ould pounce on the call button to get a channel before the other waiting subscribers. The failure of IMTS was not a technical failure, but rather a technology overwhelmed by demand for mobile telephone service. There is something amusing about seeing de- mand overwhelm our cellular technology that has capacity thousands of times greater than IMTS. The cellular carrier has direct control of the cost-versus-quality tradeoff. Use a fre- quency a little more often, and the call quality is a little lower-and the cost is also a little lower. As we will discuss in Sec. 25.4, a radio channel used every seventh cell pro- vides higher call quality than the same radio channel used every fourth cell. However, 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. Analog Wireless Telephony (AMPS) Key Radio Concepts using the channel every fourth cell allows each cell to service seven-fourths (1.75) times as many simultaneous calls. The cellular engineer can exercise this tradeoff on a small scale as well as system- wide. In a high-density region that needs a few extra channels, it may be worth a few noisy calls to serve many more subscribers. There is no high end of mobile telephone call quality. Optimization is more about making more calls good than about making some calls great. Making a good call better is an insignificant improvement, whereas making bad calls good is of major impor- tance. An AMPS call at a 10-dB signal-to-interference (S/I) ratio is lawful, 15 dB is okay, 20 dB is good, and 25 dB is great. Going from 25 to 40 dB S/I is an almost impercepti- ble improvement. The important cellular engineering issues are at the low end of the SI range. 6.4 The AMPS FM Channel The AMPS channel is a 30-kHz FM radio channel. The full AMPS channel is a two-way channel with 45 MHz between the two directions. The higher frequency, around 875 MHz, goes from the bas