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Wireless Underground Sensor Networks (WUSNs) are natural extensions of the established Wireless Sensor Network (WSN) phenomenon and consist of sensors buried underground which communicate through soil. WUSNs have the potential to impact a wide variety of applications including precision agriculture, environmental monitoring, border patrol, and infrastructure monitoring. The main difference between WUSNs and traditional wireless networks is the communication medium. However, a comprehensive wireless underground channel model for WUSNs has not been developed so far. In this thesis, the Soil Subsurface Wireless Communication (SSWC) channel model is developed based on an extensive empirical study in a large agriculture field. The results of the experiments provide important insights for the model, which have not been available in the wireless communication literature. The SSWC channel model captures the signal attenuation and bit error rate (BER) in underground settings based on five components: (1) The dielectric soil model estimates the soil permittivity based on soil parameters including soil moisture. (2) The direct wave model captures the attenuation of the line-of-sight signal between sender and receiver. (3) The reflected wave model considers the attenuation on the signal which is reflected at the soil surface before reaching the receiver. (4) The lateral wave model estimates the attenuation of a third front of waves that potentially reach the receiver. Due to the fact that a significant portion of the lateral waves’ propagation occurs over-the-air, this form of transmission is an excellent option to extend the communication range without increasing the power consumption. (5) The signal superposition model captures the phase shifting between the mentioned waves, the resulting attenuation, and the bit error rate. The SSWC model is validated through extensive underground experiments. To the best of our knowledge, this is the first channel model for the underground to underground communication in WUSNs with comprehensive set of features. The SSWC channel model is fundamental for the development of cross-layer communication solutions for WUSNs and for the development of underground to aboveground and aboveground to underground channel models for WUSNs.