# Spectrum Awareness for Cognitive Radios Supported by Radio Environment Maps: Zonal Approach +
## 4 Test Scenario and Exemplary Maps  
In order to investigate the impact of the interpolation techniques on the REM quality, numerous tests were conducted for UHF frequency band. In total, there were 21 test frequencies assigned by the governmental authority. The comparative analysis for three selected frequencies, i.e., 303.05 MHz, 863.5 MHz, and 1997 MHz, is presented in [43]. The selection of frequencies was made in the following way: Two boundary frequencies and one frequency in the middle of the band. In this paper, we selected the highest frequency (1997 MHz), as it is the most adequate for broadband communication systems. The experiment was conducted in the following way. First, measurements were taken in a real environment with 39 sensors to get input data, and then exemplary maps were created using different construction techniques. In the next step, the analysis of calculated RMSE for various arrangements of sensors was made. The initial distribution of 39 sensors is shown in Figure 2. There were Non-Line-of-Sight (NLOS) conditions for the following sensors:  
* P6-P8, P10, P11, P24, P25, P27, and P29--the average height of the forest separating sensors is about 25 m,
* P2, P18-P22, P30, and P31--the approximate height of the buildings separating sensors is between 12 m and 15 m,
* P1, P3-P5, P9, P12-P17, P23, P26, P28, P35, and P36--the approximate height of the buildings and single trees separating sensors ranges from 8 m to 10 m.  
For the sensors P32-P34 and P37-P39, Line-of-Sight (LOS) conditions could be observed. The area presented in Figure 2 was divided into four zones that reflect the distance of the sensors from the TX antenna and the signal levels measured by the sensors. Zone 1 represents the sensors located around the TX antenna with the received signal level higher than \(-90\) dBm. Zone 2 includes sensors that measured a signal lower than \(-90\) dBm and were deployed east of the TX antenna. This zone is an example of an urbanized area. Zone 3 also contains sensors with the signal level lower than \(-90\) dBm, but located west of the TX antenna. This zone represents partly forested and partly rural area. Zone 4 was created at the bottom of the map and exemplifies a distant area at the edge of the radio range. All the sensors within this zone measured very low signal levels, lower than \(-100\) dBm. This division of sensors into four zones was used in our further analysis. To assess the quality of the maps created with the selected interpolation techniques, the calculated RMSE values were compared for three groups, each with a different number of measurement sensors used for interpolation, called: Scenario 26 (26 sensors out of 39), Scenario 20 (20 sensors out of 39), and Scenario 13 (13 sensors out of 39). The remaining sensors were used as control sensors to validate the result of the interpolation. Moreover, each of the three scenarios consisted of three tests (Test_a, Test_b, and Test_c) with different sets of measurement sensors used for interpolation. This means that there were three different deployments of measurement sensors for each scenario. Such an approach to the problem of data validation is known in the literature on the subject as the repeated random subsampling validation or Monte Carlo cross-validation. In order to be in line with this method of validation, measurement sensors were selected randomly for the interpolation process. Finally, for each test the difference between the measured and the interpolated signal level was compared for all the control sensors. It should be noted that the measurement (training) and control (validation) sensors did not overlap in any of the tests. The sets of the control (validation) sensors for each test are presented in Table 1.  
The measurement system that was used to collect input data for the purpose of creation of REMs is presented below.