Multipath propagation characterization for terrestrial mobile and fixed microwave communications

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Doctoral thesis (article-based)
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38, [102]
Helsinki University of Technology Radio Laboratory publications. Report S, 256
Multipath propagation is a key issue studied throughout this thesis, and it causes dispersions in delay, frequency and spatial domains. These are dominant phenomena in both terrestrial mobile and fixed wideband communications. In this thesis, multipath propagation mechanisms including diffraction, refraction, reflection and scattering are studied when radio waves interact with dielectric and metallic objects, or an atmospheric duct. Measurements were also performed for empirical modelling and validation of the theoretical work carried out in this thesis. By using physical optics (PO) method, the attenuation by double knife edges with ground reflections is solved for the first time under a general formula of the attenuation by multiple knife edges with ground reflections derived in this thesis, and some important and interesting conclusions are obtained. The attenuations by curvilinear-topped obstacles and by multiple flat-topped obstacles are also presented in closed forms. The results are the simplest and easiest ones available now, and they can be applied for field strength predictions both in mobile and fixed microwave communications. Based on three-ray (direct, reflected and super-refracted) and two-ray (direct and super-refracted) multipath models for plane and spherical earth, respectively, frequency selective fading (FSF) and depolarization due to clear air are studied by simulations and experiments for terrestrial line-of-sight (LOS) microwave links and dual-polarized communication systems. Novel simulation methods have been introduced and applied based on the fact that the amplitudes and excess delays of the rays are functions of the (modified) refractive index gradients which are random variables with exponential and normal distributions inside and outside the duct in lower atmosphere, respectively. Some important empirical or semi-empirical models and parameters are presented at 5 GHz based on large amount of measured data in indoor and outdoor environments. The results include path loss models, excess delay and rms delay spread, spatial and frequency correlations, window (sector) length of averaging fast fading components, path number distribution, and tapped-delay-line (TDL) channel models. These empirical or semi-empirical parameters and models are the latest results achieved at 5 GHz, and they are of great importance in designing of future wireless local area networks (WLAN), especially the TDL models are developed for the first time in this frequency band. Using a general autocorrelation function derived in this thesis for three-dimensional (3-D) scattering environments, a novel theoretical modelling method is developed to study the propagation mechanisms of different types of Doppler spectra observed in measurements. The 3-D autocorrelation function is connected to the probability density functions (PDF) of the angles of arrival (AoAs) of the scattered waves and the antenna radiation patterns in the azimuth and elevation planes. This is a new work which tries to define and explain the physical reasons of 3-D Doppler spectra from propagation point of view. A new computer simulation method for wideband 3-D received signal level in an urban environment is developed under the general assumptions of the distributions for path number, amplitude, excess delay etc. This simulation method can provide detailed fading characteristics for wideband mobile communications in a specific urban environment.
multipath propagation, diffraction, frequency selective fading, depolarization, WLAN, radio channel modeling and sounding
Other note
  • X. Zhao and P. Vainikainen, Multipath Propagation Study Combining Terrain Diffraction and Reflection, IEEE Transactions on Antennas and Propagation, vol. 49, no. 8, pp. 1204-1209, Aug. 2001.
  • X. Zhao, P. Vainikainen, and J. Kivinen, Diffraction over Typical-Shaped Terrain Obstacles, Journal of Electromagnetic Waves and Applications, vol. 13, no. 12, pp. 1691-1707, Dec. 1999.
  • X. Zhao, Y. Zhang, P. Vainikainen, and Y. Xie, The Experimental and Computer Simulation Studies of Frequency Selective Fading in Line-of-Sight Terrestrial Microwave Links, Radio Science, vol. 36, no. 6, pp. 1393-1403, Nov.-Dec. 2001.
  • X. Zhao and P. Vainikainen, Computer Simulation of Multipath Depolarization due to Clear Air, Microwave and Optical Technology Letters, vol. 23, no. 1, pp. 4-7, Oct. 1999.
  • X. Zhao, J. Kivinen, P. Vainikainen, and K. Skog, Propagation Characteristics for Wideband Outdoor Mobile Communications at 5.3 GHz, IEEE Journal on Selected Areas in Communications, vol. 20, no. 3, pp. 507-514, Apr. 2002.
  • J. Kivinen, X. Zhao, and P. Vainikainen, Empirical Characterization of Wideband Indoor Radio Channel at 5.3 GHz, IEEE Transactions on Antennas and Propagation, vol. 49, no. 8, pp. 1192-1203, Aug. 2001.
  • X. Zhao, J. Kivinen, P. Vainikainen, and K. Skog, Characterization of Doppler Spectra for Mobile Communications at 5.3 GHz, accepted for publication in IEEE Transactions on Vehicular Technology, May, 2002.
  • X. Zhao, I. T. Rekanos, and P. Vainikainen, A General Computer Simulation Method for 3-D Received Signal Level in Wideband Mobile Communications, Microwave and Optical Technology Letters, vol. 32, no. 2, pp. 119-122, Jan. 2002.
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