## 2 Materials and Methods
### Frequency Band Selection  
The choice of the operating underground frequency was crucial, as attenuation caused by the soil increases with frequency. Investigating signal propagation through the soil as a transmission channel under challenging conditions and analyzing the quality of the electromagnetic waves were among the most complex aspects of this study. Many studies have indicated that the UHF frequency band is promising for underground communications. To validate this, we meticulously designed an experiment involving LoRa modules [15]. The experiment entailed burying LoRa modules, as shown in Figure 2, in holes 70 cm deep and spaced 2 m apart. We set up six holes with a total distance of 12 m between the first and last hole, ensuring comprehensive coverage. The LoRa modules were configured to send messages from the LoRa transmitter to the LoRa receiver over the 433 MHz Radio Frequency. The goal of this experiment was to determine the suitability of the 433 MHz frequency for underground communication. Figure 3 illustrates the thoroughness of our experimental setup:  
Figure 2: LoRa receiver and transmitter. The settings for the LoRa modules, chosen to achieve underground communication, are shown in Tables 1 and 2:  
The soil characterization was intended for buried sensor communication; we selected the lower frequency (433 MHz) for reduced propagation loss. We successively tested these outdoor nodes for underground-to-underground data transmissions under various operating conditions and radio parameters, as shown in Table 2. The results show stable packet reception between the RX (receiver) and TX (transmitter) nodes operating at 433 MHz, with no packet losses. The wireless transmission in the LoRa standard (ITU-T Y.4480) [16] was carried out on specific ISM frequency bands, including 433 MHz, 865 MHz, and 915 MHz, on a default bandwidth of 125 KHz. This selection determined the time required to modulate the data packet using Cyclic Redundancy Check (CRC), Spreading Factor (SF), and RSSI value Figure 4. The radiated transmission power was limited, since LoRa operates within the ISM frequency bands. To achieve a broader radio range than conventional modulation types, such as Frequency Shift Keying (FSK), the sensitivity of the LoRa receiver was significantly improved, so as to successfully receive and decode a useful LoRa signal even up to 20 dB below the noise level. Higher frequencies result in shorter wavelengths; however, 433 MHz and 868 MHz can exhibit similar Radio Frequency (RF) transmission performance, because many factors influence this performance. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Center frequency & 433 & MHz \\ Bandwidth & 125 & KHz \\ Coding rate & 4/8 & Bit \\ Spreading Factor (SF) & 12 & – \\ Output power & 14 & dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: LoRa module settings. Figure 4: LoRa receiver and transmitter response. 433 MHz LoRaWAN Network : This network operates in the 433 MHz frequency band, commonly used for short-range, low-power communications worldwide. It offers good underground penetration through obstacles and is suitable for remote control Wireless Underground Sensor Networks (WUSNs) and IoT applications. * 868 MHz LoRaWAN Network : This network operates in the 868 MHz frequency band, allocated for short-range, low-power communications in Europe and certain other regions. It offers an extended range compared to the 433 MHz network, making it a preferred choice for IoT, smart city applications, and industrial monitoring. LoRa transmits over license-free Megahertz Radio Frequency bands: 169 MHz, 433 MHz (Asia), 868 MHz (Europe), and 915 MHz (North America). LoRa enables very Long-Range wireless data transmission, with communications reaching ten miles or more in rural areas using low power. In Tunisia, both 433 MHz and 868 MHz are permitted. Since the soil characterization was intended for buried sensor communication, we selected the lower frequency (433 MHz) for reduced propagation loss.