无线通信基础(双语)_教学课件_3课件.ppt

Review of Last Class; Fading;Radio Propagation Model “... characterization of radio wave propagation as a function of frequency, distance and other conditions.”; General ray-tracing method (simulation) Multiple ray-tracing with propagation effects (reflection, diffraction, LOS path, scattering, etc) Might consider building material (steel, concrete, brick, etc) Empirical method On-site measurement Curve-fitting Statistical method;Chapter 2 ---- Lecture 2;Illustration;2.4 Large-scale path loss and shadowing;2.4 Large-scale path loss and shadowing; The Friis Equation Free-Space Path Loss; No interference, no obstructions There is no loss of energy in free space, but there is attenuation due to the spreading of the waves;Where: Pt = total power radiated by an isotropic source Gt = transmitting antenna gain Gr = receiving antenna gain d = distance between transmitting and receiving antennas ? = wavelength of the carrier signal = c/fc fc = carrier frequency;What troubles some people when they see this question is that the path loss also increases as the square of the frequency. While it is true that absorption of RF by various materials (buildings, trees, water vapor, etc.) tends to increase with frequency, remember we are talking about “free space” here.;Effective area of an isotropic antenna;1. Free space power flux density (W/m2) power radiated over surface area of sphere S ＝ Pt / (4? d2) By covering some of this area, receiver’s antenna “catches” some of this flux.;2. Effective area of an isotropic antenna The power received by an antenna of effective area A is given by: PR=S·A However, from electromagnetic theory, we note that the effective area of an isotropic antenna in any direction is given by: A= ?2 / (4? ) Thus PR=S·A=Pt ?2 / (4? d) 2;3. Directional Radiation While the isotropic antenna is a useful illustrative device, most antennas are not isotropic. Instead, they h