 from a noisy radio environment. If we have two FM sig- nals modulated around the same frequency, then there is some ratio between them that ensures that the stronger signal is the one we hear. We call this the capture ratio of an FM receiver, and a typical capture ratio is 1.0 dB. That means if the desired signal is 60 percent of the power at the receiver and somebody else's signal is 40 percent, then a typical FM receiver should produce the desired signal. 1.10.2 Digital modulation AM and FM are analog modulation schemes in that some physical property of the radio wave is modulated in some proportion to a physical property of the meaningful 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 S (t) S (t) Signal Signal Modulation V (t) Modulation Figure 1.8 AM and FM modulation schemes. signal. The contrast, of course, is with digital modulation, where the signal is turned into a sequence of binary digits, bits, and those bits are transmitted by modulation of the radio wave. Since CDMA, the subject of this book, is digital modulation, let us con- centrate on digital modulation. In digital modulation, we are adding bits to the radio signal. A bit is the smallest unit of information, represented by one of two states at a particular location. In writing, we usually represent bits as having a value of 0 or 1. Actually, the meaning associated with a bit could be a number (built from Os and 1s) or a choice such as true/false or a repre- sentation of anything, including the human voice, through some particular code. There are two general approaches to digital modulation, coherent, and noncoherent. In coherent modulation, a pilot or reference signal is used. A pilot signal is a broadcast signal that is modulated with a known pattern sent from the same transmitter. The re- ceiver already knows what to