## 3 Materials and Methods
### Analysis of Independent Research Variables and Data Collection  
In this research, the following 17 independent variables or predictors selected from the set of research data provided by the M:tel operator are observed: (1) Cell; (2) Downlink (DL) PRB Usage Rate; (3) Average CQI; (4) DL ReTrans Rate; (5) UL ReTrans Rate; (6) DL IBLER; (7) UL IBLER; (8) Cell Traffic Volume DL; (9) Cell Traffic Volume UL;(10) Cell Downlink Average Throughput; (11) Cell Uplink Average Throughput; (12) Average DL User Throughput; (13) Average UL User Throughput; (14) UL Average Interference; (15) DL_QPSK.TB.Retrans; (16) DL_16 QAM.TB.Retrans; (17) DL_64 QAM.TB.Retrans. The M:tel BL mobile operator provided the data for research purposes based on the official Request. The Request specified the necessary variables related to KPIs, radio channel properties, utilization of physical resources, number of users, eNodeB parameters, topology and signal parameters in the observed research geo-space [19]. From the obtained database, the values of the variables for the period of data collection between 1 January 2021 and 15 January 2021 and with a one-hour sampling frequency were extracted in an Excel file for the purposes of this research. By inspecting the data, empty cells (missing values), unusual values equal to zero and unusual values even several thousand times less than usual were observed and filtered. The final database was formed and consisted of a total of 31,143 measurements for each of the observed independent variables. According to the supervised learning paradigm, the total data set is divided into two parts: (1) model training data, consisting of 21,756 measurements (instances or vectors) or 70% of the total data set, and (2) model testing data, consisting of 9387 measurements or 30% of the total available data set. Cell  
The access LTE network of the M:tel operator in the area of the observed three-segment road consists of a large number of eNodeBs that provide the connection of the UE with the rest of the 4G network. Their locations are represented by red squares in Figure 2. According to the number of mobile users, it is obvious that the highest density of eNodeB deployment is in the vicinity of BL city [19]. Also, in Figure 2, based on the colors and the map legend, areas with different levels of signal attenuation can be identified. Specifically, it refers to the areas between \(-\)126 dB and \(-\)90 dB and areas between \(-\)90 dB and 0 dB. Each of the eNodeBs covers one or more cells with a signal, and a total of 87 cells can be identified in the area observed. Figure 1: Geographical area of research. The smallest unit of radio resources in the LTE network that can be allocated to a user is called a Physical Resource Block (PRB). It consists of 84 resource elements (7 symbols of 0.5 ms duration \(\times\) 12 subs of 15 kHz each). When available but unused PRBs are not sufficient to serve all active users, it can cause degradation of quality of service (QoS). DL PRB Usage Rate represents the ratio of the average number of used physical blocks in the Physical Downlink Shared Channel (PDSCH) and the total number of DL PRB available, multiplied by 100. PDSCH represents a DL physical shared channel whose priority function is the transmission of user data, but also the transmission of data essential for control, and DL system information [19]. Average CQI  
The CQI can have a numerical value between 1 and 15, which the UE sends over the uplink connection to the base station. Based on the received CQI value, the eNodeB selects the appropriate Modulation and Coding Scheme (MCS), thereby defining the data transmission rate in the communication channel. This means that each CQI value is mapped to a specific MCS: Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM, 16 QAM, 64 QAM) [19; 20]. DL ReTrans Rate & (5) UL ReTrans Rate  
When the communication between the base station and the UE is not established in the first or any subsequent attempt, data resending or retransmission is performed.