14]. For its operation, an SS based on CR must perform the process of finding spectrum availability in the PS; then, this spectrum information is passed through a set of policies to obtain an effective spectrum availability (ESA) that can be used opportunistically and intelligently [14]. Therefore, for the operation of CR systems within an SB, it is necessary to identify and quantify the ESA. To find the available spectrum segments, different strategies can be used, which are consolidated into two broad groups: one of them using information provided by specialized databases and the other using local measurements [10]. However, by using the information of the databases or by using internal spectrum measurements, the internal spectrum availability information would only indicate the presence of white space, i.e., it indicates either the presence or absence of the incumbent operator. For example, determining the building's inner white spaces on the TV band known as TV white space (TVWS) does not guarantee that a non-licensed operator can effectively use the building's internal available spectrum because they could cause interference to primary users. Indoor TVWS and ESA are similar but not the same because the TVWS is found using the power level measurements of the Primary Band and a threshold level, while ESA determination requires that when the SS transmits using the TVWS, it does not interfere with the building TV receiver device's internal deployment. Thus, for obtaining the ESA, it is necessary to consider an additional condition, the minimization of the possible local interference that CR-based short-range wireless systems (SR-CRWS) could produce on the incumbent user network deployed inside the SB [10; 15]. Because CR must work with reusable available spectrum, the ESA determination inside a building has become a real and current challenge. Moreover, determining the ESA would have the benefit of allowing the operation of a private wireless service or system that could be planned in the interior of an SB without the need to use new spectrum bands. Such interior systems could be SR_CRWS, such as cognitive wireless networks and cognitive wireless home networks, using any current technology (e.g., Wi-Fi, GSM, or LTE) [9]. Since the SSs proposed here will operate inside buildings, they need to use a frequency segment that can quickly propagate through internal walls. That is, a spectrum that can overcome inner obstacles using low transmission power levels is required--obstacles such as the interior walls of a building, for example. The band assigned to TV broadcast fulfills such requirements [16; 17]. This work uses TVWS for efficient spectrum reuse because it allows broadband connectivity and IoT connectivity, providing solutions for private networks, smart grids, and smart buildings, among others [18]. Nevertheless, it is very important to clarify that this procedure can be replicated for the other primary bands. Considering that, this article proposes a validated measuring mechanism that allows the estimation of the lower available amount of ESA inside an SB located in a smart city dense urban area with an appropriate inner deployment of incumbent users.