## 2 Related Work and Technologies
### TV Whitespace  
TV whitespace (TVWS) refers to TV channels located between frequency bands not used for TV broadcasting in certain regions. TVWS are parts of the radio frequency spectrum not used by transmission, also called interleaved spectra [22]. In global, TVWS are also referred to as currently unoccupied portions of the spectrum in the terrestrial region in television frequency bands in the Very High Frequency (VHF) and Ultra High Frequency (UHF) TV spectra (either analogue or digital, especially in the UHF band). In a simpler way, the TVWS spectrum represents a large part of the UHF spectrum (300 MHz-3 GHz), that is, hundreds of MHz, which in some countries also include VHF, which is available in a specific geographic region and can be used in a shared way. This spectrum can be used by primary (licensed) users or by secondary users who, using non-licensed equipment, can share the spectrum with digital TV transmitters, among other types of users. The amount of terrestrial whitespace available depends on several factors, such as geographical characteristics, the level of potential interference in the incumbent TV broadcast service, TV coverage objectives and related planning and use of television channels [29]. In the work presented in [22], the authors refer to the importance of TVWS, focusing their study on the application component and key areas in the application field. Starting by proving that TVWS has excellent penetration in buildings and good propagation characteristics, which, in turn, makes a TV band an innovative platform with great potential in a wide range of important applications, whether used indoors or abroad, the authors show that it is of great interest to investigate not only the quantity of TVWS and characterize its main properties, but also to evaluate the real applications of TVWS in reality. The TVWS use cases discussed in the study are particularly focused on wireless broadband access in rural environments, future wireless home networks, WLAN wireless services and smart grid network/smart meter communication. In [30], several pilot projects are presented, namely in Africa, Europe, Asia and North America, mostly in rural areas [31], showing great potential. However, after several tests, the question of the applicability of TVWS was left unanswered. It is not clear why the TVWS tests were defined years ago, but they did not result in any commercial applications. This is relevant when considering the power of the restricted market for telecommunications operators, implying that both governments and regulators are not interested in the implementation of this technology. For example, in 2013-14, there was a movement to implement Microsoft-funded TVWS in Bangladesh, yet no regulatory movement was expressed by the Bangladesh government in regard to TVWS at that time. The reason believed to be behind this decision concerns all operators being busy with their 4G licensing during that period. Adding to this problem is the fact that the commercial deployment of TV blanks, especially 470-698 MHz, are not allowed to be used for research purposes in many countries, as they present a risk of security and interference with other sectors of commercial activity [30]. The works presented in [32; 33] focus on the need for the existence of a geographic database of previous TVWS available in different countries, since the frequencies available are different from country to country and within countries (from region to region), each of which has their own policies and different regulatory regimes. It is therefore necessary to safeguard the spectrum of frequencies that are used by security forces, emergency and commercial entities, without any type of interference. In [34], the authors describe external field measurements in TVWS carried out in Munich, Germany.