Multipath Wave Propagation and Fading（多路径波传播和衰减）.pdf

Chapter 5 Multipath Wave Propagation and Fading 5.1 Multipath Propagation In wireless telecommunications, multipath is the propagation phenomenon that re- sults in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. In digital radio communications (such as GSM) multipath can cause errors and affect the quality of communications. We discuss all the related issues in this chapter. 5.2 Multipath Small-Scale Fading Multipath signals are received in a terrestrial environment, i.e., where different forms of propagation are present and the signals arrive at the receiver from transmitter via a variety of paths. Therefore there would be multipath interference, causing multi- path fading. Adding the effect of movement of either Tx or Rx or the surrounding clutter to it, the received overall signal amplitude or phase changes over a small amount of time. Mainly this causes the fading. 75 5.2.1 Fading The term fading, or, small-scale fading, means rapid fluctuations of the amplitudes, phases, or multipath delays of a radio signal over a short period or short travel distance. This might be so severe that large scale radio propagation loss effects might be ignored. 5.2.2 Multipath Fading Effects In principle, the following are the main multipath effects: 1. Rapid changes in signal strength over a small travel distance or time interval. 2. Random frequency modulation due to varying Doppler shifts on different mul- tipath signals. 3. Time dispersion or echoes caused by multipath propagation delays. 5.2.3 Factors Influencin