n attribute of nature to be dealt with. And once nature is unkind enough to create multipath, the rake filter is the best way to deal with it. We are objecting to the mythology that CDMA somehow turns this obstacle into a kind of virtue that makes the system better. The best we can hope for is to keep multipath from making the system worse. 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. The CDMA Principle The CDMA Principle near-far problem, that signal paths vary considerably, as much as 80 dB if one sub- scriber is 100 times closer to the base station than another. We use power control in CDMA to adjust transmitters so that the received power levels are balanced. In AMPS and GSM, we are happy with power levels matching within a few decibels. This is nowhere near good enough for CDMA. A CDMA channel running at 85 percent of its full call capacity will overwhelm a call received at just 1 dB below optimal power. 10 Because CDMA requires each user to have a proscribed fraction of the signal pie, safety margins are not going to solve problems of inaccurate power control. Adding a 1.5-dB safety margin to the power-control algorithm only serves to reduce system ca- pacity by 30 percent. 11 CDMA relies heavily on the power-control algorithm doing an effective job of managing power levels. When the mobile telephone starts a call, it sets its transmit power based on mea- surements it makes of the forward channels. This initial choice of transmit power is done without base station participation, so we call it open-loop power control. Once the call radio link is established, within the two-way data stream is a flow of power-control messages going in both directions, power-up and power-down messages. This feedback- driven process is called closed-loop power control. Sitting here, godlike, with full knowledge, we can solve the CDMA equations in Sec. 26