Two important issues of the research of radio propagation for the design of mobile communications systems and for better understanding of the propagation mechanisms were addressed in References [1] through [3]. The first is received-signal prediction for the coverage planning. The second is radio-channel characterization and modeling for broadband wireless communications systems.
Paper [1] presents a practical aspect of over-rooftop multiple-building forward diffraction from a low source, including two formulations and an introduction of their application. In particular, the low-loss formulation for multiple-diffraction exposes a factor, which accounts for strong over-rooftop propagation. Both low and high-loss formulations take the advantages of the uniform geometrical theory of diffraction and physical optics and have the major advantage of significantly shortening the computation time over existing formulations. They behave well particularly in and near the transition zone and are written in explicit forms additionally for soft-boundary that corresponds to vertical polarization transmission and reception in the horizontal plane. The application in the vertical plane for the total received signal prediction is introduced, using the formulations for hard boundary corresponding to vertical polarization transmission and reception.
The lengthy time spent in ray-trace computation is a major existing problem with mobile radio propagation prediction for urban micro-cellular environments. The use of two-dimensional (2-D) methods in a horizontal plane appears to be a solution for site-specific prediction for both transmitter and receiver lower than the surrounding buildings. It was seen that fast 2-D methods, which do not require a building height database, provide good accuracy in many cases. However, further studies on multiple-diffraction are required, because after the first diffraction there is a difficulty in determining and successfully using the second and later diffraction. Paper [2] presents fast 2-D diffraction modeling in the horizontal plane for site-specific propagation prediction in urban micro-cellular environments. Comparison with measurements and simulation of published results validate the 2-D modeling that significantly reduces the computation time and overcomes the limitation and difficulty of existing techniques for multiple-building diffraction. The modeling makes three contributions which are (1) extension of an expression for multiple building forward diffraction and use of the extended formulation, (2) equivalent source technique for parallel-street multiple diffraction, and (3) inclusion of reflections from curved surfaces of street building corners. Additional comparisons of three-dimensional model prediction with measurements for an urban line-of-sight environment place the work in a more complete context. The results show that an increase in the number of rays may not enhance the accuracy of ray-trace model prediction.
The work reported in Reference [3] has a two-fold purpose: (1) to model wide-band propagation in urban micro-cellular environments and (2) to verify fast two-dimensional ray methods used which do not require a building height database. The techniques of wide-band measurements and modeling also provide better understanding of mobile radio propagation mechanisms. The report presents wide-band propagation modeling in the shadow, i.e., out-of-sight, regions of urban micro-cellular environments and comparisons of model predictions with measurements. The results show it feasible to use two-dimensional ray methods in the horizontal plane for propagation predictions in side and parallel street environments. It is important to select proper rays whose relative phases and amplitudes are on the orders considered. The ray methods were applied in the modeling of wide-band propagation.