e noise. To understand what something is, it is often useful to know something it is not. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Signal Quality Radio Signal Quality Wireless Telephone Transmitter Building Figure 2.1 Fading due to multipath transmission. The general term distortion is frequently used in the audio world, but one hears less frequently in radio. Analog telephone listeners know noise as a rushing or hissing sound on the line, terference as hearing fragments of another conversation, and distortion as garbled speech. These same three impairments affect radio signals as well. 2.1.4 Multipath Radio waves can go in straight lines, and this is how we usually visualize them travel- ing. However, they often travel on the bounce, and we use the term multipath for the phenomenon of receiving several copies of the transmitted signal. There are two multipath issues to deal with. First, the combination of the same ra- dio wave at different phases causes dramatic shifts in the cumulative amplitude, and second, the signal may be received repeatedly at different times. We use the term multipath fading for the varying amplitude due to multipath trans- mission. Fading is easy to understand if we picture a single-frequency broadcast from a single source being received on two paths, as shown in Fig. 2.1. Where one path is half wavelength longer, the two signals will tend to cancel each other out because they are out of phase. The same is true if the difference in path length is 1.5 wavelengths or any other difference that adds the multiple paths out of phase with each other. A collection of many paths of similar radio path gains at a single frequency produces a Rayleigh distribution, and we call the single-frequency multipath-fading phenomenon Rayleigh fading. For the mathematically curious, here are a few facts about Rayleigh fading.