 conventional reuse. There are three good reasons for this. First, CDMA systems are a technology evolution from FDMA and TDMA, and understanding the historical insights helps us understand the technology. Second, CDMA is strange and often counterintuitive, and we feel that it is often easier to under- stand what it is by understanding what it is not. And third, there are 700 million conventional reuse wireless telephones in use today, and a CDMA engineer should be well versed in their tech- nology as well. 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. Conventional Reuse Principles Capacity and Quality Principles -103dBm Signal -120dBm Noise Figure 27.1 A 17-dB S/N ratio with Rayleigh fading. At any given moment, the AMPS channel provides excellent audio quality as long as the signal exceeds the noise plus interference by the capture ratio described in Sec. 1.10.1. Since the capture ratio is only about 1 dB, the determining factor in voice qual- ity is the effect of Rayleigh fading. Consider a noise-limited setting, a 30-kHz narrowband channel with a moving trans- mitter or receiver. It does not matter which end is moving because the changing radio path generates a moving Rayleigh distribution in either case. According to the statis- tics of a Rayleigh distribution, the fraction of the time the signal is below the noise level is the average ratio of noise to signal. When the average signal-to-noise (S/N) ratio is 17 dB, the picture looks like Fig. 27.1. The Rayleigh fading signal dips below the noise level about 2 percent of the time. Listening tests at Bell Telephone Laboratories found that three-quarters of the listeners were pretty well satisfied with such a call, as we discussed in Sec. 18.3. In an interference-limited setting, the same 17-dB ratio of signal to interference (S/I) may be somewhat worse because the interference is also va