also added by radio emissions from the sun and outer space. Receiver amplifiers have their own sources of thermal noise from their own compo- nents that usually overwhelms the natural radio thermal noise. These can be reduced by better amplifier design or by putting the amplifiers closer to the antenna terminals so that the cable losses come after a stage of amplifier gain. This alone can reduce the receiver noise by several decibels. Most radio noise is additive white gaussian noise (AWGN), so let us examine each of these four words in turn. Temperature is measured from absolute zero. 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 Key Radio Concepts Additive noise is simply added to the signal. As elementary as this sounds, there are other things that happen to radio signals. We use amplifiers to multiply radio signals, processing equipment to modulate a radio carrier, and other equipment to filter fre- quencies. A medium such as air, or air with raindrops in it, reduces the radio signal, of- ten reducing some frequencies more than others. The characterization of radio noise as additive is an important aspect of its behavior. White noise is signal with equal energy at all frequencies, statistically speaking. In the sound medium, white noise has a hissing shhhh quality. One audio example of a nonwhite noise is hum, clearly concentrated at 60 Hz (in the United States). Audio en- gineers use pink noise that has equal energy per musical octave. It has a more rushing sound than white noise because it has comparatively more low-frequency content. Gaussian noise follows the statistical normal or gaussian distribution. The mathe- matical properties of gaussian distributions are an important part of radio engineering theory. Hum is an example of nongaussian noise, but we would be hard-pressed to find a good example of white noi