ow us to use our favorite analogy for explaining CDMA. Consider a few dozen peo- ple in a small room all talking in pairs. Each listener knows his or her speaker's voice and can tune out the other voices that interfere with his or her own conversation. This tuning-out ability has limitations and the ability of these listeners to understand their speakers runs out if we have too many people talking at the same time. In CDMA, we assign each digital stream its own distinct voice in the form of a digital code, and all of these streams coexist on the same radio channel all at the same time. Spread spectrum came from military research where resistance to enemy jamming was the major design issue. There are several spread-spectrum technologies, and CDMA happens to be resistant not only to deliberate outside interference but also to other users on the same channel. 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 Engineering Concepts Radio Engineering Concepts 1.4 Bandwidth as Real Estate We define radio territory by land area and frequency range. A transmission license au- thorizes its owner to transmit only within a specified geographic region and between lower and upper frequency bounds set in the license. These licenses are regulated by governments in just about every country in the world today. Here in the United States, the Federal Communications Commission (FCC) gives out broadcast licenses based on the service being offered and the technology being used. In the United States, it would be fair to say that regulation of the radio spectrum is tighter and more restrictive than mineral rights but looser and freer than airspace, which is controlled minute by minute by the Federal Aviation Administration (FAA). The allocation of radio frequency carries with it the obligation not to transmit on any other frequency bands. More frequency bandwidth