# Spectrum Awareness for Cognitive Radios Supported by Radio Environment Maps: Zonal Approach +
## 2 Related Works  
Licensed spectrum users, in the literature referred to as primary users (PUs), do not use radio spectrum in a continuous manner or use spectrum resources in all locations. This poses a very important technical and organizational challenge related to the efficiency of the usage of spectrum resources. A relatively new proposal in this field is the concept of cognitive radios, which is associated with the opportunistic access to the spectrum by secondary users (SUs) [6; 7; 8]. This approach concerns the detection problem of the transmission from PU [9; 10; 11] and the rules of sharing spectrum resources with, for example, cognitive radios. Reuse of the spectrum, however, must not interfere with the transmission made by PUs and must, therefore, be carried out in accordance with the principles of compatibility [12; 13]. The generic idea of the coexistence between PUs and SUs is shown in Figure 1. Two primary networks are composed of PU transmitters and sets of PU receivers (PU #1 and PU #2). The range of Primary Network #1 is presented in purple color, while the range of Primary Network #2--in blue-grey color. There are three exemplary CR networks that do not overlap. CR 1 network is deployed within the range of Primary Network #1, while CR 2 network within the range of Primary Network #2. CR 3 network is arranged in the area that is covered by the ranges of both PU transmitters. In general, SUs are allowed to use resources assigned to PUs operating in the same area, however, they must obey a set of specific rules to avoid disturbing PUs. As a minimum, cognitive radios have to know the transmission pattern of PUs or have to recognize the transmission from PUs. In early stages of CR concept development, the US federal regulations related to the reuse of the so-called TV White Spaces (TVWS) imposed a strict requirement that emissions coming from PU should be detected above the level of -114 dBm [14; 15; 16]. Such a rigorous demand practically precluded the reuse of radio spectrum by CRs. For this reason, in the second decade of this century, new solutions were sought. One of the subjects of the World Radiocommunication Conference WRC-12 were cognitive radios. The main conclusion was that for implementation of CRs, no additional global regulations were required and that the start-up of CRs may be carried out on the basis of the current ITU Radio Regulations. Moreover, in this regulation, it was stated that at least it is necessary to outline the rules for the protection of licensed users (systems) and to define the conditions for cross-border coordination of CR systems operating in neighboring countries [17; 18]. One of the solutions aimed at making the operation of CRs possible was the concept of the centralized database, which would store all the registered transmitters with their geographical coordinates, characteristics, and transmission patterns [19]. Although such a concept created better opportunity for spectrum sharing, it took into account static PU transceivers only. On the other hand, the market of mobile devices is still growing and therefore, there is a need to include mobile transmitters in the database too. To meet the need the concept of the Radio Environment Map (REM) was proposed. As a general rule, REM is considered as a database which provides comprehensive and up-to-date information on radio spectrum. It is assumed that this information is composed of results of measurements, geographical features, available services, spectral regulations, positions and activities of radios, policies adopted by the user and/or service providers, and knowledge from the past [20; 21]. It is expected that measurements are conducted in a continuous manner to update the database regularly and to get a recent and reliable picture of the radio spectrum. For this reason, REMs are perceived as a useful and powerful tool supporting frequency assignment authorities.