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Analysis of Specular and Off-Specular Scattering in Millimeter-Wave and Terahertz Applications

Ghani, Hossein | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58828 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Ahmadi Boroujenir, Mehdi
  7. Abstract:
  8. In this dissertation, the problem of wave propagation and scattering modeling in multilayer structures with rough interfaces in the terahertz and millimeter-wave frequency bands is investigated. Due to the short wavelengths in these bands, the surface roughness of materials attenuates the specular component and generates extensive off-specular scattering, which poses challenges for the design of sixth-generation communication systems and non-destructive testing. In the first part of this research, two matrix frameworks are developed to predict the specular component. First, the Rough-Surface Transfer Matrix Method is presented, which directly embeds Gaussian roughness statistics into two-by-two matrices. While maintaining linear computational complexity, this method eliminates the need for iterative solvers and provides suitable numerical stability at high frequencies. Furthermore, the effective layer method is introduced, which, by replacing the entire multilayer stack with an effective half-space, accelerates computations for inversion problems by up to 2.5 times compared to full-wave methods. In the second part, to overcome the limitations of existing models in power splitting, a hybrid multilayer scattering model is introduced. This model predicts the angular distribution of the non-specular scattered power by combining a recursive method for specular fields and the Kirchhoff scalar approximation for non-specular fields, utilizing a physics-based power splitting factor. For rigorous validation of the model, a measurement setup based on a Terahertz Time-Domain Spectroscopy system was implemented, and the model's results were compared with laboratory data. In this regard, the statistical parameters of the materials' surfaces were extracted using the photogrammetry method, and the measurements were conducted with a high dynamic range. Simulation results demonstrate a root-mean-square error reduction of up to approximately 90% compared to classical models. Finally, by conducting a parametric analysis, the concept of a scattering envelope is introduced to determine uncertainty bounds. These achievements provide a tool for THz network designers to estimate the link budget in non-line-of-sight scenarios, as well as for NDT specialists to determine the detection threshold of subsurface defects
  9. Keywords:
  10. Electromagnetic Scattering ; Multilayer Structure ; Transfer Matrix Method ; Terahertz Band ; Millimeter Wave ; Nondestructive Test ; Sixth Generation (6G) Communications

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